Miniature solid-state switched spiral generator for the cost effective, programmable triggering of large scale pulsed power accelerators

This paper presents the design and testing of several different configurations of spiral generator, designed to trigger high current switches in the next generation of pulsed power devices. In particular, it details the development of spiral generators that utilize new ultrafast thyristor technology as an input switch, along with a polarity dependent output gap to improve the efficiency of the spiral generator design. The generator produced 50 kV from a 3.6 kV charging voltage, with a rise time of only 50 ns and a jitter of 1.3 ns—directly comparable, if not better than, a generator employing a triggered spark gap as the input switch. The output gap was constructed in house from commonly available components and a 3D printed case, and showed remarkable repeatability and stability—simple alterations to the output gap could further reduce the rise time. The entire spiral generator, along with control and charging electronics, fitted into a case only $210\times 145\times 33\mathrm{mm}$.

Figure 1

General schematic of a spiral generator, with input switch and pulse sharpening output gap. Red represents conductor (usually copper tape), blue insulation (usually Mylar). Generally, 10 or more turns are used. $V_{\mathrm{charge}}$ is the charging voltage of the stripline, $N$ is the number of turns of the spiral, ${ϵ}_{\mathrm{ff}}$ is a measure of the efficiency of multiplication, and $V_{\mathrm{output}}$ the output voltage.

Figure 2

(a) Complete spiral generator utilizing spark gap input switch and commercial self-breakdown spark gaps on the output (in the center of the spiral winding). (b) Spiral generator utilizing thyristor based input switch and custom polarity dependent output gap. Note in (b) the generator is pictured with test leads and a gas supply to the output gap—these are removed prior to usual use. Also visible in (b) is the Kovalchuk ball gap underneath the spiral that it is designed to trigger.

Figure 3

Picture of finished spiral stripline after impregnation with high voltage insulating epoxy.

Figure 4

Schematic of the circuit used to trigger the RU87–7 miniature spark gap, when it was used as an input switch to the spiral winding. The TTL input signal triggers the MOSFET via an NPN and PNP transistor. This in turn discharges a 200 V capacitor to ground via a $1:20$ air core transformer, the output of which triggers the miniature spark gap.

Figure 5

(a) Schematic of thyristor circuit used for the input switch. (b) Trigger circuit that supplies the thyristor circuit in 5(a). (c) Circuit with six thyristors in series, used in early experiments. (d) The more optimized circuit with four thyristors.

Figure 6

Design of the polarity dependent output gap developed in the project.

Figure 7

A typical test configuration, in this case using optimized the four thyristor input switch and output gap. $C_L$ represents the capacitance of the load (the triggering electrodes of a Kovalchuck ball switch).

Figure 8

Output pulses of three versions of the spiral generators. (a) Using the miniature triggered spark gap input switch and $+5.4\mathrm{kV}$ charging voltage. (b) Using the initial six thyristor input switch and $-4.5\mathrm{kV}$ charging voltage. (c) Using the four thyristor input switch and $-3.6\mathrm{kV}$ charging voltage. For experiments with an output gap, “OG” marks its breakdown. Note the start time of 100 ns simply reflects when the scope was externally triggered, not the thyristors.

Figure 9

Relationship between the pulse parameters and the charging voltage for the different configurations of spiral generator. (a) Amplitude; (b) rise time; and (c) jitter.

Figure 10

Comparison between experiment and simulation for spiral generators using six thyristor and four thyristor (optimized) input switches. With the six thyristor switch the charging voltage was $-4.5\mathrm{kV}$, with the four thyristor switch $-3.6\mathrm{kV}$. A 15 pF capacitive load was employed for the simulations.

Figure 11

Effect of different value capacitors placed in parallel with the load on the output characteristics of a spiral generator utilizing a six thyristor input switch. The charge voltage in all experiments was $\sim 5\mathrm{kV}$.

Figure 12

The output voltage, rise time, and jitter measured at the load for four different output gaps all using the spiral generator with a six thyristor input switch.