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摘要:
激光加速器可以输出具有独特品质的质子束，例如µm尺寸、ps脉冲长度和高峰值电流。强流粒子束的空间电荷力效应较强，对面向应用的束流传输提出了挑战。通过二维PIC模拟研究了激光加速后与质子速度接近的电子的影响。采用椭球模型估算空间电荷力的影响，比较不同电荷分布的差异。结果表明每束团质子数超过1010后空间电荷力显著影响质子束传输，甚至严重破坏束流品质。空间电荷力的影响在20 ps后显著减弱，离开靶约1.2 mm。
Abstract:Laser accelerators can provide proton beams with unique qualities, such as micron size, picosecond pulse duration and high peak current, and have been demonstrated for various applications and for scientific research purposes. The effect of the space charge force in high peak current beams is strong and raises challenges for application after beam transportation. We performed two-dimensional particle-in-cell simulations and studied the influence of electrons that have velocities close to that of the protons after laser acceleration. We employed ellipsoid models with different charge distributions to estimate the effects of the space charge force. Results demonstrate that space charge will affect beam transmission, and even lead to complete transmission failure if the number of protons per pulse exceeds 1010. The influence of the space charge force diminishes greatly after 20 ps, which corresponds to approximately 1.2 mm from the target.
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Key words:
- laser acceleration /
- proton beam /
- space charge force /
- high brightness
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Figure 2. Beam expansion with a waterbag distribution. Plots show the charge distribution on grid points in the zx plane at 0 ps, 20 ps, 50 ps, 100 ps after the laser interaction with the target
Figure 3. Influence of space charge in different charge distribution models on the evolution of electric fields and beam parameters within 100 ps after the laser interaction with the target
Figure 4. Influence of space charge for different values of total charge on the evolution of electric fields and beam parameters within 100 ps after the laser interaction with the target
Table 1. Proton and electron numbers for simulations with different targets
thickness/μm density/(g/cm3) proton number electron number aluminum 2.5 2.7 4.22×107 7.63×1010 polymer 1.2 1.186 9.91×107 3.87×1010 DLC 0.005 3 2.15×107 2.78×108 
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