Adjustable Parameter Design of Nanosecond High Voltage Pulse Power Supply Based on Marx Generator
Marx generators have served as the workhorse technology for generating high voltage pulses since their invention in the early twentieth century. The basic principle involves charging multiple capacitors in parallel at a moderate voltage, then switching them to series connection to produce a voltage that is the sum of the individual capacitor voltages. For nanosecond pulse applications, the Marx generator design must achieve extremely fast erection time, the time required for all stages to switch to series connection, while providing adjustable output parameters including amplitude, pulse width, and repetition rate.
The erection process in a Marx generator determines the pulse rise time and is critical for nanosecond pulse generation. Traditional Marx generators using spark gap switches achieve erection times in the tens of nanoseconds range, suitable for pulses of similar duration. Faster erection requires switches with faster turn on times, such as semiconductor switches or triggered spark gaps with specialized designs. The erection time also depends on the circuit inductance in the discharge path and the capacitance of the stages.
Stage switch selection affects both the erection speed and the achievable repetition rate. Spark gap switches can handle high voltages and currents but have limited repetition rate due to the time required for deionization between pulses. Gas filled switches with hydrogen or other fast deionizing gases can achieve higher repetition rates. Semiconductor switches including thyristors, MOSFETs, and diodes offer the fastest switching and highest repetition rates but may have voltage and current limitations that require series and parallel combinations.
Adjustable amplitude in a Marx generator based pulse supply can be achieved through several approaches. Varying the charging voltage changes the output amplitude proportionally, providing continuous adjustment over the design range. Switching out stages reduces the number of series capacitors and thus the output voltage, providing discrete amplitude steps. A combination of coarse adjustment through stage selection and fine adjustment through charging voltage control provides flexible amplitude programming.
Pulse width adjustment in Marx generators is more challenging than amplitude adjustment because the pulse width depends on the load characteristics and the circuit parameters. The pulse width is approximately determined by the product of the total series capacitance and the load resistance for resistive loads. Adding peaking capacitors or pulse forming networks can provide more precise pulse width control. Active pulse chopping using an output switch can terminate the pulse at a programmed time, providing precise width control independent of the load.
Repetition rate capability depends on the charging system and the switch recovery characteristics. The charging system must restore the stage capacitor voltages between pulses, with the charging time decreasing with higher charging current capability. The switches must recover their blocking capability after conducting, with recovery time varying with switch type and the conduction conditions. The maximum repetition rate is limited by the longer of the charging time and the switch recovery time.
Nanosecond pulse applications impose stringent requirements on the output waveform quality. The pulse should have fast rise and fall times, flat top with minimal droop, and minimal overshoot or ringing. These characteristics depend on the generator impedance, the load impedance, and the circuit parasitics. Impedance matching between the generator and the transmission line to the load minimizes reflections that could distort the pulse. Careful layout minimizes parasitic inductances that cause ringing.
Triggering systems for Marx generators must reliably initiate the erection process with low jitter. The first stage switch is typically triggered externally, with subsequent stages triggering through the overvoltage created when earlier stages switch. Triggered spark gaps or semiconductor switches provide the initial switching with precise timing. The trigger system must provide adequate trigger energy and voltage to ensure reliable triggering over the operating range.
Load considerations for nanosecond pulse supplies include the load impedance, the load type, and the load connection. Resistive loads absorb the pulse energy and determine the pulse shape through the resistance value. Capacitive loads require charging during the pulse, affecting the pulse shape. Transmission line loads require impedance matching to prevent reflections. The generator design must accommodate the expected load characteristics or include provisions for load adaptation.
Thermal management in high repetition rate Marx generators addresses the heat generated in switches, resistors, and other components. Average power dissipation scales with the pulse energy times the repetition rate. Cooling systems remove the heat to maintain component temperatures within acceptable limits. Thermal design must consider both the peak temperatures during pulses and the average temperature rise over extended operation.
