Abstract
A linear transformer driver (LTD) generator capable of delivering up to 0.9 MA current pulses with 160 ns rise time has been assembled and commissioned at University of California San Diego. The machine is an upgrade of the LTD-III pulser from Sandia National Laboratories, consisting of 40 capacitors and 20 spark gap switches, arranged in a 20-brick configuration. The driver was modified with the addition of a new trigger system, active premagnetization of the inductive cores, a vacuum chamber with multiple diagnostic ports, and a vacuum power feed to couple the driver to plasma loads. The new machine is called compact experimental system for $Z$-pinch and ablation research (CESZAR). The driver has a maximum bipolar charge voltage of $\ifmmode\pm\else\textpm\fi{}100\text{ }\text{ }\mathrm{kV}$, but for reliability and testing, and to reduce the risk of damage to components, the machine was operated at $\ifmmode\pm\else\textpm\fi{}60\text{ }\text{ }\mathrm{kV}$, producing 550 kA peak currents with a rise time of 170 ns on a 3.5 nH short circuit. While the peak current is scaled down due to the reduced charge voltage, the pulse shape and circuit parameters are close to the results of the cavity and power feed models but suggest a slightly higher inductance than predicted. The machine was then used to drive wire array $Z$-pinch and gas puff $Z$-pinch experiments as initial dynamic plasma loads. The evolution of the wire array $Z$ pinch is consistent with the general knowledge of this kind of experiment, featuring wire ablation and stagnation of the precursor plasma on axis. The gas puff $Z$ pinches were configured as a single, hollow argon gas shell, in preparation for more structured gas puff targets such as multispecies, multishell implosions. The implosion dynamics agree generally with 1D magnetohydrodynamics simulation results, but large zippering and magneto-Rayleigh-Taylor instabilities are observed. The CESZAR load region was designed to accommodate many load types to be driven by the machine, which makes it a versatile platform for studying $Z$-pinch plasmas. The completion of the CESZAR driver allows plasma experiments on energy coupling from LTD machines to plasma loads, instability mitigation techniques and magnetic field distributions in $Z$ pinches, and interface dynamics in multispecies implosions.
Highlights
The machine described in this work is based on the linear transformer driver (LTD)-III cavity received from Sandia National Laboratories, where it was extensively tested with resistive loads, but it was not configured with a power feed to drive plasma loads [23]
CESZAR can operate at a charge voltage Vc Æ 60 kV, producing peak currents in the range of 400– 500 kA with a 160–200 ns quarter period, depending on the load impedance
Short-circuit tests done at Vc Æ 60 kV show a slightly higher inductance than the circuit model and a comparable peak current
Summary
Linear transformer drivers (LTDs) are a promising pulse generator configuration, as they do not require pulseforming lines like conventional systems based on Marx bank technology to produce short (100–300 ns) pulses. The use of many bricks in parallel results in a circuit with low inductance, suitable for producing fast, high-current electrical pulses. Ferromagnetic cores can be placed inside the cavity to prevent current from taking a parasitic current path around the cavity enclosure This parasitic current path is electrically in parallel with the path that includes the power feed and load. The machine described in this work is based on the LTD-III cavity received from Sandia National Laboratories, where it was extensively tested with resistive loads, but it was not configured with a power feed to drive plasma loads [23]. The machine was modified with a new trigger system, an active premagnetization circuit for the ferromagnetic cores, a vacuum chamber for diagnostic access, and a vacuum power feed to propagate the current pulse to the load. V, we summarize our work and provide some concluding remarks
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