Fred's World of Science: Mosfet Research


In 1998-9 I was working full time at Fermilab while finishing my undergrad degree at the U of C. The research I did that year was mostly concerning the development of a new type of modulator for an experiment. The experiment is called the Electron Compressor, and is essentially electron cooling and space charge cancellation with an electron current density lens in the Tevatron accelerator at Fermilab. The current of an electron beam had to be modulated with a wideband signal from 100kHz to 15MHz at roughly 10kV. Due to the geometry of the electron gun, the impedance of the modulation electrode was so low, we needed about 20kW continuous power. This led to a number of technical difficulties.

I designed a new system of amplification using high power mosfets in class A topology, capable of high voltage, high current, large bandwidth, and high efficiency. I submitted the research to the Particle Accelerator Conference in 1999. My poster was accepted, and I presented it that year in NY. Here is the abstract from the paper, and a picture of the 4-mosfet driver board.

MOP143 Electron Gun Beam Extraction with MOSFETS
F. Niell, Fermilab, Batavia, IL

Electron beams with an energy range of 10keV at 2A requiring intensity modulation with bandwidth from 120kHz to 15MHz pose an interesting problem to the RF designer. Driving the extraction electrode of an electron gun can be a challenge with these requirements. The straight forward approach is the construction of a vacuum tube, single ended amplifier capable of delivering the anode voltage to the extraction electrode of the gun. However, vacuum tubes of this size and power are becoming increasingly expensive, and as the technology dies, the reliability of new components comes into question. The logical alternative is the implementation of solid state electronics. However, the 15MHz 10kV requirements pose a problem, as the solid state high voltage, high current technology does not exist in a single package. A new system of driving the extraction electrode of a Pierce type gun was developed using MOSFETs. Arranged in ultra-reliable "bricks" of series-parallel arrays, the MOSFETs allow for highly efficient amplification-mode operation. Amplitude linearity to 0.1% is achieved. The proposed system, capable of producing 10kV at 10kW will be implemented in the Elecron Compressor experiment at Fermilab.

Note the large white ceramic mosfets from DEI. Pretty cool, huh?

Copyright July 1998

U Mich