Reply To: Fusion Oil

[asymmetric_implosion]

Z pinches are one of the most common implosion type radiation devices. The Z-machine at Sandia National lab is the largest such device in the world operating at nearly 26 MA with a rise time of less than 100 ns. A large PF device is 3 MA with a rise time time of up to 10 us. Z-pinches are largely used for nuclear weapons effects simulations and fundamentals studies of dense matter (stellar stuff) as they produce copious amounts of x-ray radiation. The preferred mode of operation is to use wires of high Z metals strung in two layered cylindrical arrays of many fine wires. A typical ring of wires can be more than 200 individual wires. Gas puff technology is viable for such devices and on occasion layered shells of gases are used as the radiating material. Published works suggest the Z-pinch devices are under performing compared to PF devices with a pure deuterium fill. A number of hypothesis exist as to the cause.

The plasma focus was an accidental discovery that happened to mimic the Z-pinch. The advantage of the PF is the less expensive pulse power system as it operates with a slower rise time thus less voltage to drive the plasma. The two primary stages of operation, the axial flow phase and the radial implosion phase, seem to produce a more stable pinch in a plasma focus. The Z-pinch is a pure radial implosion. The PF never took off for a few reasons: some political as the chief advocate for PF devices in the US had personal problems that made him appear unreliable, the PF devices tended to shatter insulators at increasing current which remains an issue in the classic PF design, and historically poor performance compared to Z-pinch devices.

The political issue was unfortunate but people are involved in science and some times our issues get into the work. The shattering insulators was a perception issue as much as a technical issue. No one in the Z-pinch world thinks twice about polishing key components and rebuilding switches are every shot. A drop in piece of ceramic doesn’t seem so bad by comparison. There are also effective alternatives to a solid, plasma facing insulator. More needs to be done but the initial results done in the 1990s were promising. The historically poor performance was a result of poor understanding of the PF process. Most people looked for thermal fusion which the PF does not do well. The non-thermal fusion, fusion driven by fast ions generated in the pinch, was frequently put down by those in the fusion community as unreliable and unpredictable. I think recent work has caught the attention of more open minded folks and showed them that reproducible and predictable are at hand. There is also a bit of a sense of that fusion must be hard. If our best minds have not cracked it yet, it must be very hard. I don’t know that I agree but I think there are options of fusion that are yet to be explored using the best knowledge of modern plasma physics. Is there an approach that is vastly to superior to all other approaches? I don’t know but it seems less and less likely that NIF or ITER are going to reach the goal of fusion energy and be economically viable in the current financial climate.