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May 07, 2010

Progress to Nuclear Fusion with Pulsed Power Improvement at Sandia

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Physicist Dr. Franklin Felber has taken a big step towards meeting two of the greatest technological challenges of pulsed power for fusion – current amplification and pulse compression.

A wire-array flux-compression cartridge installed on Sandia’s Saturn pulsed power generator doubled the current into a 3-nH load to 6 MA and halved its rise time to 100 ns . The current into the load, however, was unexpectedly delayed by almost 1  μs. Estimates of a plasma flow switch acting as a long-conduction-time opening switch are consistent with key features of the power compression. The results suggest that microsecond-conduction-time plasma flow switches can be combined with flux compression both to amplify currents and to sharpen pulse rise times in pulsed power drivers.
The full article is on arxiv.

The conduction time and power compression that can be achieved with a PFS (Plasma flow switches) are limited by: (i) the requirement for the channel transit time to be shorter than the diffusion time; (ii) the number of Rayleigh-Taylor e-foldings, which is proportional to the square root of the channel length; (iii) inductive energy lost to kinetic energy and heating of the plasma; and (iv) parasitic inductance of the channel.

Numerous practical advantages might result from in-serting a PFS or its equivalent near a channel entrance, as in Fig. 3, including: (i) good shot-to-shot reproducibil-ity; (ii) control of the conduction time and opening time; (iii) reduced entrainment and loss of magnetic flux; (iv) protection of the PRS (plasma radiation source) load from the long voltage pulse; and (v) compatibility with flux compression.

In summary, wire-array cartridges were installed on Saturn to amplify the current and sharpen the pulse by flux compression. Unexpectedly long delays of almost 1 μs were observed between the Saturn pulses and the currents into the loads on three shots, including one shot in which the load current was doubled and its rise time halved. The long delay and the sharp pulse rise time are consistent with a naturally occurring PFS, which opened only after a plasma was ejected from the channel into the load volume. The results suggest that microsecond-conduction-time PFSs might be particularly well-suited to be combined with flux compression in amplifying currents and sharpening pulse widths in existing and next-generation pulsed power drivers. If so, the results could provide a significant step towards the long-term goal of applying flux compression to achieving substan-tial savings in the capital cost of next-generation facili-ties for radiation simulation and for energy production by Z-pinch-driven ICF (inertial confinement fusion).


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