We report on experimental measurements of how an externally imposed magnetic field affects plasma heating by kJ-class, nanosecond laser pulses. The experiments reported here took place in gas cells analogous to magnetized liner inertial fusion targets. We observed significant changes in laser propagation and energy deposition scale lengths when a 12T external magnetic field was imposed in the gas cell. We find evidence that the axial magnetic field reduces radial electron thermal transport, narrows the width of the heated plasma, and increases the axial plasma length. Reduced thermal conductivity increases radial thermal gradients. This enhances radial hydrodynamic expansion and subsequent thermal self-focusing. Our experiments and supporting 3D simulations in helium demonstrate that magnetization leads to higher thermal gradients, higher peak temperatures, more rapid blast wave development, and beam focusing with an applied field of 12T.
The electric field strength between the cathode and anode (i.e., the voltage) of a pulsed power machine is one of the most important operating parameters of the device. However, to date, accurate and precise voltage measurements on these high energy pulsed power systems have proved difficult if not virtually impossible to perform. In many cases, the measurements to be performed take place in an environment cluttered with electromagnetic interference (EMI), radio frequency interference (RFI), and electron pollution, and there is the potential for electrical discharge (or arcing), there is limited physical access, or the measurement area is deemed unsuitable due to radiation safety concerns. We report on an electro-optical-based approach to measuring strong, narrow-pulse-width electric fields that requires no interfering metallic probes or components to disturb the field to be measured. Here we focus on device theory, operating parameters and a laboratory experiment.
Calculation of the power flow from the 36 pulse forming lines to the vacuum region of Saturn has always been complicated by the three-dimensional structure of the rod and bottle connections to the vacuum insulator stack. Recently we have completed a 3-D calculation of the bottle configuration and found a large error in previous impedance estimates. We have used this calculation to determine impedance and to construct a 2-D model of each of the 36 bottles of each level of the insulator using the Transmission Line Matrix (TLM) technique. These TLM models are then used in a 2-D model for each of the three levels of the insulator. Each model starts at a measured forward-going pulse in the water tri-plate and ends at the Brehmstrahlung load at the center of the machine. Because of long transmission line lengths and short pulse lengths, each level can be considered independent of the others. A combination of the three models then represents a quasi-3-D model of the load region of the machine. The results of these calculations agree well with measurement and thereby provide confidence in simulation predictions for those areas where measurements are not possible. Details of the 3-D bottle calculation, the TLM model, and results of the load region simulations using this model are given.
Saturn is a short-pulse ( 40 ns FWHM) x-ray generator capable of delivering up 10 MA into a bremsstrahlung diode to yield up 5 × 10^12 rad/s (Si) per shot at an energy of 1 to 2 MeV. With the machine now over 30 years old it is necessary to rebuild and replace many components, upgrade controls and diagnostics, design for more reliability and reproducibility, and, as possible upgrade the accelerator to produce more current at a low voltage ( 1 MV or lower). Thus it has been necessary to reevaluate machine design parameters. The machine is modeled as a simple LR circuit driven with an equivalent a sine-squared drive waveform as peak voltage, drive impedance, and vacuum inductance are varied. Each variation has implications for vacuum insulator voltage, diode voltage, diode impedance, and radiation output. For purposes of this study, radiation is scaled as the diode current times the diode voltage raised to the 2.7 power. Results of parameter scans are presented and used to develop a design that optimizes radiation output. Results indicate that to maintain the existing short pulse length of the machine but to increase output it is most beneficial to operate at an even higher impedance than originally designed. Also discussed are critical improvements that need to be made.