Jiahui Yin

Effect of Cavity-Triggering Sequences on Output Parameters of LTD-Based Drivers

This paper explores the feasibility of improving the load current parameters by optimizing the cavity-triggering sequences for a linear transformer driver (LTD) module with 60 cavities connected in series. The mechanism behind this was analyzed. The results demonstrate that it is necessary for the improvement in the output parameters of the LTD to postpone the closure of the switches in the cavities later than the arrival of the electromagnetic pulse propagating downstream. However, this requires the switches to withstand pulsed voltages higher than their charge voltage before closing. When the LTD switches withstood a pulsed voltage peak of about 400 kV for several tens of nanoseconds before closing, the LTD module could generate a current pulse with a rise time of 12.5 ns and a peak of 1340 kA on the matched load.

Optimized Design of Azimuthal Transmission Lines for the Cell Driven by Two PFLs in Induction Voltage Adders

In this paper, a full-size 3-D electromagnetic model (without electrons) and an equivalent circuit model of a single cell in magnetically insulated induction voltage adders has been established. The simulation results of these two models agree well with each other. On the basis of these two models, azimuthal transmission lines for a cell driven by two pulse forming lines (PFLs) have been optimized. The optimization process aims at matching the impedance to the feed PFLs, ensuring uniform potential distribution along the insulator-stack, maximizing the symmetric current flow around the bore, and minimizing the waveform distortion of the injected pulse. A formula is presented to estimate the azimuthal impedance and an asymmetric coefficient is also defined to quantify the asymmetric extent of the feed pulses. The profiles of the total four candidate azimuthal lines have been simulated and analyzed. The simulation results indicate a relatively low feed asymmetry for all the four profiles. However, considering the waveform quality of the cell output voltage, the azimuthal line profiles with four points connected or wholly connected to the cathode plate are the most suitable for induction cells with two PFL feeding. And the corresponding coefficients

Effect Analysis of Switch Prefire in Linear Transformer Drivers

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Trigger Method Based on Secondary Induced Overvoltage for Linear Transformer Drivers

are 3.1% and 7.6%, respectively. It is concluded that, when optimizing azimuthal lines for cells with two PFL feeding, the principal consideration should be matching the impedance instead of symmetrizing the current flow.

2.8-MV Low-Inductance Low-Jitter Electrical-Triggered Gas Switch

In this paper, the circuit model capable of dealing with magnetizing characteristics of the magnetic cores and switch prefire is established for linear transformer driver (LTD) cavities in PSPICE. Through simulation, the effects of switch prefire on other components in LTD and its output current pulse have been analyzed associated with the variation of energy loss in magnetic cores. Simulation results indicate that a switch prefire in one cavity would produce oscillations added to the charge voltage of the other switches in this cavity and cavities adjacent immediately. The magnitudes of such oscillations have an intimate relation with the magnetic core loss equivalent resistance. The loss equivalent resistances in the range from 4 to 6 have produced moderate voltage oscillations. Experiments on the 300-kA LTD cavity developed by the Northwest Institute of Nuclear Technology proved that such oscillations added to the charge voltage of other switches are likely to increase the breakdown probability of them.

Low voltage pulse injection test of a single-stage 1 MV prototype induction voltage adder cell

In this paper, a triggering method based on secondary induced overvoltage for a linear transformer driver (LTD) is proposed and investigated by simulation. A detailed circuit model established for the LTD is chosen to study the effect of some factors on the feasibility of this triggering method. The considered factors include the permeability, equivalent resistance of energy loss, saturation of magnetic cores, and the delay of gas switches in LTD cavities. Simulation results indicate that high equivalent resistance and high permeability of magnetic cores are helpful in realizing this trigger method. However, the resistance plays a leading role. An equivalent resistance of more than 1.5 Ω favors the realization of this trigger. However, a relative permeability of even 25 has not produced a fatal menace to this triggering method. The feasibility of this triggering necessitates the available volt-second product of the present magnetic cores to be expanded twice. As for gas switches, the interval given by secondary induced overvoltage would likely be long enough to complete their breakdown. From simulation results, it is shown that LTD switches could be rapidly closed under the action of such an overvoltage pulse with a rise time of less than 10 ns.

Circuit Model of Magnetically-Insulated Induction Voltage Adders Based on the Transmission Line Code

Megavolt-class-triggered switches play a key role in large-induction voltage adder accelerators. This paper introduces a 2.8-MV, low-inductance, low-jitter, electrical-triggered, gas SF6 switch. The main section is a V/n switch and the pretrigger section is a trigatron switch. Based on electric field simulation, experiment, and analysis, the structure and spacing of gaps, location of trigger disk, diaphragm, and switch inductance are all discussed. The primary test indicates that the switch can be operated in the range of 0.85-2.76 MV with the pressure of the SF6 changing from 0.1 to 0.9 MPa. The self-breakdown voltage is linearly related to the logarithm of the pressure, as the gap spacing and the voltage rise time are fixed. Three working modes were observed: self-breakdown, self-triggered, and ex-triggered. In the triggered modes, the switch jitter is <;3 ns and the switch inductance is usually below 150 nH.

Note: Multi-gap gas switch with low trigger-threshold voltage by mounting resistors and capacitors in parallel with switch gaps

Low voltage pulse injection tests have been done on a 1 MV prototype induction cell. The tests mainly aim at measuring azimuthal uniformity of feed currents and evaluating cell electrical parameters. Tests are conducted under two cases that the cell is fed by a single pulse and two pulses, respectively. The results indicate that, in the case of the single-point feed, the best current uniformity with an azimuthal variation of 19.3% is acquired when azimuthal lines connect to cathode plates with lower half circumferences. In the case of two-pulse synchronous feed, the current uniformity becomes better, and a current with an azimuthal variation of 11.8% is achieved. Moreover, the effects of asynchronous feed on current uniformity are also experimentally investigated. The results imply that, for given injecting pulses with the duration of 70-80 ns, a time deviation less than 30 ns could be acceptable, without obvious degradation on the feed uniformity and current addition. In addition, the influences of the current uniformity and amounts of feed pulses on cell equivalent inductances are evaluated. The experiment results show that, the equivalent inductance would nearly keep a value of 130 nH as the current variation is less than 50%. However, the extreme asymmetry of feed currents or the increase of amounts of feed pulses would produce additional inductance.

Note: Novel trigger pulse feed method for mega-volt gas switch

Based on the transmission line code (TLCODE), a 1-D circuit model used for the analysis of magnetically-insulated induction voltage adders (MIVAs) was developed. Using cells improved from those of the JianGuang-I facility, a 10-stage MIVA was conceptually designed, and its equivalent TLCODE circuit model was described in detail with consideration of the magnetic insulation. To verify the effectiveness of this model, simulation results were compared with those of a PSPICE model (with no electron emission) under both synchronously triggering mode and ideal time-sequence triggering mode. The comparisons show that calculation results of these two models are accord with each other perfectly (within 0.1%). In addition, features of the calculation results were particularly analyzed and verified by the existent literatures. Considering the magnetic-insulation process, the inner stalk of the 10-stage MIVA was designed, and its output parameters were simulated with various electron emission thresholds (150, 200, and 250 kV/cm). Furthermore, qualitative analysis was done to present the models abilities of the magnetic-insulation treatment.

Experimental Study and Electromagnetic Model of a 1-MV Induction Voltage Cavity

To reduce the trigger threshold voltage of the multi-gap gas switch used for linear transformer drivers, a method is proposed by mounting resistors and capacitors in parallel with the switch gaps. Based on the circuit model of the six-gap gas switch, the gap voltage distribution during the triggering process is analyzed. When the multi-gap gas switch is triggered, the voltage distribution between gaps is mainly determined by the stray capacitance between electrodes. In such condition, the trigger voltage is not fully applied on the trigger gap, and as a consequence, a higher trigger voltage is required for obtaining a low jitter. The effects of capacitor parameters on the triggering characteristics of the switch are experimentally investigated. Compared with the original switch design, the results indicate that at a charging voltage of ±80 kV and operating at 60% of the self-breakdown voltage, the trigger voltage is reduced from 110 kV to 75 kV while the 3.2 ns jitter of the switch is preserved.