Short-circuited Diode Research Articles

Characterizing the HERMES-III Extended Magnetically Insulated Transmission Line

Modeling and simulation of the HERMES-III extended magnetically insulated transmission line (MITL) terminated by a diode with an annular cathode and a planar anode was performed with an electromagnetic particle-in-cell code, EMPIRE-PIC. Self-limited MITL theory was developed and implemented in order to model select shots in EMPIRE by allowing one to transform an experimental current trace into a voltage waveform that is input into EMPIRE. Simulating shots 11 132, 11 133, 11 134, 11 135, 11 136, and 11 146 in EMPIRE shows good agreement with experimental current traces at several locations along the MITL. New calibrations for the anode current shunts are proposed based on low-power shots fired into a short-circuited diode configuration (short-shots). The new calibrations show improved agreement with simulation results.

Lateral distribution of the energy losses of an electron beam in a substance at normal incidence of charged particles on the sample surface

The results of developing a new universal function describing the lateral distribution of energy losses of an electron beam in a substance are presented for local electron-probe methods, including cathodoluminescence, the method of electron beam induced current (EBIC) in the circuit of a short-circuit diode, and X-ray spectral microanalysis. An analytical expression for the one-dimensional function ?(x) of the distribution of electron energy losses is obtained. It is necessary to calculate the distributions of information signals in the sample surface plane and makes it possible to reliably control the realized spatial resolution of experiments for any materials in a broad range of beam electron energies.

测量1 MA/100 ns脉冲大电流的微分环

The Qiangguang-Ⅰ (QG-Ⅰ) accelerator can produce high current pulses with rise time of about 100 ns and amplitude of about 2 MA. The load current is usually monitored by self-integrating Rogowski coils. In order to verify the obtained results of the coils, a differential loop with fast response time, relatively simple design, and appropriate performance against electromagnetic noise, has been developed. Calibration experiments show that the differential loop has a response time of about 1.2 ns, a response frequency range from 10 kHz to 100 MHz, and a sensitivity of 6.1310-11(Vs)/A. The fast response may help to monitor transient variation of the load current related to some rapid physical processes. The differential loop has been tested within short-circuit diode of QG-I accelerator and compared with self-integrating Rogowski coils. The current waveforms obtained through numerical integration of the loop signal exhibit good agreements with those from the coils, with an amplitude deviation less than 10%. Comparison of the results confirms the availability of the loop, and also verifies the reliability of the load current measurements on the accelerator.

Counterstreaming electrons and ions in Pierce-like diodes.

The dynamics of Pierce-like diodes is investigated for ions moving with arbitrary velocities opposite to as well as in the direction of electron propagation. By application of an integral formalism that is able to account for mobile streaming ions, equilibrium solutions for the diode in an external circuit are derived. The stability of the uniform equilibrium for the short-circuited diode and for counterstreaming species is investigated. In this case, new oscillatory unstable branches appear that destabilize the diode for all values of \ensuremath{\alpha}, where \ensuremath{\alpha} is the Pierce parameter. This contrasts with the diode with co-moving ions being stable for sufficiently small values of \ensuremath{\alpha}. These new branches coincide with the Pierce-Buneman modes for initially resting ions. If \ensuremath{\Vert}${\mathit{v}}_{\mathit{i}0}$\ensuremath{\Vert}/${\mathit{v}}_{\mathit{e}0}$ is increased, where ${\mathit{v}}_{\mathit{s}0}$ are the injection velocities (s=e,i), a stronger destabilization is observed for counterstreaming in comparison to co-streaming with maximum growth rate in the range of ${\mathit{v}}_{\mathit{i}0}$\ensuremath{\approxeq}-2${\mathit{v}}_{\mathit{e}0}$ for a hydrogen plasma. It is furthermore shown that the familiar picture of waves (Fourier modes) must be substantially modified to meet the physics of bounded plasmas.

Formation and Stability of Virtual Electrodes in a Cylinder

Two-dimensional computer experiments have been performed to determine whether a stable virtual electrode can be formed inside an evacuated cylinder by the injection of charged particles from the outside. This question is important to the feasibility of certain inertial confinement schemes for fusion reactors. The charges are injected with constant velocity uniformly from the surface of the cylinder and directed at the axis. Above a certain current, depending on the injection velocity and dimensions, dc solutions exist corresponding to the formation of a virtual electrode at a calculable radius within the cylinder. Such analytic solutions involving virtual electrodes are not necessarily realized in practice as they were not in the one-dimensional computer experiments of Birdsall and Bridges on the short-circuited diode. The problem discussed here, although related to the diode, is different both in geometry and in the manner of injection. It is interesting that our computer experiments show the formation of a stable virtual electrode for a wide range of input current. Such variations of potential that do occur are transient and decay to less than ±2% after ten single-particle transit times. Experiments have also been conducted with the current injected from four separate guns. Stable virtual electrodes are formed in this case opposite each gun.

Low frequency noise suppression in space charge limited solid state diodes

The noise suppression in solid state space charge limited diodes is calculated in two ways. In the first theory the noise in the open-circuited diode is attributed to carrier density fluctuations in the potential minimum. This gives an expression for the noise that contains an unknown parameter. In the second theory the noise in the short-circuited diode is considered as a convection current fluctuation in the potential minimum that is partly suppressed by corresponding fluctuations in the depth of the potential minimum. This gives an expression for the noise that does not contain any unknown parameter. The results indicate that the noise of the device is much less than thermal noise of the differential conductance of the device, and that the noise resistance R n of the device has the low value R n = 1 2 ( kT qI a) . G80 G80 V

High-frequency effects of the potential minimum on noise

A physical model of the compensation phenomenon in space-charge reduced shot noise is discussed in some detail and applied first to a planar diode of infinite spacing short-circuited to ac effects. Analysis of both the time response and frequency response shows that parameters of the potential minimum are very important to the degree of noise compensation or over-compensation, and for certain parameters the internal feedback mechanism of returning charges may cause a self oscillation without external stimuli, or in other words a building-up to saturation levels of noise phenomena in these frequency ranges. For other parameters of the minimum there appear to be very complete compensation phenomena possible, but practical parameters for this were not found in the infinite spacing case. The infinite spaced short-circuited diode gives results in exact agreement with the frequency domain analysis of the open-circuited diode as analyzed by Siegman and Watkins. Some additional approximations may be made in this analysis for qualitative explorations, particularly important being the consideration of only a single velocity class of noise perturbation. The short-circuited diode of finite spacing is analyzed in both the frequency domain and time domain, and an approximate correction to the above results found as a function of total transit angle across the diode. In particular, the factor produces appreciable decreases in the reduction factor in the vicinity of certain transit angles across the diode, and as the Tien dips occur in these general ranges, the finite diode spacing is believed to be a strong candidate for explanation of these dips.