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Differential Beam Current and Errant Beam at SNS W. Blokland, C. Peters,N. LuttrellESS, Lund, Sweden, November 2015ORNL is managed by UT-Battelle for the US Department of Energy#Errant Beam at SNSIntroduction

CCLSCL, b=0.61SCL, b=0.81DTLRFQSourceMEBTSNS AcceleratorPower on Target1.4 MW at 1.0 GeV (1.2 MW now)Pulse on Target 1.5 E14 protons (24C)Macro-pulse in Linac~1000 mini pulses of ~24mA avg over 1ms at 60HzErrant Beam at SNS: any beam outside the normal operation envelope:Abrupt beam lossesBeam current drops or missing mini pulsesToo high peak density beam on target#Errant Beam at SNSExisting Loss Monitors abort in 16-30 s but, unexpectedly, also short losses in SCL lead to accumulating damageSuspected mechanism: beam hitting cavity surface releases gas or particulates followed by ionization or redistribution, creating an environment for arcing. The arcing can damage the cavity surface Errant Beam Committee to investigateData collected from loss monitors, RF waveforms and the modified (60Hz and archiving of data) LabVIEW-based Beam Current MonitorsDifferential Current Monitor: Why is even short errant beam ( 90% of BLM trips were due to Warm Linac RF faults- RF faults occur at different times during the pulse#Errant Beam at SNSThe majority of trips originate in the Warm LinacRF Adjustments: gradient changes, resonant frequency changes, and preventative maintenance on vacuum systems fault frequency reduced by more than a factor of two. SCL downtime was reduced by a factor of six

FY12-1FY12-2FY13-1BUT still need to reduce impact of remaining errant beam (losses or no losses) either upgrade all BLMs and MPS system or implement single differential current monitor to abort as fast as possible, down to about 6s with special hookup to MPS.Errant beam loss from 30 to 15 events a day#Errant Beam at SNS

CCLSRF, b=0.61SRF, b=0.81RFQProtecting the Super Conducting LinacThe choice was to implement a differential BCM with the last toroid before, and the first toroid after the SCLWe use the PXI Platform with LabVIEW Real-time and FPGASourceMEBTHEBT01CCL102AmplifierDTLAmplifierAttenuatorsDigitizerSNS Linac

Beginning of CCL102 current waveform (long pulse)AbortHEBT BCM01 current waveform (short pulse)ElectronicsBuildingsLayout of Differential Current Monitor#Errant Beam at SNSProcessing: FlexRIO FPGASampling at up to 100MhzPipeline delay 15 clocks cycles: 150nsOffsetDelayScaleOffsetDelayScaleDroopDroopDown-streamUp-streamPreprocessDifferenceSumCompareUpstream previousDownstream previous si>?Y>?Y>?Y>?YAlarm.. s4 s5 s6 s7.. s4 s5 s6 s7.. s4 s5 s6 s7Dif + -Dif + -Dif + - si si>?Y.. s4 s5 s6 s7Processing must account for difference in cable lengths, calibration, baseline drifts, and droop.#Errant Beam at SNSFlexRIO FPGA processing

Preprocessing codeTiming Decoding codeFeatures:Send three waveformsAny of the preprocessing stagesDifferential signalsSliding windows outputsAlarm status on each sampleWhich threshold exceededSNS Timing LibraryReceive timing link and data link data [4]. We dont need a separate (and custom) timing decoder!#Errant Beam at SNSReal-Time ProcessingReal-time code @60Hz:Archiving of errant beam waveforms on FTP serverCalculate statisticsInterface with control system using native LabVIEW EPICS [5]automatic creation of PVs and console screens

All time-critical s-level signal processing, including abort, is done in FPGAConsole screen for DCM#Errant Beam at SNSExperiment: Abort TestTemporary change to the Machine Protect System (A. Justice)DCM aborts in middle of the beam pulse: ~8.5s abort time = 2 to 3x improvementThrough normal MPS: ~14 s

Reaction TimeDCM pulls abortBeam goneCurrent and previous beam pulseCalculated best:1s for DCM processing2.5s signal cable delays1 s abort cable delay1 s chopper abort1.5s beam propagation7s. Shorter cables and better sensor locations can get us as low as 6 s#Errant Beam at SNSMPS delay testDCM downstream signal disconnected: 13.8 s total for beam shutoff3.2 s for DCM abort (small pulses)10.6 s for MPS trip after receiving abort + time of flight

(Alan Justice)~3 s in rack#Errant Beam at SNSResults: Difference between locationsExample of the errant beam we want to abortAnalyzed to be due to a Warm Linac RF drop

Alert on small window differenceAlert on pulse-to-pulse downstreamAlert on pulse-to-pulse upstreamAlert to MPS

Waveforms:UpstreamDownstreamDifference after short sliding window Alert was given in