A nanosecond pulse power supply for grid-controlled electron gun used in HALF

Hefei Advanced Light Facility (HALF) uses a grid-controlled thermionic cathode gun as its electron source of the linear accelerator. The nanosecond grid-controlled power supply is an important part of the electron gun power supply system, and its performance will affect the quality of the beam. To achieve the requirements of the electron gun for nanosecond pulse power supply, we use the series-parallel multi-stage avalanche transistor scheme to achieve nanosecond fast pulse output. The test results show that the power supply not only can meet the requirements of HALF for its electron gun but also have good waveform repeatability and amplitude stability.


Introduction
Hefei Advanced Light Facility (HALF) is a diffraction limit storage ring light source in the vacuum ultraviolet and soft X-ray bands [1], which is now started to construct by the National Synchrotron Radiation Laboratory (NSRL) in the University of Science and Technology of China (USTC).HALF uses a 2.2 GeV RF linear accelerator as an injector [2].And it is designed to operate in the top-off injection mode [3].To make the injector more stable, reliable, and longer life, we choose the thermionic cathode grid-controlled electron gun (EG) scheme.
As we all know, applying a high pulsed voltage to the grid of the EG can control the strength of the electron beam output and its pulse width.Therefore, developing a nanosecond pulse power supply (nano-PPS) with high stability and reliability for specific needs is of great significance for the smooth construction and high-performance operation of HALF.In HALF, the main design requirement of the nano-PPS is to generate a gate pulse with a bottom width of less than 1 ns and an amplitude of over 600 V.
Nano-PPS usually uses switching devices like avalanche transistor (AT), drift step recovery diode (DSRD), fast ionization diode (FID), and so on.In fact, the performance level of DSRD and FID is higher than that of AT, but unfortunately, they have not been commercialized, so it is difficult to buy a separate DSRD or FID on the market.As a commercial device, AT has the advantages of stable supply, low price, long life, and fast switching speed, which has attracted wide attention from scientists in recent years.
At present, there is much research [4][5][6][7][8] on AT applied in different fields.However, most of the waveforms in the literature are not suitable for the requirements of the nano-PPS of the EG in HALF.Some of them do not reach the bottom width of less than 1 ns, some focus only on the extremely fast pulse front edge or high pulse amplitude and the shape of the waveform is not required, while the EG requires the gate pulse to have both a fast rising and falling edge and a flat top of a certain width.
Aiming at the demand of the nano-PPS for the EG of HALF, a series-parallel multi-stage AT scheme is selected to realize the output of the nanosecond fast pulse.In the design, we use several novel designs, such as the use of AT for fast pulse high-voltage trigger, impedance taper structure, and compensation circuit for waveform optimization.The experimental results show that the nano-PPS can meet the requirements of the EG in HALF and it has good waveform repeatability and amplitude stability.

System of the nanosecond pulse power supply
The nano-PPS system designed in this paper is shown in Figure 1.The front-stage DC high voltage power supply can output 0~4kV DC voltage to charge the transmission line (TLine).The small signal generated by the signal source forms a large trigger signal through the multi-stage ATs in series, and then it will trigger the series-parallel AT network.Finally, a nanosecond pulse wave will be generated on the load.The pulse width is related to the length of the TLine, and the fast pulse front edge is determined by the fast conduction velocity of the AT.The pulse amplitude can be increased to several kilovolts by using ATs in series.The output current of the circuit can be improved while using ATs in parallel.

Circuit of the nanosecond pulse power supply
To obtain a fast pulse front (hundreds of picoseconds) and to prevent the trigger from being disturbed and misdirected, we use a different set of AT circuits to trigger the main pulse circuit, which can achieve a faster pulse front and more stable triggering.
Forty ATs, two in parallel per stage and 20 stages in series, make up the main impulse circuit.The trigger consists of 16 stages of ATs connected in series.The type of AT used in all circuits is FMMT415.The energy storage capacitor is a 100pF high-voltage ceramic capacitor.The trigger pulse with a certain amplitude is obtained by discharging the energy storage capacitor, which acts on the main pulse discharge circuit.The schematic of the entire nano-PPS is shown in Figure 2. We use a novel tapered impedance TLine discharge structure that utilizes the distributed capacitance and inductance of the copper-clad board to form a TLine.Starting with the upper transistor unit connected to the load, the impedance gradually decreases from 50Ω to 5.5Ω.The copper coating area and the length of each stage of the AT's pin are different.The range of each stage of the capacitor is: 0.8pF~3.2pF,and the corresponding copper-clad area is 30mm 2 ~120mm 2 .
The advantage of using a tapered impedance structure is that it maintains a fast pulse front and also maintains enough current on the rising and falling edges to give the pulse waveform a somewhat flat top.

Simulation of transmission line discharge
To evaluate the performance of the tapered impedance TLine discharge structure, we use PSPICE software to simulate.We simulated the TLine discharge structure of constant impedance, tapered impedance, and tapered impedance with a 100nH compensation inductor.The simulation results are shown in Figure 3.It can be seen from Figure 3 that the TLine discharge structure of tapered impedance is better than that of constant impedance.Among all, the waveform of the tapered impedance TLine discharge structure with a 100nH compensation inductor is the best.Though its output amplitude is smaller than that without an inductor, the zero-crossing point moves forward, the pulse width decreases, and the flat top becomes a bit smoother.

Circuit measurement
The prototype is tested after all the components are welded.The circuit connection of the prototype test is shown in Figure 4.The oscilloscope uses Tektronix MSO64, with a bandwidth of 4 GHz.The measured attenuator is 50 dB.Firstly, we measure the trigger.The voltage is -4.248V, and the actual output is -1342 V. Its leading edge is about 2.682 ns.And its pulse width is about 20 ns.The trigger waveform is shown in Figure 5. Secondly, we test the pulse forming main circuit.The voltage is -2.397V, and the actual output is -757.452V. Its leading edge is about 400 ps.The bottom pulse width of it is about 850 ps.The pulse output waveform is shown in Figure 6.Thirdly, we test the amplitude stability of pulse output, measuring the waveform stability of 1027 sampling points within 80ps.We can know from Figure 7 that the average pulse amplitude is -793.50Vand its standard deviation is 16.863V.Calculation of pulse stability is 2.12% p R  .Last but not least, we test the long-term stability of the nano-PPS.The nano-PPS is triggered 10 times per second and operated for 8 hours, during which time each output is recorded.The results are shown in Figure 8.Although the amplitude of the waveform fluctuates, the waveform repeatability is pretty good.

Conclusion
In this paper, a nano-PPS based on AT is designed to achieve the requirements for EG in HALF.To ensure trigger stability while obtaining the fast pulse front, we use a 16-stage series AT to trigger the pulse-forming main circuit composed of a 20-stage series-parallel AT.At the same time, tapered impedance structure and compensation inductor are used to optimize the waveform.The test results show that the output waveform of this nano-PPS not only has a pulse front of hundreds of picoseconds and a bottom width of less than 1 ns but also has good amplitude stability and waveform repeatability, which can attain the requirements of EG in HALF.

Figure 1 .
Figure 1.System block diagram of the nano-PPS.

Figure 2 .
Figure 2. Circuit diagram of the nano-PPS.

Figure 4 .
Figure 4. Connection diagram of the nano-PPS.