Abstract
Keywords
1 Introduction
Ultra-intense laser interacting with plasma is a rapidly developing field attracting much attention. It has been demonstrated that multi-gigaelectronvolt electrons[1], approximately 100 MeV ions[2–4], and copious secondary radiations from extreme ultraviolet bursts[5,6] to gamma rays[7,8] can be generated from the complex interactions. In these studies, targets play the core roles as they determine the properties of the plasma. Many kinds of targets[9] have been employed in the experiments. Gas targets, generated from a supersonic nozzle[10] or contained in a cell, have been widely used for electron acceleration in laser wakefield acceleration scheme. Solid targets, in contrast, are widely employed for ion acceleration. Micrometer-thick metal or plastic foils were firstly used to accelerate protons in target normal sheath acceleration (TNSA)[11] scheme. Later on, other schemes such as radiation pressure acceleration (RPA)[12] and relativistic induced transparency (RIT)[13] were explored in experiments thanks to the successful fabrication of free-standing nanometer-thin foils made of diamond-like carbon (DLC)[14], plastic or metal foils[15,16].
In addition to gas targets and solid targets, foam targets were also employed in laser–plasma experiments for years[17–23]. Ionized by the pre-pulses or the rising edge of the main pulse, foam targets can evolve into plasma with electron density around the critical density (
In a previous work, the fabrication method of CNF was briefly described[16]. Free-standing single-layer CNF targets or double-layer targets composed of CNF and DLC foils can be produced with this method. However, the maximum area and the uniformity of the CNF still need to be improved. Most of all, ultrathin plastic or metal foils cannot be used as the substrates to deposit CNF owing to their low melting temperature, which limits its applications in laser-driven super-heavy ion acceleration. In this paper, we report the advance in the fabrication and characterization methods of CNF. The improved synthesis methods enable the fabrication of large-area and uniform CNFs as free-standing films or the deposition of CNFs on nanometer-thin plastic and metal foils, which significantly widens the application range of the CNF targets in laser–plasma experiments.
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2 Synthesis and characterization methods of CNFs
2.1 Synthesis method
The CNFs were synthesized through the floating catalyst chemical vapor deposition (FCCVD) method. The prototype FCCVD system used for the synthesis of CNFs is composed of a high-temperature furnace, a gas delivery system, a heating unit for catalyst, and a quartz tube. In our lab, the PID controlled heating unit can keep the temperature of catalyst from 50°C to 200°C with a precision of 0.1°C. The furnace is from Nabertherm Inc. and can heat the quartz tube to 1100°C with a precision of 1°C. The growth process of CNTs is as follows. The catalyst (ferrocene and sulfur) in the heating unit first sublimates at the entrance of the quartz tube, and then is carried into the reaction zone by the mixed gas of argon and methane. At the reaction zone, the catalyst agglomerates as nanoparticles that serve as the seeds of CNTs. Fed by the carbon atoms from methane, short nanotubes start growing out of the seeds and become longer in the gas flow. Eventually, long CNTs are carried out of the high-temperature zone by the gas flow and deposited on the substrate placed at the end of the furnace. Energy dispersive spectroscopy (EDS) measurement shows that the final CNF target consists of carbon (
However, such a standard method based on the prototype FCCVD system cannot meet all the requirements for laser–plasma experiments. For example, double-layer targets consisting of CNF and ultrathin metal or plastic foils are ideal targets for laser-driven super-heavy ion or proton acceleration. However, we found the temperature at the deposition zone is over 600°C owing to the hot gas flow and thermal radiation, which is too high for plastic substrates. In addition, the melting temperature of free-standing nanometer-thin metal foils is typically lower than that of corresponding bulk metal by hundreds of degrees. This means that ultrathin metal foils cannot be used as the deposit substrate.
To solve these problems, we upgraded the prototype FCCVD system and further developed the synthesis method. A schematic drawing of the new system is shown in Figure 1(a). We first increased the diameter of the quartz tube from 36 to 62 mm, which enabled the deposition of larger-area CNF. Moreover, we added a water-cooling component at the end of the quartz tube as shown in Figure 1(b). It is a three-segment steel pipe cooled by flowing water. The segmented structure of this pipe can reduce the thermal radiation from the central part of the furnace. The cooling water driven by a circulating chiller can efficiently cool down the high-temperature gas flow. As a result, the temperature of the deposition zone drops to below 100°C, which is low enough for ultrathin plastic and metal foils. It is also optional to use only one or two segments of the pipe to adjust the deposition temperature. In the pipe, a specially designed target frame, as shown in Figure 1(c), is employed to mount the silicon wafer or the target holder for deposition. It can support holders up to 5 cm wide.
Figure 1.(a) Setup of an FCCVD system equipped with the water-cooling component. (b) A photo of the water-cooling component. (c) Target frame used to fix the target holder or the Si wafer in the deposition zone.
2.2 Thickness and density measurement methods
The density of CNFs can be determined from their areal density (mass per unit area) divided by their thickness. We used Si wafers as the testing substrates to measure the properties of the CNFs for a given synthesis condition. The wafer was mounted on the frame and positioned at the deposition zone. After a deposition lasting for several to tens of minutes, the Si wafer covered by CNF (Figure 2(a)) was taken out and cooled down to room temperature for the measurement.
Figure 2.(a) An as-prepared testing CNF target. (b) Part of the testing CNF target having been wiped off. (c) The testing target where the Cu powder has been sprinkled on the upper surface. (d) Schematic diagram of the thickness measurement method of CNFs. Image of the morphology of the surface of (e) the Si wafer and (f) the CNF under confocal microscopy. (g) SEM image of a Cu particle on the surface of CNF. (h) SEM image of the cross section of a CNF with thickness of 152 ; the red dashed lines indicate the boundaries of CNF. (i) Comparison between measurements from SEM (yellow bars) and confocal microscope (green bars).
The areal density was measured by an ultra-microbalance with a precision of 0.1
A widely applied method to measure the thickness of a foam is to observe the cross section of the foam with scanning electron microscopy (SEM). However, this method is time-consuming and costly. We used a quick and accurate method to measure the thickness of CNFs by using 3D confocal microscopy (3DCM) instead. A schematic diagram of the method is shown in Figure 2(d). We first sprinkle Cu powder on the CNF sample, as shown in Figure 2(c). The powder contains Cu particles with diameters of 1–2
To verify our thickness measurement method, we also measured the thicknesses of CNFs with SEM for comparison. Figure 2(h) shows the SEM image of the cross section of a sample, where the boundaries of CNF can be clearly distinguished. The thickness of this sample is 152
3 Results and discussion
3.1 Control of the thickness and density
In the FCCVD process, the sublimation rate of catalyst, the flow rate of argon and methane, the temperature of reaction and deposition zone, and the deposit position all have a big effect on the properties of synthesized CNF targets. Figure 3(a) shows that the sublimation rate of the catalyst linearly increases with the temperature of the sublimation zone. By accurately controlling the temperature of the sublimation zone, the sublimation rate of the catalyst can be precisely controlled. Table 1 lists the thicknesses and densities of CNFs obtained with different synthesis parameters, where
Flow rate, | Flow rate, | Deposit | Time | Thickness | Density | ||
---|---|---|---|---|---|---|---|
Condition | Ar (sccm) | position | (min) | ||||
C1 | 120 | 500 | 4.0 | B | 20.0 | 60.8 ± 2 | 1.0 ± 0.5 |
C2 | 130 | 500 | 4.0 | B | 23.0 | 80.7 ± 3 | 2.1 ± 0.5 |
C3 | 140 | 500 | 4.0 | B | 25.0 | 89.7 ± 3 | 3.5 ± 0.5 |
C4 | 140 | 500 | 6.0 | B | 30.0 | 91.9 ± 3 | 4.6 ± 0.6 |
C5 | 130 | 1006 | 6.1 | B | 30.0 | 44.0 ± 2 | 6.9 ± 0.8 |
C6 | 120 | 1003 | 6.1 | A | 45.5 | 50.5 ± 2 | 13.1 ± 1.0 |
Table 1. Conditions of CVD method and parameters of deposited CNF.
Figure 3.(a) Sublimation rate of catalyst as a function of the temperature of the sublimation zone. (b) Thickness of CNFs as a function of the deposition time.
In addition, increasing the flow rate of methane or argon, and adjusting the substrate towards the upstream direction can also increase the density of CNF. Under the conditions that all the synthesis parameters are combined, the density of CNFs can be controlled in the range of 1–13 mg/cm3. As depicted in Figure 3(b), for given synthesis parameters of
3.2 Uniformity and morphology of CNFs
The uniformity on the thickness of the prepared large-area CNF is vital for fundamental studies and applications that require good repeatability. Using Si wafer as the testing substrate, we measured the thicknesses of a
Figure 4.(a) Thickness of CNF measured along the
We also characterized the morphology of CNFs deposited on the Si wafer using SEM as shown in Figure 4(b). The diameter of the red circle shown is 4
3.3 Capability to deposit CNF on plastic or metal foils
For the previous FCCVD system, ultrathin plastic or metal foils cannot survive in the deposition zone and are all broken during the deposition as shown in Figure 4(d). By adopting the improved FCCVD system, CNFs can be deposited on any ultrathin foil. Figure 4(e) shows a double-layer target composed of the CNF and a 200 nm plastic foil, where target holes with the halo have been shot in one experimental campaign. Figure 4(f) shows an intact
4 Summary and conclusion
We successfully synthesized large-area uniform CNF targets with an improved FCCVD method by enlarging the diameter of the quartz tube and introducing a three-segment water-cooling pipe. Simple and convenient methods of measuring the thicknesses and densities of CNFs were developed. It was found that the thickness of CNFs linearly depending on the deposition time and the density of the CNFs can be controlled by varying different parameters, such as temperature of the sublimation zone, flow rate of argon or methane, and deposition position. The improved synthesis method enables the fabrication of double-layer targets composed of CNFs and ultrathin plastic and metal foils, which are highly demanded targets for laser ion acceleration and laser-driven radiation sources.
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