The SNS four-phase LEBT chopper

Abstract

The Spallation Neutron Source front end incorporates a beam chopper in the LEBT that will remove a 295 ns section of beam at a 1.118 MHz rate (65% transmission) with less than 50 ns rise/falltime. The H , beam pulse length is one ms at a 60-Hz rate (6% duty factor). The LEBT is all-electrostatic, and the chopper incorporates four 3-kV solid-state switches driving an einzel lens, split into quadrants, with a 4-phase chopping waveform. The suppressed beam is targeted on a four-segment Faraday cup which provides on-line intensity and steering diagnostics. Results of proton beam tests will be reported. BEAM REQUIREMENTSThe Spallation Neutron Source (SNS) comprises a 1-GeV H , linac injecting a storage ring with a 1 ms injection time and single-turn extraction, operating at 60 Hz [1]. During the 1 ms injection into the ring, approximately 1200 turns are accumulated. To reduce the activation of the extraction Lambertson septum magnet, a 295 ns notch is introduced in the injected beam by two sets of choppers, operating at the ring revolution frequency of 1.188 MHz. waveform on the four segments deflects the beam sequentially in four directions, shown in Figure 1, creating the 295 ns gaps in the 1 ms beam pulse. The four electrode segments are each independently pulsed to ं 3 kV by solidstate switches, deflecting the beam sequentially along each of the 45 degree diagonals. The solid-state switches have active pull-up and pull-down, providing a less-than 50 ns chopping transitions. Figure 2 shows an early version of the split einzel electrode. IMPLEMENTATION Figure 2: Split Chopper ElectrodeThe beam is targeted onto a diagnostic plate, split along the diagonals similar to the chopping electrode itself, in the wall common to the end of the LEBT and the start of the RFQ. The four segments of the target electrode are electrically isolated and water cooled, serving as a beam current diagnostic and a beam steering diagnostic. For a 65 keV, 35 mA H , beam with a 65% duty factor and with 35% of the current removed by the 1.188 MHz chopper, the total average power dissipated on the target is 48 watts.Angular beam steering is provided by a symmetric d-c bias on each segment of up to ं1 kV. Beam position steering is provided by physically moving the entire ion source and LEBT relative to the RFQ and the diagnostic plate on a sliding vacuum seal. ELECTRONICSThe key electronic component of the LEBT chopper system is the solid-state ं3 kV bipolar switch. This switch is

Experimental performance of an E×B chopper system

“…A standard chopper solution in many facilities [16][17][18][19] is the use of transverse electric kickers to deflect the beam from the reference trajectory. This requires that an electrostatic field is built up during the beam-interception time or that an oscillating field is produced by an rf cavity or a resonant circuit.…”

Rise/fall time enhancement of the spallation neutron source linac LEBT chopper system

“…The SNS LEBT chopper design was described in [4]. The chopper incorporates four +/-2.5 kV pulsed power supplies (pulsers) which drive four quadrants of an electrostatic focusing lens floating at -50 kV.…”

Ion-source and low-energy beam-transport issues with the front-end systems for the spallation neutron source

“…6 shows a very compact electrostatic LEBT [17] designed for the Spallation Neutron Source presently under construction. Its main features include extraction at 81 kV for a final beam energy of 65 keV and two lenses separated by a ground electrode, with the second one being split into four isolated quadrants to facilitate dc steering as well as beam chopping with about 50-ns rise time [18]. The last of the electrodes acts as target for the chopped beam portions and is split into four quadrants as well, allowing to measure transverse beam offset on-line.…”