SNS Beam Loss Monitor Electronics Final Design Review

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SNS Beam Loss Monitor ElectronicsFinal Design
Review

• D. Gassner, R. Witkover
• Brookhaven National Laboratory

2
SNS BLM Electronics Outline

• Requirements
• Dose Rate Estimates
• Gain and Threshold Control
• Dynamic Range
• System Block Diagram
• Cable interface
• Analog Circuit Schematic
• Front end Stage
• Fast Loss Integrator (MPS Trip) Stage
• Viewing Stage
• 1 W/m stage
• Thermal Drift
• VME Digitizers
• Data Acquisition
• HV Bias
• Summary

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BLM System Electronics Requirements

• Provide measurements of loss for tuning
• Protect the machine from high activation due to
  slow, low-level losses
• (1W/m criteria)
• Protect the machine from radiation damage due to
  fast high-level losses
• MPS Input (Beam Permit/Inhibit Link)
• Not for personnel protection.

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Dose Rate Estimates

• Slow Loss 1 W/m criteria
• Corresponds to 10- 4 loss distributed around
  248 m Ring.
• Beam-off activation approximately 100 mrad/hr at
  1 ft.
• Rule of thumb Multiply by 1000 to get beam-on
  dose rate (100R/hr).
• Same as 0.5 R/sec during the 1 ms SNS pulse.
• Need to resolve 2 decades below, or 1 of 1 W/m.
• What is maximum high-end loss?
• 1 local loss (of 2 MW) 20 kW requested.
• Pre-integration to extend dynamic range.
• Total range is equivalent to 21-bits sign
• Low-end resolution limited by noise and BW, upper
  end by detector and/or electronics saturation.

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Required Gain and Threshold Control

• Programmable Gain (local remote)
• Beam intensities
• Linac and HEBT range 15-38 mA
• Ring increases 103 during macro-pulse
• RTBT can vary 103 between single turn, full
  intensity
• Detector shielding by beam line components
• Beam energy dependence
• Programmable loss limit thresholds
• Individual channels
• Separate thresholds for Fast and 1 W/m losses
• Trip outputs can be masked through MPS for studies

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Electronics Dynamic Range

• Fast Response for Beam Abort
• For 10us rise time, BW would be 35 kHz. (550 pA
  noise scaled from RHIC)
• This limits resolution to 30 Rad/hr.
• Intention is to observe large fast losses 106
  Rad/hr
• Signal Upper Limit
• 1 of 2MW, in Linac and HEBT, uniformly over
  pulse 644uA
• Lost fast in one place (RTBT) 644 nC (very
  large)
• Lower Limit
• 1 of 1W/m 324 pA
• Total dynamic range of 126dB (324pA 644uA)
• Need separate signal paths for Fast and Slow
  losses.

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System Block Diagram
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BLM-Cable Interface

• In order to balance rise time due to signal cable
  length variations, a BLM-Cable interface panel
  will be included which will hold capacitors at
  the transition from the coax to mass termination
  connectors. Example
• Belden 9054 is 16 pf/foot, runs will vary from 75
  to 300 feet, yielding 1200 4800 pF.
• 470 Ohm AFE input R, rise times will vary from
  1.2 to 5us (assume 2.2 X RC rise time)
• We can extend the dynamic range for RTBT fast
  loss signals by pre-integrating the signal by
  choosing input caps for a 50-100 usec time
  constant. This would allow us to get to several
  times 0.1 single point loss without saturating
  the 1 W/m Viewing stage electronics.
• NOTE Belden 9054 is a low Tribo-electric cable
  which significantly reduces noise due to
  friction. It is essential when measuring signals
  in the nA or lower region.

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Analog Front End
80kHz
80kHz
1.5kHz
1.5kHz
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Front End Amplifier Stage

• Trans-impedance amplifier
• Burr-Brown OPA627BP
• low offset voltage, Vos 100 µV max (un-trimmed)
• Drift 0.8uV/0C, max
• Bias current, Ib 1 pA
• GBW 16 MHz. Open loop gain 120dB.
• Selectable gain settings
• Normal Linac 62k Ohm
• Normal Ring 6.2k Ohm
• High Controlled loss 1k Ohm
• Noise voltage gain is set by the resistors Rf
  and Ri, 6200/470 13.2, but the signal is only
  set by Rf, (IC ideal current source). RHIC noise
  10 pA for 10 Hz BW. Using a 100 kSa/S ADC, BW
  will be limited to 50 kHz. Scaling between these
  cases gives an equivalent noise current of 0.71
  nA and an output noise voltage of 4.36 µV.
  Johnson Amp noise 4.1 µV.
• Thermal Drift calculated worst case is 100 µV,
  actual data (better) shown later.
• Trim Pot
• Protection Diodes

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Analog Front End
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Fast Loss Integrator (for MPS)

• Experience at LANSCE has shown that a beam
  inhibit signal should be based on integrated dose
  rather than dose rate. Hardware damage is
  normally due to amount of energy rather than rate
  of energy deposited.
• An integrator will be used to provide a signal
  to a comparator to generate a signal for the MPS
  when the programmable reference is exceeded.
• Response time 10 us (input RC, FE, this stage
  response.)

• Leaky Integrator vs.
• Simple
• Adequate ?
• Non-linear
• Residual offset

• Gated Integrator
• More complex
• Gating Reset
• Charge injection
• Precise

16.6 ms
Circuit components values related to MPS
Comparator range (unknown).
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Analog Front End
80kHz
80kHz
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Viewing Stage

• Features
• Isolation to drive 100kS/s digitizer
• Remote gain setting X1, X10 and gain readback.
• Change viewing gain without changing MPS trip
  threshold.
• Noise Calculated 110 µV (trim pot this stage. With trim pot, better 20 µV.
• Drift See upcoming plot
• Saturation problem in RTBT, solved by
• Added capacitors in Cable Interface, 50-100us
  time constant
• Jumper selectable AFE signal routing
• Linac Ring routed directly from first stage.
• RTBT routed through integrator.
• Should yield several times the 0.1 single point
  loss without saturating the electronics.

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Viewing Stage Data Acquisition

• Data Acquired
• Baseline (1 msec) BLM signal (1 msec ion
  collection time)
• Digitizer 16 bits (incl sign), 100 kSa/s
• 2 bytes/sample, 500 bytes/SNS cycle
• /-5 V, LSB 152.6 µV
• Only selected BLMs send full data (10 max for
  entire system).
• Sum of losses per BLM, per macro-pulse, sent once
  per second.
• Input to waterfall type display.
• 1000 point FIFO history at the console level for
  use in the event of an abort.

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Analog Front End
80kHz
80kHz
1.5kHz
1.5kHz
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1 W/m Stage

• Philosophy
• Reduce noise by reducing BW.
• 1kHz allows enough settling time so digitizer can
  acquire baseline.
• Reduce amplitude of fast loss spikes so we dont
  saturate.
• Gain added ( X 10) so we can use 16-bit ADC
  rather than 24 bit ( see slide )
• 1 W/m loss during the cycle (32.4nA X 6.2k X 10)
  will yield 2mV.
• 1 of 1W/m 20uV
• Trim Pot
• Data Baseline (1 msec) Beam On (1 msec)
  Residual signal (1 msec)
• Digitizer
• Slow loss viewing 16 bits (incl sign), 100
  kSa/s
• 2 bytes/sample, 600 bytes/SNS cycle
• /-5 V, LSB 152.6 µV
• Data accumulated over 10 second or longer
  interval
• Processed to compare against a 1 W/m reference
  (alarm) in IOC.
• Pre-averaged low-level data will be available as
  a waterfall, or a strip chart display.

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Thermal Drift
1 LSB
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SNS BLM Digitizers
Simulated slow loss signal measured by a 16 and a
24 bit digitizers (Yongbin Leng)
1 W/m loss generates 32.4nA, through 6.2k
200uV, for 1 ms pulse. Times 10 gain. 1 of
1W/m yields 2 x 10-8 V.S.
1 1W/m
600 pulses 10 seconds
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VME Implementation
Non-VME, Linear power supplies
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Data Acquisition and Control

• Controls Interface
• Readback front end gain jumper setting
• Set Readback Viewing Stage Gain (X1, X10)
• Timing for Integrator (MPS) gating
• Set loss threshold, fast loss (trip), slow loss
  (alarm)
• Digitized loss data (fast and slow)
• Gain and calibration data folded into resolved
  data.
• Set and readback of High Voltage, On/Off bits
• System test

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HV Bias

• ISEG HV Bias Supply
• VME High Voltage Power Supplies in 2 slot width
• 1 channel and 2 channel versions in same
  dimensions
• LCD display for voltage or current
• variable rate of change of output voltage
• switched polarity
• integrated protection and control circuits
• output overload and short circuit protected
• SHV connector on front side
• Full control via VMEbus
• EPICs drivers
• 1.2k/channel
• Model VHQ 204L, 0-4kV, 1mA

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SNS BLM System - Summary

• BLM Electronics
• Provide loss measurements for tuning. Three
  local gain ranges.
• Low level signals (1W/m)
• Expected low-end signals within the range
  proposed system.
• Bandwidth limit 1kHz.
• Process data over 10 second interval.
• Goal 1 of 1W/m.
• Higher level signals (larger fast losses)
• Wideband (50kHz) viewing.
• Additional remote gain control.
• Fast RTBT loss signals pre-integrated to extend
  dynamic range.
• Machine Protection (Integrated loss)
• 10 µsec response, programmable thresholds and
  masks.

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Back-Up Slides

• Rack Layout
• Analog Front end (Leaky integrator)

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Rack Layout

• Location Ch FBLM Crates Note
• DTL_DiagCab401 5 1
• CCL_DiagCab301 12 1
• CCL_DiagCab302 12 1
• SCL_DiagHB13Cab01 29 1
• SCL_DiagHB1Cab02 30 1
• HEBT_DiagCab07 40 3 2 1
• Ring_DiagCab05 55 12 2 2
• Ring_DiagCab06 44 0 2 3
• RTBT_DiagCab04 28 3 1 4
• Note 1 12 HEBT ch located in Ring racks
• Note 2 Includes HEBT ch beyond ground break
• Note 3Includes 12 ch from RTBT before ground
  beak
• Note 4 12 RTBT ch located in Ring racks

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Analog Front End (LI)
36Hz
80kHz
80kHz
1.5kHz
1.5kHz