A novel low energy fast chopper for H-injectors

Abstract

The Fermilab H-injector will be upgraded from a Cockcroft-Walton system to a 750 keV RFQ (radio frequency quadrupole) system. Part of the upgrade is the addition of a fast chopper in the LEBT (low energy beam transport) just before the RFQ. The novelty of this chopper is that it reflects rather than deflects beam. It also has low capacitance < 10 pF so that the rise and fall times of the beam pulse are dominated by cabling and the MOSFET switches. In fact, the measured rise and fall time of the chopped beam have been measured and they are better than 150 ns at 15 Hz.

Experimental performance of an E×B chopper system

“…Mechanical choppers, like disk choppers [8,9] or Fermi choppers [10,11], which are widely used for noncharged particles, have typical operation frequencies in the subkilohertz range, which are too low for the FRANZ case. An alternative concept is to charge the central electrode of an Einzel lens to higher voltage than the accelerating voltage of the beam, thus reflecting the beam [12,13]. However, for the relatively high initial beam energy of 120 keV and the high repetition rate required for FRANZ, this concept does not seem feasible.…”

Commissioning and Operation of the FNAL Front end Injection Line and Ion Sources.

“…The Einzel Lens was presented as a possible solution that could be mounted near to the RFQ and be able to stop the 35 keV beam [3]. In this position, the de-neutralization effects on the Hbeam would be minimal because the lens is very short.…”

“…The cube that the source sits in has a volume of 1000 in 3 . This volume is reduced by the source can sitting re-entrant in the cube, leaving a volume of roughly 800 in 3 A new test stand is under construction, and once completed will allow for these tests to be redone with similar settings to the operational sources and beam line. It will be a mockup of the operational cube and LEBT with the ability to attach emittance probes, multiwire cans, and other diagnostics.…”

“…The source cube opens up to the LEBT through a 1 in diameter hole. The LEBT beam pipe through to the RFQ has a volume of 584 in3 as it is 46.55 in long by 4 in diameter. The LEBT has two 350 L/s turbo pumps that provide the vacuum.The test stand where these sources were run for these tests contains a much larger vacuum volume of 18 ft 3 (31,104 in 3 ) shown inFigure 44.…”