Michael A. Lutz

The Glow to Arc Transition - A Critical Review

Only recently has a well-substantiated model of the low-pressure glow to arc transition become available as a result of a series of highly controlled experiments performed during the period 1958 to 1966. This model cites insulating particles at the cathode surface as responsible, due to their charging and subsequent breakdown which produce an initial burst of vapor. This paper consists of a review and critical evaluation of these experiments and an elucidation of the model.

The Gamitron - A High Power Crossed-Field Switch Tube for HVDC Interruption

A high power crossed-field discharge device has been developed for use as a high voltage direct current interrupter. This device operates at low pressure (0.05 Torr), conducting current at a fixed voltage (~500 V) only in the presence of a weak magnetic field (~100 G) which is substantially perpendicular to the electric field between the electrodes. When the magnetic field is removed, ionization ceases and current interruption results. Physical phenomena occurring in this device have been investigated, including the glow-to-arc transition, gas cleanup, and high voltage breakdown. Based on the results of these investigations, switch tubes have been developed and successfully tested at the 2 kA, 100 kV level, with recovery rates in excess of 2 kV/?s. The availability of such devices will make possible many applications including HVDC circuit breakers, ac current limiters, and practical inductive energy storage.

Gridded Cross Field Tube

A triode crossed field tube has been operated as a high voltage on-off switch tube. A third, partially transparent (grid) electrode is interposed between the anode and cathode and electrically tied to the cathode by a grid leak resistor. High voltage is first applied to the anode and cathode; the magnetic field is then raised to the conduction level (~ 0.01 Tesla). Ignition does not occur because the magnetic field is too low in the grid-anode gap and the electric field is zero in the cathode-grid gap. Pulsing the grid positive relative to the cathode (~ 1 kV) then results in breakdown of the cathode-grid gap; plasma comunication between the two gaps then fully ignites the tube and closes the main power circuit. Grid ignition has been achieved at 50 kV as well as grid ignition followed by current interruption against 10 kV. These levels were limited by the use of a modified, laboratory tube and not by the physics of the technique.

The Crossed Field Switch Tube-A New HVDC Circuit Interrupter

A crossed field (Penning) discharge device has been developed which can perform HVDC interruption. The discharge is controlled by a magnetic field, the removal of which causes the plasma to decay in a time ~10?s. The present single tube capability is 100 kV/2000 A with a recovery rate of 2 kV/?s. The tubes are capable of series and parallel operation, opening the door to many HVDC applications requiring forced current interruption at performance levels far in excess of those of a single tube.

Feasibility of a High Average Power Crossed Field Closing Switch

Results of an experimentalprogram to determine the feasibility of using a crossed field device as a high average power triggered closing switch are reported. The tube contained coaxial, cylindrical electrodes and was triggered by pulsing a magnetic field (<0.1 T) to a value sufficiently high to trap electrons and initiate conduction. Holdoff voltages up to 60 kV were achieved, with peak (circuit limited) currents as high as 20 kA and pulse durations in the range of 1 to 100 ¿s. Ignition jitter was ~0.1 ¿s, and the voltage recovery rate was 2 kV/¿s after 20 kA conduction and a 50 ¿s deionization time. These single and double shot data indicate that it should be possible to build such a device to operate at high average power levels for use in high power (MW) modulators.

High Power On-Off Switching with Crossed Field Tubes

A crossed field tube is used as an on-off switch to produce square, 25 ¿s wide current pulses at 40 kV and 3.2 kA. Operation is also demonstrated at 120 Hz at an initial 50 kV and 1.3 kA for a ten pulse train. Calculations indicate that continuous operation at over 150 kW of average power is possible with a suitably designed cooling system. A crossed field tube operated in this way has a higher current capability and lower dissipation than hard vacuum tetrodes and could replace such tubes in higher power, lower impedance modulators.

Current Interruption at Powers up to 1 GW with Crossed-Field Tubes

This paper describes the current interruption and voltage holdoff capabilities of a wide range of crossed-field tubes (XFTs). The XFTs investigated have active electrode areas ranging from 96 to 8000 cm2 with mean diameters from 2 to 56 cm. For the larger XFTs, currents up to 10 kA have been interrupted at voltages up to 100 kV, corresponding to a switched power of 1 GW. Experiments have shown the peak interruptible current to vary linearly with tube radius. The mechanism responsible for the limitation may be related to the self-generated magnetic field, but other factors are also shown to play a role.