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The Front-End Systems (FES) of the Spallation Neutron Source (SNS) project have been described in detail elsewhere [1]. They comprise an rf-driven H -ion source, electrostatic LEBT, four-vane RFQ, and an elaborate MEBT. These systems are planned to be delivered to the SNS facility in Oak Ridge in June 2002. This paper discusses the latest design features, the status of development work, component fabrication and procurements, and experimental results with the first commissioned beamline elements.

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

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