FF-1 upgrade reassembly complete

The upgrade, which involved reinforcing insulation on the device’s transmission plates and around its switches, prepared it for firing up to 45 kV. With the advice and design help of LPP contractor John Thompson in San Diego, the team came up with and implemented a half dozen ways to make the Mylar plastic sheets that insulate the current more effective. Re-assembly was completed only nine days after we received the last parts from machinists.

Amgad (right) and fellow KSU grad student Mohamed Ismail stack layers of green boratron above FoFu’s freshly reassembled and reinforced main insulator, made up of carefully cut sheets of Mylar (white), which are sandwiched between new aluminum plates (grey).

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.

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).