Abstract
1 Introduction: why the electromagnetic pulses are so important
Generation of electromagnetic waves was first demonstrated by Heinrich Hertz in 1887 and since then has become a leading subject of research, with an enormous range of applications covering radio communications, electronics, computing, radar technology and multi-wavelength astronomy. The accessible spectrum of electromagnetic emissions continuously extends toward shorter waves from radio waves to microwaves, to optical and X-rays[
Our review does not aim to cover all the issues related with the development and applications of pulsed electromagnetic sources. We address here the particular problem of microwaves generated during the interaction of powerful laser pulses with solid targets, in the domain extending from radiofrequencies (MHz) to terahertz. These electromagnetic pulses (EMPs), which are regularly detected in laser–target interactions with laser pulses from the femtosecond to the nanosecond range, are recognized as a threat to electronics and computers, and have stimulated the development of various protective measures. This situation has, however, significantly evolved since the invention of chirped pulse amplification (CPA) in lasers[
The main source of strong GHz emissions has been identified as the return current flowing through the support structure to the target, charged by the intense laser–target interaction. Controlling the geometric and electrical characteristics of the target support has therefore become the major EMP mitigation approach. The understanding of the physics of EMP generation has substantially advanced very recently, and other mechanisms of EMP generation have been identified. Among the related main research topics, we mention: the excitation of chamber resonant modes; the characterization of secondary EMP sources; the scattered radiation. These processes are discussed in Sections
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A full comprehension of the physics of EMP generation and the mechanisms of their operation will enable the creation of temporally and spatially controlled electromagnetic fields of high intensity and wide distribution. This would lead to the new and significant employment of laser–plasma interactions for powerful and versatile radiofrequency–microwave sources, which will be of direct interest to particle-acceleration schemes[
This review paper summarizes the recent knowledge and experience gained by scientists working with high-power laser systems in many laboratories worldwide. Section
2 Physics of EMP generation
2.1 Target polarization
The principal source of electromagnetic emissions is charge separation and target polarization under the action of a laser pulse. Strong laser fields ionize the atoms and create a plasma, which expands from the target surface. As the laser pulse interacts essentially with electrons, the plasma is far from thermodynamic equilibrium. The electrons are heated and accelerated by the laser pulse and their average energy is much higher than that of ions. Moreover, a relatively small proportion of the electrons are accelerated to energies much above the average and may leave the target[
The target potential
In order to evaluate the charge accumulated on the target, the target capacity
It is important to know how fast the charge is accumulated and how long it can be maintained on the target. The temporal characteristics of the current define the spectral domain of emission and the field amplitude. There are two characteristic times defining the charge accumulation: the laser pulse duration and the cooling time of hot electrons. The hot electrons are primarily cooled through collisions with atomic electrons in the target. The cooling time of MeV electrons on a solid target is on the ps timescale. For example, the cooling time of a 1 MeV electron,
2.2 Mechanisms of electromagnetic emission
2.2.1 Terahertz emission
Electromagnetic emissions are produced at all stages of the laser–target interaction. However, we are specifically interested in the emissions that are produced during the electron ejection process, that is, during and after the laser pulse on the characteristic time of electron cooling, which is about a few ps. The corresponding frequency is in the domain going down from 1 THz. The amplitude of EMPs in that domain is highly significant, and these frequencies are the most damaging for electronic circuits. Two principal sources of EMP emission can be identified: the first is related to the ejected electrons and the second to the return current.
In the case of ps or sub-ps laser pulses, the duration of electron ejection
In addition to the EMP emission during the hot electron ejection, the bunch of ejected electrons may induce secondary dipoles while flying near sharp metallic objects in the interaction chamber or striking the chamber walls[
2.2.2 Gigahertz emission
Emissions in the domain of frequencies lower than 30–100 GHz are produced on a timescale longer than 30–100 ps and related to the relaxation of the charge accumulated on the target during the laser pulse interaction. Let us consider an example of a metallic target in the form of a disc of diameter
The system of a target and a stalk attached to the ground is an example of a linear antenna. It may emit signals over a broad frequency range depending on the temporal shape of the feed-in current, but in our case of interest for a current pulse length that is shorter than the antenna length, the characteristic wavelength of emission is four times the stalk length,
Assuming there are no other objects in the near-field, the intensity of EMP emission at the main frequency of the target support structure can be estimated using the formula for a linear half-wavelength antenna[
Equation (
The role of the conducting stalk in EMP emission can be demonstrated in the following numerical experiment performed with the electromagnetic code SOPHIE[
Figure
The intensity of GHz emission can be affected by changing the stalk material and/or reducing the velocity of the propagation of the current. By using a dielectric stalk, one increases its resistance and consequently reduces the return current[
This simple analysis also explains why the ps laser pulses are much stronger emitters in the GHz domain, compared to the ns pulses. The former accumulate a big charge for a short period of time and discharge it in a short and intense current pulse. In contrast, the latter induce a relatively weak continuous current and consequently a much weaker emission. The authors of Ref. [
The EMP signal can be significantly enhanced if a long and a short laser pulses interact with the same target. In Ref. [
Among multiple sources of this emission, we mention the secondary polarization charges induced by ejected electrons on the conducting parts of the chamber[
2.3 Modeling of the electron emission
Ejection of energetic electrons is identified as the dominant source of target charging. This process is shown schematically in Figure
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The theoretical model developed in Refs. [
The electric potential is represented as a sum of the thermal potential created by the electrons in the Debye layer near the target surface and the positive charge left on the target surface by escaped electrons:
Figure
This model demonstrates dependence of the charging process on the laser and target parameters. The number and energy of hot electrons depend primarily on the absorbed laser energy, intensity and focusing conditions. The conventional estimate of hot electron average energy, given in Equation (
We discuss now generation of the neutralization current
The target size also has an impact on the charging process. Let us consider a cylinder with its axis aligned with the laser. It is characterized by thickness
The target thickness can vary from large values where hot electrons never reach the rear side to very small values
The target crystalline structure also affects the electron mean free path and consequently the accumulated electron charge. In experiments with targets made of different allotropes of carbon in Ref. [
In the study described in Ref. [
2.4 Numerical modeling of the EMP emission
Because of the large disparity of temporal scales, the process of electron emission needs to be simulated in several subsequent steps by using different numerical tools. First, the hot electron production during the interaction of an intense laser pulse with a solid target depends strongly on the quality of the target surface at the moment of laser pulse arrival. It may be modified by the laser prepulse and affect the absorption of the main laser pulse. The preplasma formation and its expansion from the solid target surface is described with a radiation hydrodynamic code on the ns timescale. Secondly, as the main laser pulse interaction with the plasma and hot electron generation are kinetic processes, they are simulated in detail with a relativistic particle-in-cell (PIC) code. This fully kinetic simulation is however limited to a characteristic time of the order of 1 ps and to a spatial size of a few tens of microns. Moreover, the electron collisions are described in a simplified manner. For these reasons, at the third step, the electron distribution calculated with a PIC code is transferred to a Monte Carlo particle transport code describing the propagation of hot electrons in the solid target, their collisions and secondary reactions. It provides the number and the energy distribution of the escaped electrons.
Numerical simulations reported in Ref. [
The current decreases by an order of magnitude in 2 ps after the laser pulse and the emission zone is limited effectively by the radius of
A Monte Carlo transport code describes single particle motion in matter, but it does not account for collective effects and self-consistent electromagnetic fields. Therefore, it cannot describe the electromagnetic emission. The fourth stage of EMP modeling was performed with a large-scale electromagnetic PIC code SOPHIE[
The GHz emission was described in additional numerical simulation with the code SOPHIE on much larger temporal and spatial scales and by taking into account the boundary conditions in the whole experimental chamber, including the target, stalk and all other elements. Figure
A similar numerical model of EMP generation caused by electron emission is described in Ref. [
The numerical simulations discussed so far confirm the theoretical estimates discussed in Section
2.5 Intense transient fields due to deposition of charged particles
Charge emitted by intense laser–target interactions can be efficiently deposited onto objects present within the chamber and, in particular conditions, may give rise to the generation of very large transient electric fields, even rather far from the interaction point. A scheme of this phenomenon is shown in Figure
This was demonstrated for energetic petawatt-range laser–matter interactions[
The AD-80D(R) D-dot differential electric field sensor[
For shot #29 (269 nm target thickness, 386 J laser energy and
Through a process of accurate cable frequency-domain de-embedding (see Section
Figure
Proof-of-principle numerical simulations were performed by CST Particle Studio three-dimensional (3D) PIC code to get a suitable description of the field development due to charged particle dynamics in the considered setup. The parabola was modeled as a thin silver layer on a thick glass cylinder, mounted on a stainless steel annular holder. Secondary-electron emission and superficial charge deposition were computed on all surfaces. Space-charge effects were also calculated, but the overall bunch charge was kept to low values to minimize them. For each particle species, emission was uniformly distributed within a
The optimization process was performed to get a suitable qualitative fit to the experimental data of D-dot probe shown in Figure
In experiments of this type, intense UV, X and
2.6 Methods of description for EMP signals
2.6.1 Modal structure of the fields in the vacuum chamber
The duration of EMP fields extends over a time much longer than the laser pulse. The average dimensions of a vacuum chamber used in experiments of laser–matter interaction is up to a few meters, and thus the microwave electromagnetic waves undergo multiple reflections on the objects usually present within the chamber, and especially on its walls, floor and roof. Consequently, the quasi-modal structure of fields in such a resonant cavity is settled out after tens of reflections, corresponding to an overall transient time of a few hundreds of ns. The electromagnetic field inside the vacuum chamber can be mathematically represented as the weighted sum of an orthogonal set of proper modes[
The determination of the coefficients of this expansion is obtained by resolving a system of linear equations. In the most general case, these coefficients are functions of time. They contain the coupling integrals[
For a primary EMP pulse of short time duration and broadband spectrum, the cavity acts as a microwave filter. A given excited resonant solenoidal mode persists for a long time depending on its quality factor
It is also possible that persistent EMP signals might be due, for particular time intervals, to sources of field placed within the chamber and with specific time and spatial profile, having a frequency content that does not necessarily match one or more of the resonant modes. This can occur for transient field sources, and in this case their fields would be represented by the expansion of harmonic and irrotational vectors in Equation (
For a hollow chamber having a simple shape, it is possible to determine the eigenfunctions and eigenvalues analytically. However, conductive objects present in a real experimental chamber may significantly change the modal distribution. These situations can be analyzed with 3D electromagnetic simulations[
The modal structure of the electromagnetic fields is also modified by hot electrons and plasma expanding from the target. They move rapidly from the interaction point, fill the experimental chamber and influence the space and time characteristics of transmitted and reflected electromagnetic waves. In particular, expanding plasma may reflect EMP waves with wavelengths longer than the critical wavelength associated with the electron density. Thus, within the experimental chamber, a time-varying volumetric distribution of critical regions may be created for each EMP wavelength[
2.6.2 Time-domain and spectral-domain analysis of EMP signals
The EMP signals and discharge currents measured on different laser facilities have a complex temporal structure. A suitable way to describe the time-domain measurements is the amplitude envelope approach[
The EMP signals generated in the laser–target interactions have a rather fast rise and a slow decay, similar to EMPs generated in nuclear explosions[
Fast Fourier transform (FFT) is commonly used to analyze the spectral content of the signals. However, as can be seen from the example shown in Figure
2.7 Experiments and modeling of EMP signals on several laser facilities
2.7.1 EMP experiments on Vulcan Petawatt laser facility (RAL)
The Vulcan laser facility was one of the first petawatt lasers commissioned in Europe, operational since the early 2000s at the RAL in the UK. It delivers pulses with duration
First measurements of the EMP generated by the Vulcan Petawatt laser were made inside the vacuum chamber in December 2003[
The waveforms shown in Figure
The magnitude and frequency of the EMP signal were calculated for an ideal rectangular target chamber. The response of a real target chamber is different due to the effect of equipment inside the chamber, which causes a shift of the resonant frequency and excitation of harmonics.
The Vulcan Petawatt target chamber is a rectangular box of height
Mode | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|
E-field | [m] | [m] | [m] | [m] | [MHz] | [pF] | [kV] | [kV/m] | [A/m] | [A/m] |
E–W | 2.2 | 2.0 | 4.6 | 3.97 | 76 | 11.2 | 14.3 | 7.2 | 8.2 | 17.1 |
Vert. | 2.0 | 2.2 | 4.6 | 3.67 | 82 | 9.2 | 17.3 | 7.9 | 8.3 | 19.2 |
N–S | 2.2 | 4.6 | 2.0 | 2.96 | 101 | 2.1 | 75.7 | 16.5 | 32.0 | 29.4 |
Table 1. Values of different parameters calculated for the fundamental modes of Vulcan Petawatt chamber.
Another experiment that included mode characterization of the same Vulcan Petawatt chamber was performed in 2015[
Table
Expected | Measured Frequencies (MHz) | ||
---|---|---|---|
Frequencies | EO Ch1 | EO Ch2 | |
(MHz) | (North–South) | (East–West) | D-dot |
Not detected | Not detected | ||
Not detected | |||
Not detected | |||
Not detected | Not detected | Not a | |
sharp peak | |||
Not detected | Not a | ||
sharp peak |
Table 2. Frequencies of the expected harmonics and detected spectral peaks in the Vulcan experiment. Superscript E-W or N-S indicates the mode axis and numbers 1, 2 and 3 indicate the harmonic order.
The D-dot probe results show good correspondence with the expected theoretical values and electro-optic measurements for the 148.5 MHz contribution. An agreement can be also observed for the 202 MHz and 228 MHz harmonics. It is generally difficult to make reliable comparisons between different EMP diagnostics unless they are in identical locations. The presence of metallic objects in the target chamber results in a complex EMP field topology; field strengths and relative amplitudes of spectral components can vary greatly at different positions within the chamber.
Temporal variation in mode frequencies in the Vulcan chamber. Measurements were performed with pulses from the Vulcan laser focused onto a flat target with an
Time–frequency analysis with a scanning time window was applied to the EMP measurements to reveal the temporal information associated with the different frequency components in the signal, which would be lost with a standard frequency analysis (see Section
The frequencies in the range corresponding to the resonant modes of the chamber are expected to be present for the lower energy shot, but were too weak for the Möbius loop to detect. With the higher laser energy, more electrons were ejected from the target[
2.7.2 EMP experiments on ABC laser facility (ENEA)
A set of experiments was carried out for studies of modal field distribution on the ABC laser facility operating at the ENEA laboratory in Frascati, Italy, at a fundamental wavelength of 1054 nm with 3 ns pulses[
A first-order representation of the electromagnetic field distribution within the chamber was obtained by analytically modeling it as a hollow spherical cavity of diameter System.Xml.XmlElementSystem.Xml.XmlElementSystem.Xml.XmlElement
Two types of probe were used in the experimental campaign: a wideband monopole (WM) antenna[
Shot | #1525 | #650 | |
Target | Al | CH | |
Thickness | 1520 | 140 | |
Laser A | Energy [J] | 41 | 76 |
Intensity [PW/cm | 0.7 | 1.3 | |
Laser B | Energy [J] | 25 | 62 |
Intensity [PW/cm | 0.4 | 1.1 | |
WM inside | Peak–peak amplitude [V] | 60 | 78.5 |
Energy [nJ] | 107 | 610 | |
SWB inside | Peak–peak amplitude [V] | 179 | 256.1 |
Energy [nJ] | 783 | 6300 | |
WM outside | Peak–peak amplitude [V] | 4.24 | 16.3 |
Energy [nJ] | 0.884 | 33 |
Table 3. Laser energy and intensity, target thickness, and the measured energy and peak–peak amplitude of detected signals for two shots on the ABC facility.
Figure
The frequency spectra in Figure
The general loose correspondence between spectra of signals inside and outside the chamber (apart from some localized and low amplitude frequency components) and the presence of intense components at low frequency, with maximum up to
Figure
The FFT gives information on the spectral content of a signal on the whole analyzed time interval. We applied also a time–frequency analysis STFT, as described in Section
2.7.3 EMP experiments on Asterix IV laser facility (PALS)
Asterix IV is an iodine laser system delivering a pulse of 300 ps duration with energy up to 1000 J at a wavelength of 1334 nm[
The target chamber was modeled as a resonant cavity by the finite element method using the COMSOL Multiphysics software. The calculated resonant frequencies and field distributions inside the target chamber are different from a hollow cavity because of the presence of optical and diagnostic systems, vacuum ports, etc. Inside the chamber, the electromagnetic field is given by the emission patterns of EMP sources and the chamber response at resonance frequencies. Moreover, it has been shown[
The loop antenna measures the time derivative of the magnetic flux,
In addition to the target chamber geometry, the target holder system has to be taken also into account because it acts as an EMP-emitting antenna powered by the return target current neutralizing its positive charge (see Section
2.7.4 EMP experiments on Shen-Guang III (LFRC) and Shen-Guang II Upgrade (NLHPLP) laser facilities
The Shen-Guang III laser facility (SG-III) is the largest laser driver for inertial confinement fusion research in China. It has 48 laser beams and can deliver 180 kJ ultraviolet laser energy in 3 ns[
The Shen-Guang II Upgrade (SG-II-UP) facility has eight laser beams of total energy 24 kJ and duration 3 ns at the third harmonic (351 nm) for implosion, coupled to a petawatt beamline delivering 1 kJ energy in 1 ps at the first harmonic (1053 nm) for the generation of a relativistic electron beam[
Figure
Figure
The size of the spherical chamber determines its lower resonance frequency of 0.11 GHz[
Such a high-frequency EMP could be generated by the return current through a target holder if it would be a 10 cm long metallic stalk connecting the target and a well-grounded conducting plate[
In order to understand the spectral evolution of the radiation, we simulated the dynamics of the ps pulse in the chamber with a two-dimensional electromagnetic code. Figure
Figure
Figure
Figure
2.7.5 EMP experiments on DRACO laser facility (HZDR)
The EMP was investigated at the DRACO 150 TW laser facility at Helmholtz-Zentrum Dresden-Rossendorf. This is a double CPA Ti:sapphire system delivering 30 fs pulses with energy on target up to 3 J. Intensities up to
Figure
The measurements showed that the integrated EMP signal scales generally linearly with the laser energy, and that the signal parallel to the laser polarization is systematically higher than that recorded in the direction orthogonal to the laser polarization, and shows a slightly steeper slope.
3 Methods of EMP diagnostics
3.1 Challenges of measuring EMP fields in laser–matter interaction experiments
Mechanisms of generation of the EMP transient fields were discussed in detail in Section
Generally speaking, an ideal EMP probe should give no perturbation to the field to be measured, have high sensitivity, dynamic range and bandwidth, and be capable of surviving intense fields. Moreover, since the EMP fields practically never have a predetermined direction, it should be capable of measuring more than just one field component in the same position at the same time, with good selectivity between the different components. Since EMPs are generally not plane waves, to have a complete characterization, both fields,
One main concern for EMP measurement is the necessity to effectively separate the EMP signal correctly detected by a given sensor, from the background EMP fields acting as high-intensity noise on the full readout system. This background field can be directly coupled with the digitizing and storage devices (oscilloscopes) or can penetrate within the link (usually coaxial cables) where the measured signals are traveling, and then adding to them.
The signal
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The stored signal
Since the EMP fields outside the chamber can be roughly estimated to scale with the square of the distance, long transmission links can be used to move the digitizing and storage devices far away from the chamber, at distances where the residual EMP background is much attenuated. As described in Section
The EMP sources are located within the experimental chamber, and it is thus obvious that the related field strength is maximum in this region and increases if approaching the source point. Remarkable EMP levels were detected also in the exterior region due to ineffective shielding of the chamber. It is instructive, therefore, to separate the two regions.
3.1.1 Interior of the experimental chamber
The interior of the chamber with vacuum up to
Current densities and charge distributions can be generated on the sensor primarily from Compton scattering of
Here is a list of some countermeasures[
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In the past, notable knowledge was acquired in the development of sensors for EMPs caused by nuclear explosions. In these experiments, sensors were placed at a large distance from the emission point, and far-field plane-wave conditions (Equation (
3.1.2 Exterior of the experimental chamber
The environment exterior to the experimental chamber has EMP fields, which can still be rather intense but reduced with respect to those in the interior region. There is presence of particle and
Measurements in the exterior of the chamber are usually simpler. In some cases, probes can be placed with a good separation from other objects, and Equation (
The EMP fields are generated in the interior region of the vacuum chamber. If the chamber had been a perfect Faraday cage, no electromagnetic field would escape. The main issue is the presence of many leakages on the chamber surfaces, which allows the field to exit the chamber. Dielectric glass windows and vacuum flanges can be fairly transparent to radiofrequencies and microwaves with wavelength
Another source of leakage can be associated with the vacuum feedthroughs of the cables. If they are placed on a dielectric window, without galvanic connection with the conductive chamber walls, a multiply connected waveguide is actually achieved. This has no frequency cutoff, similar to the case of a coaxial cable or a twisted-pair transmission line, and any field is free to propagate through it[
An EMP wave propagating in the chamber, approaching one of those possible open doors to the exterior region, will be partly reflected back to the chamber, and partly coupled with the door, and thus transmitted. For large wavelengths and at large distances, the field transmitted through the hole can be approximated to and modeled as a spherical wave if no obstacles are present[
3.2 Conductive probes for EMP fields
Any conductor placed in a region where an electromagnetic field is present becomes a source of a current, with features related to the applied electromagnetic field. In specific structures, this current can be driven to a waveguide (transmission line), where the associated electromagnetic wave travels with low attenuation up to the place where it can be observed and stored. The IEEE Standard Definitions of Terms for Antennas (IEEE Std 145-1983) defines an antenna as ‘a means for radiating or receiving radio waves’[
Historically speaking, the first types of these devices were just simple dipolar antennas and resonating loops[
One of the important parameters for an antenna is the working bandwidth, defined as ‘the range of frequencies within which the performance of the antenna, with respect to some characteristic, conforms to a specified standard’[
According to Rumsey’s principle, there are several possible prototypes that can meet these basic features: log-periodic, spiral, helical, volcano smoke, Alpine horn, biconical, etc.[
Here is a list of features for an ideal probe for EMP measurements[System.Xml.XmlElementSystem.Xml.XmlElementSystem.Xml.XmlElementSystem.Xml.XmlElementSystem.Xml.XmlElement
Nevertheless, in many experiments useful information on EMP fields can be effectively achieved by classical antennas. We describe here some typical prototypes, which meet many of the requirements. The relation
3.2.1 Probes for the electric field
D-dot probes were classically designed and optimized for the measurement of the time derivative of the electric flux density. In particular, under the hypothesis that the sensor is electrically small, the voltage at the device output can be written according to Equation (
A suitable sensor for measuring electric field intensity is the parallel plate dipole (PPD). One example, built in the form of a parallel plate capacitor, is shown in Figure
3.2.2 Probes for the magnetic field
The characteristic relation for magnetic field probes is also presented by Equation (
Another common structure is that using the Möbius configuration [
3.2.3 Probes for the neutralization current
Experimental investigation of the return current requires a collection of current probes to measure currents flowing between the target and the ground through the target holder system. This diagnostic procedure is in general a complex problem because the target holder system not only acts as a short-circuit conductor of the target charge, but also as an antenna transmitting the EMP in the high-frequency band corresponding to the holder geometry, as well as an antenna receiving other EMP modes associated with resonant frequencies of the interaction chamber and accessories localized inside this chamber. The EMP emission by the interaction chamber and accessories is caused by currents neutralizing the charge delivered by the expanding plasma to them. Thus the target current has two components: the first one is associated only with neutralizing the target charge, and the second high-frequency one is associated with the EMP signal emitted and received by the target holder. The high-frequency components of the target current can dominate when the plasma is produced with laser intensities
A resisting target probe is advantageous for plasmas produced with low-intensity lasers. However, the resistivity of the shunt should be as small as possible to minimize its influence on the observed current. Alternatively, in the case of a small resistance of about 1
The experimental observations show a complex structure of target currents, the durations of which are much longer than the durations of the laser–matter interaction[
As the target holder system acts as a receiving antenna, the EMP fractions that are emitted by the interaction chamber and by accessories inside the chamber interfere with the transient target current. For this reason, only the beginning of the observed target current is not associated with these secondary EMP fractions because the hot electrons and slower plasma hit the walls of these objects with a delay up to a few tens of nanoseconds. To avoid the direct impact of the probe on the measurement of the current flowing through the resistance, because it becomes part of the antenna, an inductive probe was developed[
Figure
3.2.4 Use of conductive probes in EMP measurements
As described in Section
In the case of probes sensitive to the time derivative of electric or magnetic fluxes, it is necessary to perform a time integration to retrieve the desired field from the measurement, but this operation can be rather challenging. The electric or magnetic field is derived by the sensor, according to its transfer function of Equation (
Use of long cables. As mentioned in Section
Indeed, this filtering feature can be used as a simple way to equalize the signal before digitization, as demonstrated in Ref. [
Figure
The delayed signals are also cleaned of the frequency components higher than a few hundreds of megahertz. In addition, for some frequency bands, the cables behaved also as time integrators. Figure
In the same campaign, measurements of the neutralization current flowing through the target holder (see Section
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These are the conditions to date more engaging for the correct detection of EMP fields on the interior of the vacuum chamber. Practically all the issues discussed in previous Section
3.2.5 Conducting probes inside the vacuum chamber of petawatt lasers
We describe here the methods used for electric field measurements in experiments on planar thin plastic targets with the Vulcan Petawatt laser at focused intensity beyond
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Even if countermeasures were taken for the
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This led to the conclusion that the possible
As explained in Section
3.3 Dielectric probes for the EMP fields
As explained in Section
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An alternative way of measuring single vectorial components of EMP electric fields generated by intense laser–matter interaction, capable of overcoming some of these issues, was proposed in Ref. [
The fully dielectric construction of the detector and the absence of conductive links to the oscilloscopes cancel the
These types of measurements allow for the detection of single field components with high selectivity with respect to the others. The associated electro-optic probes can have low dimensions and invasiveness. Indeed, their effective permittivity may induce a local perturbation on fields measured in vacuum. This remains much lower and localized than for conductive probes, and with offline calibration can be estimated for de-embedding operations. The intrinsic low sensitivity of the electro-optic techniques is generally not detrimental when dealing with high-intensity field measurements, as for EMPs, because low-noise amplifiers can possibly be used for moderate field intensities. Nevertheless, this may be an issue for very large band signals.
The full characterization of the EMP fields should rely on the determination of both their electric and magnetic fields. Nevertheless, the electric field is more relevant for problems related to diagnostics, because it can produce ionization and discharges on materials. For this reason, the types of devices discussed here deal with electric field measurements only, and rely on electro-optic effect[
3.3.1 EMP diagnostics and measurements in experiments on ABC laser facility (ENEA)
Experiments were performed with the ABC Nd:phosphate glass nanosecond laser facility[
For each shot, the laser–plasma interaction was monitored by a large number of diagnostics. Thermal ion emission from plasma with
Figure
A continuous-wave laser probing beam having
The field detection is performed by the change of polarization state, induced by the electro-optic effect, monitored by a classical polarization-state analyzer. This is the polarization-state modulation technique[
Kapteos™ built a custom version of the EOP-P2R02-BS050 probe to adapt it to the experiment at the ABC facility. An alumina sleeve (30 mm length and 4 mm diameter) contains the whole structure (Figure
Two series of measurements were performed, both with the electro-optical probe in direct view of the target and at 85 mm distance. The probe was mounted on the
For shot #1590, maxima higher than 100 kV/m were present during the first 250 ns. In the whole campaign, it was not possible to determine the absolute field phase. For the #1590 shot, a first high ‘positive’ peak is present (FWHM
These are the first direct EMP amplitude measurements with the detector rather close and in direct view of the plasma. A maximum field of 261 kV/m was measured, two orders of magnitude higher than previous measurements by conductive probes on nanosecond lasers of much higher energy.
Simplified PIC simulations of the experiment were performed by the CST Particle Studio solver. Space-charge effects were considered, together with secondary-electron emission from Teflon and superficial charge deposition on surfaces. The target surface was the source of conical particle flows, uniform within their maximum angle of emission
Future and more accurate modelings of the experiment have to consider photoionization due to X-rays from plasma, generating a cloud of cold electrons around the external surface of the Teflon. This is expected to create a pulsed electric field, rather synchronous with the peak due to fast electrons. Effects due to charge implantation on Teflon have to be taken carefully into account, too.
3.3.2 EMP diagnostics and measurements in experiments on Vulcan Petawatt (RAL) and Cerberus (Imperial College) ps laser facilities
An experimental campaign was performed on the Vulcan Petawatt facility, shooting on
The diagnostic could also be easily converted to measure magnetic fields by simply exchanging the electro-optic crystals with a magneto-optic medium, such as a high Verdet constant glass. In this system, the field-induced time-varying polarization changes on a
Defining the bottom north–east corner of the chamber (see Figures
The crystal mounts are shown in Figure
The optical diagnostic successfully measured the EMP electric field components within the interaction chamber in both N/S and E/W directions; the temporal electric field evolution is shown with and without a numerical low-pass filter applied in Figures
A maximum field in the crystal of 10.9 kV/m was measured with the N/S probe within the resolvable frequency range of the sensor system, with a smaller field component of 5.5 kV/m measured along the E/W axis. Both measurements were made at a distance of
For the Cerberus experiment, a more compact all-dielectric single-channel diagnostic, with fiber beam transport into and out of the chamber, was developed (see Figure
3.4 Charged particle deflection for electromagnetic field probing
Both the conductive and the dielectric probes discussed so far generate a local perturbation of the fields they aim to measure. This issue can be easily accounted for during the probe offline calibration. However, it effectively acts to limit the application of these sensors to the measurement of electromagnetic fields in positions relatively far from other objects. Whenever there is the necessity to measure fields in close proximity to surfaces, they are thus not applicable. This task can be accomplished by using probing techniques involving the deflection of charged particle beams.
A technique now well established employs collimated proton beams with large energy spread, generated by the interaction of short-pulse lasers with thin foils, according to the TNSA scheme[
Proton radiography was successfully applied to measure the electromagnetic wave produced by the interaction of a high-intensity laser pulse with solid targets[
Figure
A different approach for electromagnetic field probing is to use a deflection of quasi-monoenergetic electron bunches. In this scheme, a 500 fs electron pulse with an energy of
A schematic of the experimental setup is shown in Figure
4 Methods of EMP mitigation and applications
4.1 Experiment on the Vulcan facility
4.1.1 Experimental setup
An EMP study was performed at the Vulcan Target Area West facility of the RAL[
Laser shots focused to an intensity of
Effective shielding of electronic devices against EMP is expensive, frequently impractical and requires a precise knowledge of the EMP emission frequency[
4.1.2 Dependence on laser parameters
The relationship between laser energy and EMP emission was examined using
A pulse duration study was conducted using
4.1.3 Dependence on target parameters
It has been reported in several publications that the target surface area can have a significant impact on electron and EMP emission from the target[
As laser-accelerated hot electrons are ejected from the target surface, they leave behind a positive potential that spreads over the target and prevents less energetic electrons from escaping[
Although smaller targets produce reduced EMP fields, they also change the conditions of the laser–matter interaction. Electrons heated by the laser can be guided along the target surface and produce intense fringing electric fields that alter the accelerating properties of the electrostatic sheath[
A major source of laser-driven EMP at GHz frequencies is thought to be dipole antenna emission, as a neutralizing current oscillates between the laser target and the nearest ground[
Data from the electron spectrometer shows that the number and temperature of ejected electrons with energy larger than 0.1 MeV did not change significantly for shots involving the modified stalks[
4.2 Experiment on the Orion facility
Solid target experiments were conducted on the Orion facility at AWE Aldermaston, using the 1054 nm ‘short-pulse’ beamline, capable of generating 500 J pulses in 500 fs at intensities typically in the range
4.2.1 EMP variation with the target thickness
Many of the short-pulse target shots fired on Orion have been used for proton heating, where small
4.2.2 EMP dependence on stalk conductivity
A range of methods for target mounting have been used on Orion; shots were initially fired using 60 mm long and 1 mm diameter quartz glass tubes for single target experiments, or several 23–28 mm long quartz glass stalks mounted on a common mount for multi-target experiments. Damage due to debris from the glass stalk shattering led to a switch to carbon fiber tubes of the same length for target mounting. Figure
4.2.3 EMP dependence on target dimension
In other experiments, laser shots have been fired at gold foils of various sizes. To allow comparison of rectangular and circular targets, the square root of the target area facing the beam is considered as the effective target dimension of relevance to the effective target capacitance. By examining the EMP energy generated per joule of laser drive energy as a function of target size, we observe that the EMP energy scales linearly with energy as shown in Figure
Larger targets are able to establish greater capacitance and in turn higher accumulated target charge. According to the theoretical model described in Section
As next-generation laser facilities come online, many of them plan to operate at higher repetition rates and some of them involve plans to shoot metallic tape or target arrays[
The diagnostics used to measure EMP were two probes, a B-24 full loop B-dot sensor and an FD-5 series D-dot sensor, both manufactured by Prodyn Technologies. The two probes were placed inside the vacuum chamber, close to the rear target surface normal at 173 cm, and were connected to a 12.5 GHz Tektronix DPO71254C digital phosphor oscilloscope via SMA cables (type RG402). The oscilloscope was placed outside of the experimental area to minimize direct noise pickup, and thus the SMA cables were passed through BNC feedthroughs limiting reliable frequency to approximately 3 GHz. Each of the probes used attenuators for oscilloscope protection. The
The emitted EMP energy was observed to increase as the target size increased from 2 to 20 mm, as shown in Figure
The data acquired using the B-dot probe are shown in Figure
4.3 EMP mitigation with levitating targets
As described in Section
The optical levitation traps described here are suitable for holding micro-targets in a vacuum chamber, without physical contact with external structures. This allows the realization of high-intensity, high-energy laser interaction experiments with mass-limited targets, in which the energy transport mechanisms are spatially confined. The interest in these experiments resides in the possibility of increasing the laser–target coupling efficiency, to prevent generation of unwanted X-rays from surrounding structures and to reduce EMP generation. One possible application of levitating micro-targets is an X-ray source for high-resolution imaging.
When light is reflected or refracted by small particles, photons undergo a change in momentum and this, in turn, is coupled to the particle. These changes in momentum produce forces that form the basis of optical trapping of small particles[
The light source used in the system described here was a green laser (Verdi 5 W,
Preliminary experiments at atmospheric pressure demonstrated the ability to trap droplets of saturated salt water of about
The saturated salt water droplets were found to become unstable at low pressures (300–500 mbar), and so in vacuo operation was obtained using low vapor pressure, high boiling point oils. By means of an atomizing nozzle it was possible to obtain stable levitation of
Trapping under vacuum was obtained with saturated oil droplets (Figure
Experiments on interaction of these levitated targets with an intense laser pulse were conducted with a high contrast OCPA/Nd:glass laser delivering 1 TW, 0.3 J pulses of duration of 450 fs at a wavelength of 1054 nm. Figure
Knife-edge data gave the X-ray source sizes with a spatial resolution of
Figure
Experiments with levitated targets were also performed at the Vulcan Petawatt and Cerberus facilities[
The electro-optic diagnostic was unable to detect any EMP, above the minimum resolvable field-strength limit set by experimental noise, from interactions with levitated micro-targets using either Cerberus at the few joule level or, more significantly, Vulcan Petawatt at energies exceeding 300 J, where accelerated protons of energies more than 30 MeV were observed from droplet targets. Hence, any generated EMP fields were below the experimental electrical noise level, meaning they were less than
4.4 EMP mitigation approach for proton-emitting targets
A simple method for mitigation of the EMP emission from targets used for laser proton acceleration was proposed in Ref. [
Such a target may be thought of as an electric circuit consisting of a capacitor (with capacitance
Apart from the condition on the resistance
In order to estimate the mitigation performance of this scheme, we introduce the factor
A practical test of this mitigation concept was performed at IPPLM in Warsaw, by using a 10 TW Ti:sapphire laser delivering 50 fs pulses with energy on target up to 400 mJ and intensity contrast
The EMP signal was measured with Prodyn RB230 and RB270 B-dot probes placed inside the chamber, and a custom-made Möbius loop 30 mm in diameter was placed outside the chamber in a large glass window. The laser-accelerated protons were characterized via a TOF technique using a Faraday cup and a silicon semiconductor detector placed on a long extension tube protruding from the chamber. The cage used in the test had the form of a cuboid with
Under the assumption of an isotropic electron emission, the collecting capacity of the cage could be estimated as
4.5 Mitigation techniques for the LMJ–PETAL laser system
Laser Mega-Joule (LMJ) is an MJ-scale laser facility operating with ns pulses at a wavelength of 351 nm. It was constructed in France by the CEA for defense and high energy density physics applications[
4.5.1 Design of a new target holder
Knowing that the target holder is the main EMP source, the LMJ–PETAL strategy of EMP mitigation has focused on designing a new target holder. Two major goals have been addressed:
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Electric fields surrounding the target are very intense and may induce electrical breakdown. Therefore, it is important to ensure electric contact of the target to the ground in order to discharge the target and protect diagnostics placed near the target from uncontrolled discharges. For this reason, the use of an insulating holder is not recommended. Moreover, secondary radiation produced during the laser–target interaction (UV, X-rays and electrons) may generate radio-induced conductivity on insulators. So, additional shielding of the most sensitive security equipment is an indispensable part of the EMP mitigation strategy.
A new target holder for LMJ–PETAL experiments has been designed with the help of numerical simulations, and has been fabricated and tested[
The holder was tested at a laser energy of
The discharge current intensity measurements with both types of target holders are presented in Figure
There are two reasons for the difference between the mitigation factors of the current (factor 30) and the radiated field (factor 3). First, the discharge current was measured at the bottom of the target holder where it contacts the ground. At the top of the holder, near the target, the current intensity is higher because the inductive suppression propagates along the holder with the current. Second, in addition to the target holder emission, there are other sources of EMP, which are not affected by that mitigation system.
4.5.2 EMP mitigation in PETAL experiments
An EMP diagnostic system has been developed and placed inside the LMJ–PETAL experimental chamber, at a distance of 4 m from the TCC. It is composed of five B-dot probes: four probes detecting vertically polarized magnetic fields and one detecting horizontal polarization. However, during the first PETAL campaign in December 2017, only four B-dot probes were deployed in a common setup (horizontal magnetic polarization) as shown in Figure
The EMP emission inside the LMJ–PETAL experimental chamber has been simulated by a set of numerical tools, described in Section
In the first campaign, the PETAL beam energy varied from 90 to 425 J for pulse durations of 0.5–1 ps. The measured electric field amplitude in shots with the conducting holder varied from 5 to 15 kV/m for vertical polarization and from 35 to 70 kV/m for horizontal polarization, which compares well to the simulation results. Ratios of 5–7 between the vertically and horizontally polarized fields confirm the holder current as the dominant source of EMP emission. From comparison of the shots on plastic and tungsten targets, we concluded that the EMP amplitude weakly depends on the target material and increases as the laser pulse energy with exponent 0.66. Consequently, the expected EMP amplitude is 190 kV/m at 4 m distance from the target for a laser pulse energy of 1 kJ.
In agreement with the results of the LULI experiment, in PETAL shots with the new inductive–resistive target holder, the peak electric field was reduced by a factor of 3 in the GHz frequency range: the horizontally polarized electric field amplitude did not exceed 20 kV/m at a laser energy of 400 J[
The EMP mitigation system has been also tested in joint LMJ–PETAL shots. In this experiment, four LMJ quads with a total energy of 40 kJ at wavelength 351 nm and 5 ns duration irradiated two CH discs, having 250
The EMP signals were measured for the shots with different time delays and compared to the corresponding signals measured in separate LMJ and PETAL shots. The EMP amplitude measured in a singe LMJ shot was three times smaller than the one measured in the PETAL shots with the new holder. Joint shots with time delays larger than 20 ns have shown the same signal as in the separate PETAL shots, thus confirming that charging of the PETAL target is the main source of EMP in that experiment. However, unexpectedly, the EMP signal was reduced to the LMJ level for shorter time delays[
While this phenomenon is not yet understood completely, the following explanations can be proposed. The cumulated intensity of the LMJ beams on aluminum targets in the experiment was about
Another explanation is related to the X-ray emission from the LMJ target. A few percent of the LMJ energy converted into X-rays (about 1–3 kJ) corresponds to
The ionization cross section of nitrogen atoms with 100 eV photons is
Moreover, the Rosseland mean free path of the photons emitted from the LMJ target is comparable to the protective gold foil thickness. Therefore, photons are volumetrically absorbed in the foil, delivering an energy density of about 100 kJ/cm
It is not clear for the moment why the strong EMP emission reappears for delays longer than 20 ns. The plasma recombination time is on the
Consequently, for delays shorter than plasma recombination time, the PETAL target is either less charged or its EMP emission is shielded by ambient plasma and does not propagate far away. Conversely, for longer time delays, the LMJ shot memory is lost and the PETAL target produces the same EMP as if it was standing alone. Unfortunately, due to the limited diagnostics deployed in these shots, we cannot make any further decisions on the mechanism of EMP suppression in that experiment. If confirmed, this phenomenon opens a new efficient way for EMP control in high-power laser experiments.
4.6 EMP shielding on high-power laser facilities
Assuming that the EMP event is primarily a broadband pulse of electromagnetic waves, the basic principle of the Faraday cage has proven to be an appropriate shielding countermeasure. The choice of appropriate materials and the dimensions of walls, gaps and feedthroughs is, however, often less obvious to achieve sufficient damping, especially due to the frequency spectrum which, according to experience, depends on many aspects of the experimental setup. Since modern electronic hardware and communication equipment operate in the frequency domain of tens of GHz, standardized test equipment for the characterization of setup components exists in the market. Many issues that are noted and documented in the field of high-energy laser experiments could be approached with this equipment, but so far, the reports on this are very sparse. Additionally, the complexity of modern hardware, like cameras and fast oscilloscopes, makes it hard to predict the actual sensitivity to a certain EMP field, which usually spans multiple bands on different timescales. Consequently, here we only give some general considerations that mostly rely on putting sensitive equipment in enclosures to keep EMP effects away, although the same principles also come into play when trying to contain the EMP and keep it from spreading out from its source.
When designing an enclosure, a simple ‘rule of thumb’ can be applied to get an idea about the impact of apertures in the walls, which are often unavoidable due to mechanical constraints, and the need to transfer power or signals and cooling. Considering a single aperture, the shielding effectiveness SE can be estimated as with
Apart from geometrical considerations, the choice of materials and their thickness also needs some care, because reflection as well as absorption losses play a role. Following the formalism given for example in the National Aeronautics and Space Administration report[
A dedicated test was performed at the Draco 150 TW laser at HZDR to determine the shielding effectiveness of an enclosure for a RadEye detector, typically used as a proton detector although also capable of X-ray detection. The laser energy was 3 J on target with a pulse duration of 30 fs and focused down to
The scaling of the integrated EMP signal with the laser energy is illustrated in Figure
In Section
4.6.1 Electronic equipment
Cameras/spectrometers. With the increase in the repetition rate of high-energy systems, many experiments and machine diagnostics rely heavily on CCD- or CMOS-based cameras. To maximize detection efficiency and imaging resolutions, they cannot usually be placed far enough away from the EMP source unless optical relay imaging is implemented[
Oscilloscopes. Quite often, bandwidths of a few GHz are required for photodiode or TOF measurements. Even today, such storage oscilloscopes are still rather bulky and require a considerable amount of cooling power, with forced air or even water cooling required for reliable operation when placing them into a well-designed copper housing. Sometimes, additional filtering or decoupling for the power supply line is necessary using in-line low-pass filters or decoupling transformers. Compared to cameras, digital oscilloscopes often have a higher timing precision requirement, so quite often the trigger comes into the shielding enclosure by using a fast, fiber-coupled photodiode and a pick-off from an earlier part of the laser chain.
4.6.2 Fiber communication
Although grounding and ground loops seem to become less important with rising frequency, experience shows that it is still a good idea to consider some basic principles of insulation and potential equalization or separation. This is also motivated by the fact that in many high-energy laser environments, low- and medium-frequency noise is present, originating from flash lamp discharges, Pockels cell drivers and other pulsed high-current or high-voltage equipment. Such effects can cause immediately obvious or, sometimes worse, delayed long-term damage, and will also interfere with trigger signals and data communication, which can have a definite impact on the success of an experiment. Not least due to this, many laboratories extensively use signal transmission based on optical fibers, using either commercial solutions like 1000BASE-SX for data communication or fiber-based trigger systems[
4.7 Tailored EMP suppression in the ELI Beamlines chamber
The ELI Beamlines main L4 laser will produce pulses with energy over 1.5 kJ, peak power 10 PW and duration 150 fs with a repetition rate of 1 minute[
In spite of a large effort to create an EMP resistant laser facility, it is difficult to reach a sufficient protection level for laboratory personnel and hardware because of the extreme EMP field intensities and pulse energies that may be produced in laser–matter interaction experiments. A usual approach to reduce EMP-related problems requires strict application of electric/electronic hardware shielding, and careful interconnection of subsystems and instruments with an appropriate protection/filtering while maintaining a proper topology. The protection cost scales with area, volume, complexity and the number of devices to be protected. For a large facility with many electronic devices and scientific instruments, the complete protection price may be rather high.
Broad variation of experimental setups implies a wide variation in EMP characteristics. To realize adequate prevention, protection and EMP mitigation measures, it is necessary to know the field characteristics not only close to the target, but also in other critical areas including transport tubes and compressor vessels, and points of interest inside a laboratory, in particular, locations of sensitive control/diagnostic electronics, computers, electric devices and motors, control gates and interlocks.
A full assembly of the L4-P3 system, currently in construction, includes the L4 stainless steel laser pulse compressor chamber (with volume about
Due to the large size and complexity of the L4-P3 vacuum assembly, an EMP simulation in 3D geometry required a high computing hardware performance and a large memory size. A dedicated multi-processor, multi-core server was used for large-data import, calculation, output processing and field visualization. The original engineering model of the L4-P3 system developed with Autodesk Inventor[
The EMP propagation simulations confirm that aluminum interaction chamber walls reflect the initial short pulse many times. Due to chamber asymmetries, after several reflections, a short primary pulse fills the entire chamber with apparently random, fast-changing multi-mode patterns. Because of transient excitation, the spectrum of the field in the chamber stretches down to the resonant chamber modes, although only a small fraction of the initial pulse energy goes to the chamber modes in this case. Mode coupling and secondary emissions from the chamber structures were not included in the model, and thus, no additional energy can be transferred into the chamber modes. The large metallic vacuum vessel behaves as a moderate-
The vacuum chamber acts as an EMP energy reservoir, and a large fraction of confined energy gradually escapes via a laser input port to the beam transport manifold, because dominant frequencies are higher than the beam-pipe cutoff frequency. The EMP propagates through the pipes in the form of a long amplitude and phase-modulated pulse and gradually fills the laser pulse compressor chamber. In a typical calculation with a time step of 50 ps carried up to
Simulations demonstrate that a significant amount of pulse energy is transferred from the interaction chamber to the compressor chamber and a part of the EMP escapes from the vacuum assembly. The common construction materials used for the vacuum vessel assembly do not attenuate EMP sufficiently. Figure
To mitigate EMP effects, blocking and absorption strategies were examined. Several types of radiofrequency and microwave absorption materials are currently used in metrology, research, industry, constructions and business for protection of sensitive spaces against unwanted electromagnetic fields. Unfortunately, very few absorbers on the market are suitable for the L4-P3 vacuum assembly. An EMP absorber should be compatible with a high vacuum of
Common ferromagnetic ceramics used in the electronic industry were examined for a stable, vacuum compatible, clean room compatible, nuclear activation compatible absorbing material in the MHz and low GHz domains. Materials economically viable in large quantities suitable for L4-P3 large structures were tested for compatibility. Initial tests of selected materials were performed, and an optimization of the absorbing structure for vacuum vessels is in progress.
Port | P1 | P2 | P3 | P4 | P2-BR |
---|---|---|---|---|---|
IChAux | IChL4 | LDiag | L4 compr | BackRef | |
No Abs | 16.8 | 48.1 | 6.6 | 2.06 | 20.3 |
TME | 15.6 | 50.9 | 0.16 | 0.034 | 2.7 |
P3ICh | 0.45 | 0.42 | 0.071 | 0.025 | 0.28 |
Both | 0.47 | 0.45 | 0.002 | 0.001 | 0.066 |
Table 4. EMP energy flow at the selected ports during calculation in percentage of initial EMP energy for different absorbers. See text for explanation of abbreviations.
Absorptive protection cladding inside selected vacuum vessels was used in the structural model for EMP propagation simulation. Artificial ferrite data were used for cladding. Detailed engineering models without absorber, and with absorber inside the P3 interaction chamber (ICh) and/or the TMEs were compared in pulse propagation calculations. The structural model in Figure
5 Conclusions and perspectives
The generation of EMP in laser–target interaction experiments is reviewed, and new experimental and modeling insights are presented. Two major primary sources of EMP – electron bunch ejection and the return current – are identified: the former produces EMP in the THz domain and the latter in the GHz range. The relative intensity of these two pulses depends on the laser intensity and target geometry; in general, up to 0.1% of laser energy can be transferred into these radiations. The electromagnetic energy carried with these EMPs can be confined within the target chamber for microsecond timescales, gradually dissipating due to resistive losses and energy escaping through the chamber openings. The presence of these decaying pulses is manifested in the lower frequencies of the observed field produced by multiple reflections from the chamber walls and metallic objects inside the chamber.
Methods of EMP detection are analyzed. Accurate detection of the primary EMP requires simultaneous measurements of the electric and magnetic components of the signal, which can be significantly perturbed by subsequent reflections in the chamber and secondary emissions resulting from the objects inside.
Comparison with experimental data collected on different laser facilities shows that the theoretical models and numerical simulations are capable of predicting qualitative and quantitative EMP characteristics. The amplitude of the EMP signal depends strongly on the laser pulse energy and the pulse duration. Figure
Detailed understanding of the EMP sources provides a solid background for designing mitigation techniques. Some techniques that have already been developed and tested include the use of: isolated target supports; matched resistive holders; holders of different geometrical shapes; and levitating targets. Other methods of EMP mitigation include the use of active absorbers or special shields.
This review is focused on the EMPs produced in laser interaction with solid targets. Experiments on laser interaction with gaseous targets are left out of this review, as the recorded amplitudes of EMP signals are significantly lower.
At present, the primary motivation for EMP studies is the protection of target equipment, diagnostics and personnel from deleterious EMP effects. There are, however, applications of EMPs for the generation of strong magnetic fields, acceleration of charged particles and material characterization. The physics of EMP generation and the methods of its detection described in this review can be used for further development of these applications.
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