Abstract
1 State of the art
The advent of high power lasers (HPL) working at high repetition rate (HRR) is nowadays a reality and HRR proton sources are now routinely produced with energies ranging from a few to tens of MeV. Laser-driven proton sources are characterized by a divergence that in several measurements has been proved to be related to the energy of the protons and the spatial distribution of the proton beam[
Laser-driven proton beams are becoming more and more important for applications in different fields of physics[
The possibility of extending this technique to the HRR mode of operation is nowadays a challenge in the laser–plasma community, and several laboratories and research groups are working on this. The main idea is to substitute the active RCF layers with scintillator detectors capable of transforming the ion energy deposition into light that can then be collected by an optical CCD camera. Several research groups have proposed special online configurations to imitate the RCF stack but, up to now, only a partial extension of the RCF capabilities was possible. During 2011 and 2012, two research groups from the United Kingdom and from Germany proposed scintillator-based detectors. The group from Rutherford Appleton Laboratory (RAL)[
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2 Detector design
We present a scintillator-based detector able to measure both the proton energy and its transversal spatial distribution along the propagation axis and of being set at HRR. It consists of a series of scintillators placed similarly to an RCF stack (shown in Figure
To assess the system in detail we assume a proton beam propagating in a symmetric cone emission with half-angle
The projection of the proton emission cone in the scintillator plate can be written as
The working condition can be written as
2.1 Case
The case
Figure
2.2 Case
Assuming a proton divergence with an half-angle
The result is that increasing
3 Implementation and preliminary calibration of the detector
A first detector prototype has been designed and constructed at the Spanish Center for Pulsed Laser (CLPU) in Salamanca and tested at the Centro de Micro-Análisis de Materiales (CMAM) of the Universidad Autónoma de Madrid, where a collimated proton beam up to 10 MeV is available for user access.
Figure
Base | Polyvinyltoluene |
---|---|
Density | |
Refractive index | |
Softening point | |
Light output (% of anthracene) | 65% |
Rise time | 0.9 ns |
Decay time | 2.4 ns |
Wavelength of maximum emission | 423 nm |
Bulk light attenuation length | 250 cm |
Table 1. BC-400 scintillator main properties.
The detector was placed in the middle of the interaction chamber, on the front part of a 4-axis goniometer, able to rotate
The original design of the detector uses two cameras looking from opposite sides. Due to the experimental constraints and since the proton source was very stable, two configurations of irradiation were used to image the full detector with the same camera. The odd plates with the numbers 1, 3, 5, 7 and 9 were pictured when the goniometer was in the normal position (rotation axis at
Figure
Considering a laser–ion acceleration experiment, the divergence of the particle beam has to be considered in the detector design. Indeed, increasing the total size of the detector will induce an increase of the final beam size, which, in combination with the multiple scattering, will entail a reduction of the spatial and energy resolutions. In addition, the last layers will receive a reduced number of protons per unit area, also reducing the detector sensitivity. The optimum detector design must be defined by prioritizing one parameter with respect to the other, even if a good general rule is to keep the length of the detector short to maintain a small beam size for a given number of layers. This can be done either by reducing the angle
4 Discussion and conclusion
A scintillator-based 2D ion detector for HRR experiments has been designed and built at CLPU, and tested using a proton accelerator at the CMAM in Madrid. The scintillator detector, to our knowledge, is a diagnostic device that looks very similar to an RCF stack diagnostic. Throughout the detailed analysis reported here, we have shown that it is possible to account for the laser-driven proton divergence by maintaining a compact size of detector. We have also shown that the detector can be implemented with an additional permanent magnet to remove most of the electron population, although the effect it has on the proton flux distribution at each energy will have to be mitigated.
Finally, the presented design of this 2D ion detector is promising for replacement of the classical RCF stack detector for the HRR mode of operation. It represents a new class of online detectors to support laser–plasma physics experiments in the newly emerging high power laser systems operating at HRR.
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