Multicavity linear transformer driver facility for $Z$-pinch and high-power microwave research

BLUE is a four-cavity linear transformer driver system in the Plasma, Pulsed Power, and Microwave Laboratory at the University of Michigan. Each BLUE cavity can store up to 2 kJ of electrical energy and provide an open-circuit voltage of 200 kV when using the maximum bipolar charge voltage of $\pm 100\mathrm{kV}$. The impedance of each cavity is approximately $0.5\mathrm{\Omega }$, and the rise time of the current pulse into a reasonably matched load is $\sim 100\mathrm{ns}$. The system will be used to study high-energy-density $Z$-pinch plasmas and high-power radiation sources. The facility has been equipped to handle intense bursts of x rays, nuclear fusion neutrons, and high-power microwaves (HPM). This paper reports on the construction and performance of the first (prototype) BLUE cavity and its auxiliary systems, as well as preliminary experiments using the cavity to drive resistive loads and a magnetically insulated line oscillator (MILO) HPM device. In these initial single cavity, BLUE-driven MILO experiments, 1.19 GHz microwave oscillations were successfully generated.

Figure 1

The BLUE experimental bay in the Plasma, Pulsed Power, and Microwave Lab (PPML) at the University of Michigan, with the prototype BLUE cavity configured as: (a) a bipolar Marx generator with a clear plastic lid; and (b) as a linear transformer driver with a steel lid.

Figure 2

The first assembled BLUE cavity, shown here with a clear polycarbonate lid and no ferromagnetic cores (i.e., the cavity is in a single-stage bipolar Marx generator configuration). The cavity is filled with transformer oil. The small vacuum chamber on top of the cavity houses the resistive load. The brass structure on top of the chamber is the current viewing resistor (CVR). Plug-in feedthroughs with 3D-printed parts allow high-voltage cables to be easily disconnected from the cavity without removing the lid or draining the transformer oil.

Figure 3

An L3Harris spark-gap switch with brass electrodes, capable of operation at 200 kV ($\pm 100\mathrm{kV}$) and up to 200 psi gas fill. These switches are used in the MAIZE & BLUE LTD systems at the University of Michigan. In this image, the switch is shown with a shorting wire between the UV-pin and midplane. In the BLUE cavity, the shorting wires are replaced with $3\text{−}\mathrm{M}\mathrm{\Omega }$ resistor chains for proper operation of the UV-pin.

Figure 4

Arduino Uno (ATmega328P-based) control panel for semiautonomous control of the BLUE charging and firing sequence, as well as other system functions.

Figure 5

A 20-gallon oil drum contains the three Ross HV relays used in the charging circuit. The relays are operated by the control panel (see Sec. 3a and Fig. 4) during the charging sequence. The large copper-sulfate dump resistors in this photograph were ultimately replaced with $50\text{−}\mathrm{k}\mathrm{\Omega }$ ceramic resistors.

Figure 6

The 10-gallon oil drum containing the trigger pulse generating brick, which is charged in parallel with the BLUE LTD cavities and triggered by the PT55 pulse generator mounted to the top of the drum.

Figure 7

The Pacific Atlantic Electronics PT55 module with a $+7\mathrm{kVDC}$ power supply and the newly developed $+300\mathrm{V}$ pulse generator (PT003 replacement). Together, these modules can produce a $+50\mathrm{kV}$ output pulse with $<10\text{−}\mathrm{ns}$ jitter from a logic-level ($+5\mathrm{V}$) input pulse.

Figure 8

Output pulses from an original PAE PT003 (blue) and the new $+300\mathrm{V}$ pulse generator (orange) into $50\mathrm{\Omega }$, when given a $+5\mathrm{V}$ TTL trigger signal.

Figure 9

Circuit diagram of the $+300\mathrm{V}$ pulse generator designed to trigger the PT55 mounted to the BLUE trigger pulse generator. The $+20\mathrm{V}$ and $-300\mathrm{V}$ are provided with small 12 V dc-dc converters. This generator replaces the defunct PT003 module.

Figure 10

Circuit diagram of the pre-magnetization pulse generator for resetting the ferromagnetic cores on BLUE. In this diagram, only the output to the first cavity is enabled.

Figure 11

Photograph of the internal components of the pre-magnetization pulse generator constructed for resetting the ferromagnetic cores on BLUE. This photograph shows the generator’s four positive and four negative output channels. The dark blue 3D-printed plastic structures on top of the capacitors are the screw-in breakers for connecting individual channels. In this photograph, only the cables to the first cavity are attached.

Figure 12

Output current waveforms from a single channel of the pre-magnetization pulse generator into three load scenarios: (1) into a short-circuit load (blue); (2) into the cavity for the case where the cores have already been reset or pre-magnetized (green); and (3) into the cavity for the case where the cores need to be reset or pre-magnetized (red). Also shown is a waveform produced by an LTspice model fine-tuned to closely match the experimental results.

Figure 13

The prototype BLUE cavity mounted on its side and firing into a resistive load in the bipolar Marx generator configuration (i.e., using the clear plastic lid and thus not using ferromagnetic cores). The bright flash is an undesirable arc to the white, steel support frame.

Figure 14

Nominal experimental current traces from the prototype BLUE cavity configured as an LTD at different charge voltages, acquired using a current-viewing resistor (CVR). The data have been smoothed to reduce high-frequency noise. The resistive load has resistance and inductance values of approximately $1.5\mathrm{\Omega }$ and 110 nH, respectively.

Figure 15

Experimental current trace from the prototype BLUE cavity configured as an LTD at $\pm 70\mathrm{kV}$ charge voltage into a resistive and inductive load. The experimental curve has been smoothed to remove high-frequency noise. Also plotted for comparison is the current trace from an LTspice simulation of the discharge. In the simulation, the load resistance was $1.5\mathrm{\Omega }$ and the load inductance was 110 nH, which is in good agreement with the estimated load resistance and inductance based on geometry.

Figure 16

A simple circuit model of the BLUE LTD facility. Formulas for the circuit elements, along with approximate values for a single cavity, are provided in Table 1.

Figure 17

(a) Cross-section of the MILO HPM device. (b) CST particle-in-cell simulation of the MILO in $\pi$-mode operation, with the simulated electrons (blue or green) bunching into well-formed spoke-like structures in the SWS [32]. (c) The MILO mounted to the prototype BLUE cavity.

Figure 18

Voltage signals from three $B$-dot probes in the slow-wave structure of the MILO during a discharge of the BLUE prototype cavity with a charge voltage of $\pm 70\mathrm{kV}$ (top and middle). Also plotted is the load current into the MILO, which was measured using a calibrated Rogowski coil. Microwave oscillations at a frequency of 1.187 GHz are observed for approximately 80 ns (during the period of 65–145 ns). The oscillations are well-sampled for Fourier analysis and comparison with simulation (bottom). Plotted are smoothed, normalized FFTs of the $B$-dot data, as well as normalized FFTs from CST simulations in Ref. [32] at different drive voltages.