Michigan Electron Long Beam Accelerator (MELBA)

The Michigan Electron Long Beam Accelerator is housed in an experimental bay adjacent to the Naval Architecture and Marine Engineering Building on the North Campus of the University of Michigan in Ann Arbor.

MELBA’s current source is a Marx bank. Milliamps of current charge 16 capacitors in parallel over several minutes. Once charging is complete, 14 of these capacitors release their charge in series. The other two capacitors are a voltage compensation stage that flattens MELBA’s voltage pulse allowing it to maintain a relatively uniform anode-cathode voltage for up to a microsecond. MELBA is able to produce megavolt electron pulses with currents reaching 30 kA and pulselengths as long as two microseconds.

Ceramic stack

Due to the high voltages involved in the operation of MELBA, an insulating stack must be employed to hold off the high voltage pulse from ground as we transfer it out of the Marx bank and into the experimental interaction region. The insulating stack can be seen above on the right. Each ring of the stack can hold off 100 kV. High voltages also make it necessary to keep the insulating stack and capacitor bank submerged in transformer oil.

Inside the insulating stack, the cathode stalk brings the high voltage pulse to the interaction region. This area is under high vacuum during experimental runs. The high vacuum reduces the plasma formed inside and helps to prevent arcs. In this picture, a cathode extension piece is attached to the main cathode stalk to bring the pulse into the center of the relativistic magnetron.