Lawrenceville Plasma Physics Focus Fusion Switch Success and other design upgrades ready

Left: LPP Advisor Bob Fitzgerald smiles his approval for the upgraded FoFu-1 during a visit to the lab, 8-6-2011. Right:The full complement of twelve new spark plugs.

Lawrenceville Plasma Physics nuclear fusion project has switches as a critical part of the system design. Switches have to fire repeatedly and precisely and often if the desired power is to be achieved.

A complete set of 12 re-designed switches passed all tests with flying colors, ready for higher voltages. The complete switch system fires simultaneously 18 times in a row with no misfires or prefires; New switches pass all tests at 35 kV and 40 kV.

In a major step forward for the Focus Fusion-1 experiment, LPP researchers succeeded in firing the entire bank of 12 capacitors simultaneously through our new “Mark 12” spark-gap switches. On August 7-8, the team fired the bank successfully 18 times in a row. During this series, there were no missed fires by any switch and there were no prefires–A prefire is when a switch spontaneously fires before it is triggered by the device operator. All switches fired within 40 ns of each other, and most switches fired within 10 ns of each other. While too few shots have been fired to determine the lifetime of the new switches, wear so far has been minimal.

Why is this latest switch success so important? The capacitors send energy through these switches into FF-1’s electrodes to create the filaments of current that then converge and compress themselves into the dense plasmoid. This tiny plasmoid is where the majority of fusion reactions occur. It is important that the switches all fire simultaneously so that the sheath of current filaments is symmetrical. Any asymmetry can cause some filaments to get ahead of others, preventing full compression of the plasmoid. The more the plasmoid is compressed, the faster the fusion fuel burns and the more energy is produced, so symmetry is critical. Having all the switches fire together within tens of nanoseconds helps achieve this symmetry.

The poor performance of the FF-1’s original switches, purchased from R. E. Beverly, had substantially slowed the Focus Fusion research project. The new switches, designed in January by LPP, use only the old insulators and otherwise have all new parts. While fabrication of such unique components took time, the improved performance has been worth the wait.

The successful switch firings were with the capacitors charged to 35 kV. The LPP team feels very confident that the switches will work well up to the full capacitor bank potential of 45 kV. For one thing, the operating pressure of the gas in the spark gaps was far less than the switches’ limit. So even at 45 kV, the gas pressure, which rises with voltage, will be much below that limit. Second, the switches passed a static test to see if they would spontaneously breakdown (pre-fire) at 40 kV. In late August, LPP expects to do further tests of the switches all the way up to 45 kV.

The fully assembled central electrodes with new anode (central cylinder, left) and new cathode base with knife edge for symmetrical plasma sheath formation (close-up at right).

With the switches and insulation tested, the team is now optimizing and testing the third key element of the upgrade—the cathode knife-edge. To start current flowing through the gas in the vacuum chamber, the atoms in the gas must be ionized—their electrons stripped off and made free to move. This creates the plasma, which is electrically conducting gas. The knife edge, located on the inner ring of the cathode plate, initiates the ionization.

FF-1’s first cathode had a row of tungsten pins instead of a knife edge, which proved inadequate, because the pins got loose and became uneven in height. This caused asymmetric firing (see June report). An alternative solid copper knife edge, already manufactured, was available for testing this summer. However, experiments conducted by LPP in collaboration with Texas A & M University in 2001 and discussions this month with LPP contractor John Guillory both indicated that a solid knife edge could produce inadequate ionization and trap neutral, un-ionized, gas behind the sheath of current filaments. This in turn would allow current to wander away from the sheaths, hurting their formation. So, imitating the modification successfully made in the Texas experiments, the new knife-edge has slots cut into it. These will allow the escape of gas from the region inside the knife edge, and the sharp corners of the remaining knife edge segments will increase ionization. The new modification will also be tested in August.

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