Electron beam transport in a weak plasma
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Abstract:
The propagation of an electron beam through a dilute plasma is studied experimentally. A beam is injected into a low pressure gas (P≃10−6 Torr) resulting in sufficient ions to neutralize the beam so that space charge blow-up does not occur directly. However, a low frequency beam-drift mode is unstable, which results in a radial displacement of the beam. This can produce sufficient space charge to stop beam propagation. The experimentally observed threshold is in agreement with linear theory, and the limiting current agrees with an estimate of the space charge limit for this geometry.Keywords:
Limiting
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Constant (computer programming)
Time constant
Pressure measurement
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为了研究缩比实验在气体放电中的有效性, 对缩比间隙中的低气压氩气放电进行了数值模拟. 根据气体放电相似性的猜想, 如果间隙气压p和间隙距离d的乘积为常量, 即p1d1=p2d2, 并且约化电场E/p 在两个间隙中的空间分布相同, 那么这两个放电间隙的放电特性存在相似性. 数值模拟中设置三个缩比间隙: 气隙A的长度为30 mm, 气压为1 Torr (1 Torr=133.322 Pa); 气隙B的长度为15 mm, 气压为2 Torr; 气隙C的长度为10 mm, 气压为3 Torr. 仿真结果表明, 三个间隙均为辉光放电, 并存在明显的阴极位降区. 间隙A, B, C 的阴极位降区的厚度dC分别为2.71, 1.35和0.87 mm, 相对应的pd值几乎相同, 即pdC≈2.70 Torr·mm. 这与氩气辉光放电Paschen曲线最低点(pd≈2.86 Torr·mm)相近. 缩比间隙的放电参数的特性(如工作电压、电场、电流密度、电子密度和离子密度的沿“空间”px的分布)的数值计算结果与放电相似性猜想所预计的结果一致. 所以, 可以认为放电相似性猜想适用于低气压氩气缩比间隙的辉光放电.
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Two microwave cavities operating at 2450 MHz for producing and maintaining plasmas are described. They have been tested in nitrogen, argon and hydrogen. Cavity I can be used for discharges in nitrogen between 30 mu Torr and 230 Torr, in hydrogen between 30 mu Torr and 640 Torr and in argon between 30 mu Torr and 990 Torr. Cavity II with the same gases can be used between 30 mu Torr and 500 Torr, between 30 mu Torr and 760 Torr, and between 30 mu Torr and 1500 Torr. No experiment has been made at a pressure lower than 30 mu Torr. Cavity I is very easy to handle and the stability of its discharge makes it an attractive source of free radicals to initiate chemiluminescences in kinetic studies.
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The useful range of a capacitance diaphragm gauge is extended down to 1×10−6 Torr with an uncertainty of ±10−7 Torr using computer-based averaging. Gauge linearity is measured by observing the deflection of the diaphragm by the force of gravity. Precise pressure increments are generated by tilting the gauge through a range of accurately measured angles. By this procedure the gauge is observed to be linear to within ±1% plus 10−7 Torr between 1×10−6 and 8×10−3 Torr. Additionally, the deflection of the center of the diaphragm per unit pressure is found to be 2×10−5 m Torr−1.
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Pressure measurement
Diaphragm (acoustics)
Linearity
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The interaction of titanium films with oxygen has been studied by observing their change of resistance R and workfunction phi over the range of gas pressures from UHV to 5 Torr and exposures from 0-106 Torr s. The changes in R during the fast stage of the interaction indicated that oxygen dissolved in the Ti lattice in a few seconds. The maximum increase of 1.1 eV in phi was reached at an equilibrium pressure p approximately 10-4 Torr. When p was raised above 10-4 Torr phi decreased by 0.4-0.8 eV which suggests that crystals of an oxide were being formed on the film. For p>1 Torr and exposure >104 Torr s there were further changes approximately=0.2 eV in phi which showed that the oxide was not stable. There was also a change approximately=0.1 eV due to a weakly bound adspecies which was desorbed when the gas was pumped off.
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Space charge was introduced into the PMMA (polymethyl methacrylate) samples by electron beam. The effect of electron beam on the conductivity and the permittivity of the PMMA sample were investigated, and the breakdown of the samples during charge detrapping was also studied. The experimental results show that not only space charge injection but its detrapping also makes samples breakdown. The dynamics of injected charges in the radiated PMMA samples and the mechanism of the breakdown resulted from the rapid discharge are discussed.
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The numerical study of the virtual cathode (VC) formation in the tubular helical electron flow formed by a magnetron-injection gun (MIG) in the mode of small pitch-factor under the influence of space charge is carried out.
Electron gun
Electron flow
Cavity magnetron
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For investigation of propagation dynamic of high power long pulse (0, 3–1 ms) electron beam in vacuum within pressure 5∗10−5 ÷10 Torr two experimental installation were constructed: 1. Electron energy − 300 keV, current 30–100 A, pulse 0,35 ms long; 2. Electron energy 500–600 keV, current 30–200 A, pulse 1 ms long. Two different macroinstabilities of the beam have been observed. One of them had freaquences up to 10 MHz within pressure 5∗10−5 ÷1 Torr. Second had freaquences several kiloherzes within pressure 5∗10−5 ÷10 Torr. When pressure was 5∗10−5 ÷5∗10−4 Torr monotonous increasing of beam diameter along transport way was observed depending on beam microinstability.
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Pulse duration
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Mercury
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