Differential Beam Current and Errant Beam at SNS
Differential Beam Current and Errant Beam at SNS W. Blokland, C. Peters, N. Luttrell ESS, Lund, Sweden, November 2015 ORNL is managed by UT-Battelle for the US Department of Energy
Introduction SNS Accelerator Power on Target 1. 4 MW at 1. 0 Ge. V (1. 2 MW now) Pulse on Target 1. 5 E 14 protons (24µC) Macro-pulse in Linac ~1000 mini pulses of ~24 m. A avg over 1 ms at 60 Hz DTL RFQ Source CCL SCL, b=0. 61 SCL, b=0. 81 MEBT Errant Beam at SNS: any beam outside the normal operation envelope: • Abrupt beam losses • Beam current drops or missing mini pulses • Too high peak density beam on target 2 Errant Beam at SNS
Differential Current Monitor: Why is even short errant beam (<15 µs) important? • Existing Loss Monitors abort in 1630 µs but, unexpectedly, also short losses in SCL lead to accumulating damage – Suspected 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 investigate Ion, molecule (radical), electron contaminants base material Credit: S. Kim Damage mechanism • Data collected from loss monitors, RF waveforms and the modified (60 Hz and archiving of data) Lab. VIEW-based Beam Current Monitors 3 Errant Beam at SNS
Analyzing the errant beam data Abnormal RF pulse with beam Normal RF pulse with beam Field LLRF output 558 µs in CCL 546 µs in HEBT Time (usec) Normal RF pulse with no beam 4 Errant Beam at SNS 12 µs lost in the SCL
Errant Beam Analysis Directories of data with auto select of nearest data in other directories BLM scalars from CCL and SCL BLM waveforms selected by clicking on scalars Current waveforms < 10% of BLM trips were due to the Ion Source/LEBT - BLM trips occur in the first week of a new source installation > 90% of BLM trips were due to Warm Linac RF faults - RF faults occur at different times during the pulse 5 Errant Beam at SNS
The majority of trips originate in the Warm Linac RF 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 FY 12 -1 FY 12 -2 FY 13 -1 Errant beam loss from 30 to 15 events a day BUT 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 6 µs with special hookup to MPS. 6 Errant Beam at SNS
Protecting the Super Conducting Linac • The choice was to implement a differential BCM with the last toroid before, and the first toroid after the SCL • We use the PXI Platform with Lab. VIEW Real-time and FPGA SNS Linac CCL 102 RFQ Source HEBT 01 DTL CCL SRF, b=0. 61 HEBT BCM 01 current waveform (short pulse) SRF, b=0. 81 MEBT Attenuators Electronics Buildings Amplifier Abort Amplifier Digitizer Layout of Differential Current Monitor 7 Errant Beam at SNS Beginning of CCL 102 current waveform (long pulse)
Processing: Flex. RIO FPGA Processing must account for difference in cable lengths, calibration, baseline drifts, and droop. Preprocess Difference Upstream’ previous + Dif - Sum Σ. . s 4 s 5 s 6 s 7 Compare >? Y Σ si Upstream Delay Scale Offset Droop + Dif Downstream Delay Scale Offset Droop Downstream’ previous - + Dif Sampling at up to 100 Mhz Pipeline delay 15 clocks cycles: 150 ns 8 Errant Beam at SNS - Σ. . s 4 s 5 s 6 s 7 >? Y Σ si Σ. . s 4 s 5 s 6 s 7 Alarm >? Y
Flex. RIO FPGA processing Features: • Send three waveforms - Any of the preprocessing stages - Differential signals - Sliding windows outputs Preprocessing code • Alarm status on each sample – Which threshold exceeded • SNS Timing Library – Receive timing link and data link data [4]. We don’t need a separate (and custom) timing decoder! 9 Errant Beam at SNS Timing Decoding code
Real-Time Processing • All time-critical µs-level signal processing, including abort, is done in FPGA • Real-time code @60 Hz: – Archiving of errant beam waveforms on FTP server – Calculate statistics – Interface with control system using native Lab. VIEW EPICS [5] • automatic creation of PVs and console screens Console screen for DCM 10 Errant Beam at SNS
Experiment: Abort Test • Temporary change to the Machine Protect System (A. Justice) • DCM aborts in middle of the beam pulse: ~8. 5µs abort time = 2 to 3 x improvement • Through normal MPS: ~14 µs Reaction Time Calculated best: 1 µs for DCM processing 2. 5 µs signal cable delays 1 µs abort cable delay 1 µs chopper abort 1. 5 µs beam propagation 7 µs. Shorter cables and better sensor locations can get us as low as 6 µs DCM pulls abort 11 Errant Beam at SNS Beam gone Current and previous beam pulse
MPS delay test (Alan Justice) ~3 µs in rack 12 Errant Beam at SNS DCM downstream signal disconnected: 13. 8 µs total for beam shutoff - 3. 2 µs for DCM abort (small pulses) - 10. 6 µs for MPS trip after receiving abort + time of flight
Results: Difference between locations • Example of the errant beam we want to abort • Analyzed to be due to a Warm Linac RF drop Waveforms: 1. Upstream 2. Downstream 3. Difference after short sliding window Alert was given in <1. 5 µs (could be faster with lower thresholds and still avoid false alerts) Alert to MPS Alert on pulse-to-pulse downstream Alert on pulse-to-pulse upstream DCM waveforms 13 Errant Beam at SNS Alert on small window difference
Alarm Data • Passed along with each sample 14 Errant Beam at SNS Bit Alarm 1 Any Alarm 2 Total Sum of Difference 3 Large Sliding Window Ch 0 –Ch 1 4 Small Sliding Window Ch 0 -Ch 1 5 Sliding Window Ch 0 -Ch 0’ 6 Sliding Window Ch 1 -Ch 1’ 7 MPS Latched 8 MPS Auto Reset
Results: Difference in time • Example of slow drop in current from pulse-topulse • Analyzed to be due to RF Fill problem • Pre-mortem Analysis! Waveforms for errant AND previous beam pulse AND next beam pulse: 1. Upstream 2. Downstream 3. Difference Alert to MPS DBCM waveforms 15 Errant Beam at SNS Alert on pulse-topulse downstream
Results: Temporary loss then followed by abort. (Alarm: 56) 16 Errant Beam at SNS
Results: Temporary loss & bunch shift then followed by abort (alarm 56) 17 Errant Beam at SNS
Results: Loss on next pulse Lots of issues: different beam than previous and next pulse has losses Prev & now Now & next Should we abort on no-loss difference to prevent future loss? Prevent RF learning? 18 Errant Beam at SNS Next has beamloss
Alarm histogram • Mostly alarm: 48 (different pulse lengths) • 25 cases of beam loss between CCL and HEBT in 3 days but including AP time 19 Errant Beam at SNS Alarm Value 60 Alarm Lines Typical cause all 4 alarms large loss and abort 56 all but large dif small loss and abort 48 self Ch 0&Ch 1 missing pulse before CCL 32 self Ch 1 missing pulse before CCL 28 all but self Ch 1 loss and abort 16 self Ch 0 missing pulse before CCL
Classifying Errant Beam Loss Downstream Front Partial Recovery Large • • • Middle Full No recovery Small Tree structure with 5 levels. Location Along Beam Loss Upstream | Loss in SCL Portion of Macropulse 1 st | 2 nd | 3 rd | All Nature of Loss (Mini-pulse) Partial Loss | Total Loss Beam Recovery Present | No Recovery* Nature of Recovery 20 Errant Beam at SNS Large* | Small* End
Ion Source Errant Beam Nominal beam Associated with ICS_Opr: Ion. Source: Edmp: Flt. Ctr (Very representative of this fault) Associated with ICS_Opr: Ion. Source: Foc 2: Flt. Ctr 21 Errant Beam at SNS
SCL Errant Beam Associated with SCL BLMs 22 Errant Beam at SNS Close up of loss point
DTL Power Supply Errant Beam Associated with DTL_PS: MIOC 1 A: faultcount 0 23 Errant Beam at SNS
Histogram (work in progress) 24 Errant Beam at SNS
Upstream current monitor is lost • Current transformer needs to see magnetic fields to measure current ceramic break in beam pipe ceramic • Hole burnt into ceramic causing vacuum leak! (RF electrons? ) BCM replaced with spool piece. Replacement with internal shield is long term (aka years). • No nearby current monitors that are suitable • End of DCM? 25 Errant Beam at SNS pipe hole
Current measurements using BPMs • Beam Position Monitor: FPGA code to correct log signal 26 Errant Beam at SNS + ü Use sum from all four plates ü Use demo log-amp board with band-pass filter in front ü Add correction in FPGA with exponential function ü Use existing BCM as reference for calibration We got our DCM back and extra locations! Sum signal Log signal RF Bandpass Logamp Final signals. Calibration done using downstream BCM
Plans • Add new MPS board to PXI crate with optical fiber output • Dedicated link to MPS with mask-able input 8 -9 µs total • Simplified DCM (no fancy archiving) to provide high uptime – Add decoding of intended pulse length • Linear RF envelope detectors (to avoid calibration procedure). Log-amps are very sensitive even the replacing of a cable requires recalibration • Install new toroid and/or reroute BCM cables • Second DCM for archiving – Keep the aborting DCM as simple as possible to increase robustness • Upgrade MEBT DCM to same hardware • The differential over time is also very powerful in showing anomalies – Possibly abort before losses or tell Low-Level RF to not learn on bad pulse • Soft-IOC to combine DCM data with other data to determine cause of aborts 27 Errant Beam at SNS
References REFERENCES [1] S. Kim et al. , “The Status of the Superconducting Linac and SRF Activities at the SNS, ” 16 th International Conference on RF Superconductivity, Sep 23 -27, 2013, Paris. [2] W. Blokland, “Errant Beam: Tools and Data, ” SNS Errant Beam Committee, May 2012, Oak Ridge, TN. [3] C. S. Peters, “Errant Beam Update, ” Accelerator Advisory Committee, May 7, 2013, Oak Ridge, TN. [4] R. Dickson, “Lab. VIEW FPGA SNS Timing User Guide, ” SNS, May 7 2013, Oak Ridge, TN. [5] S. Zhukov, “Pure Lab. VIEW Implementation of EPICS Communication Protocol, ” Big Physics and Science Summit at NIWeek, August 7 -8, 2012. Austin, TX. [6] W. Blokland et al. , "A new Differential And Errant Beam Current Monitor for the SNS Accelerator", IBIC 2013. [7] A. Justice, “DBCM Measurements 9/15/2015 Results”, SNS Internal Memo, September 2015 28 Errant Beam at SNS
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