Preliminary Design of the Beam Loss Monitoring System for the SNS [^1]

R. WITKOVER\*, D. GASSNER

BNL, Upton, NY,\*TechSource Inc., Santa Fe, NM

**Abstract.** *The SNS to be built at Oak Ridge National Laboratory will
provide a high average intensity 1 GeV beam to produce spallation
neutrons. Loss of even a small percentage of this intense beam would
result in high radiation. The Beam Loss Monitor (BLM) system must be
sensitive to low level, long term losses, and be capable of measuring
infrequent short high losses. This large dynamic range presents special
problems for the system design. Ion chambers will be used as the
detectors. A detector originally designed for the FNAL Tevatron, was
considered but concerns about ion collection times and low collection
efficiency at high loss rates favor a new design. The requirements and
design concepts of the proposed approach will be presented. Discussion
of the design, testing of the ion chambers, and the analog front end
electronics will be presented. The overall system design will be
described.*

# BACKGROUND

The Spallation Neutron Source (SNS)[^2] is an accelerator based intense
neutron source being built at Oak Ridge National Laboratory (ORNL) by a
collaboration of 6 national laboratories. The controls and some beam
instrumentation were consolidated so that the same detectors and
interface hardware would be used throughout the facility to minimize
costs and reduce uncertainty due to different instrumentation. BNL has
responsibility for the beam loss monitoring system for all of SNS. The
SNS H-minus ion source will inject a 60 Hz chopped beam into a 2.5 MeV
RFQ. After acceleration to 87 MeV in a Drift Tube Linac (DTL), the beam
enters the Cavity Coupled Linac (CCL), leaving at 186 MeV. The
Superconducting RF Linac (SRF) will increase the beam energy to 1 GeV.
The beam is transported to the Ring via HEBT, converted to protons using
a stripping foil, and accumulated in a single 695 nsec bunch over the 1
msec injection pulse. The bunch is transported via the RTBT line to the
spallation neutron target. The design peak beam current in the Linac and
HEBT will be 38 mA. In the Ring it will increase to more than 40 A
(average) at the end of the pulse, for a total of 1.5 x 10^14^ protons
in the single bunch. The BLM system will be designed to accommodate an
upgrade intensity of 2 x 10^14^ protons.

The baseline design will produce 1.4 MW at a 60 Hz rate. This very high
average beam power makes it crucial that uncontrolled losses do not
produce high activation, preventing hands-on maintenance. This will be
done by minimizing losses through careful design and providing beam
dumps and collimators where controlled losses can be contained. The BLM
system must minimize uncontrolled losses by providing data for tuning
the machine and inhibiting the beam when excessive losses occur.

## SYSTEM PHILOSOPHY AND DESIGN

The BLM's will be the primary tool for limiting uncontrolled losses by
providing sufficient detector coverage in all sections of the SNS. To
achieve this the BNL AGS Linac and Ring use extended ion chambers made
from hollow argon-filled, large diameter coaxial cables[^3]. At ISIS, a
machine similar in function to SNS, the same type detectors have been
used[^4]. A \"line\" detector has 1/r spatial dependence, but its length
often prevents close coupling to the beam line, lowering the response.
Since losses predominantly occur at the quad, most of the detector
receives a much lower exposure, reducing the benefit of the greater
length. The standard connectors for these cables cannot support high
bias voltage, limiting the linear range to lower dose rates. For these
reasons smaller, sealed glass ion chambers designed for the FNAL
Tevatron were used in RHIC[^5], but more detectors were required to
obtain similar coverage. In the SNS Linac, HEBT, Ring and RTBT, as in
RHIC, detectors will be placed at essentially every quadrupole
(beta-max) and at other key points. Provision will be made for a number
of movable BLMs which can be used for more detailed study.

Ion chambers were chosen as the primary distributed detector over solid
state (pin diode) and scintillator-photomultipliers. A commercial
pin-diode BLM[^6] is available but the 10 MHz maximum output rate would
not provide the dynamic range required during a 1 msec beam pulse.
Scintillator-photomultipliers do not have the long term stability needed
for an accelerator application. However, to be able to see losses within
the bunch, scintillator-photomultipliers will be installed at a limited
number of points, such as, Ring injection and extraction, and other
strategic points. Since only a relative time history is of interest for
this application, non-calibrated data is acceptable.

**System Requirements**

High level losses over several pulses can quench or damage the
super-conducting cavities in the SRF Linac. Low level losses can prevent
"hands-on" maintenance. To avoid this, losses must be limited to 1 W/m
averaged over several seconds duration, roughly equivalent to 10^-4^ of
a 2 MW beam lost uniformly around the Ring. Dynamic range must be
available to measure such long-term low level losses, yet not saturate
under short duration, high level losses. The BLM system will provide
hardware and software generated signals to indicate excessive loss and
data for tuning the beam to reduce operating losses. These
considerations lead to the requirements shown below.

> **[Detailed time history.]{.underline}** Data within a macro-pulse
> will be acquired at 100 kSa/S per channel but will be packed at the
> front-end computer (IOC) and transmitted at a 6 Hz rate.
>
> [**Total loss for each BLM per macro-pulse**.]{.underline} The data
> from each BLM will be summed at the IOC and transmitted once per
> second. This data may be presented as a "waterfall" display[^7] which
> shows the long-term history with detector location as the horizontal
> axis, time as the vertical axis, and level as color. A simple x-y plot
> of location versus loss, updated at a few Hz, will be available. The
> losses for each BLM will be kept in a 1000 point FIFO history at the
> console level for use in the event of an abort.
>
> **[Long-term, low-level beam loss alarm and display]{.underline}.**
> The long-term, low-level beam loss (\< 1 W/m) will be averaged over 10
> seconds in the IOC and compared against a 1 W/m reference. Warnings
> will be sent as this level is approached and an alarm when it is
> exceeded. The pre-averaged low-level data will be available as a
> waterfall display or a strip chart style display.
>
> **[Fast loss output for the Machine Protect System]{.underline}**.
> Each BLM electronics channel will provide an output signal to the
> Machine Protect System (MPS) to shut off the beam in the event of a
> high-level beam loss. This signal will be derived in hardware from the
> integrated beam loss during the macro-pulse and shall occur within 10
> μsec of the loss. Each channel shall have masking capability in the
> MPS to allow the use of diagnostic hardware which can cause beam loss,
> such as wire scanners. The mask will also permit studies which might
> generate a higher than normal local beam loss, and disable
> malfunctioning BLM channels which would prohibit beam operation. A
> beam inhibit signal shall also be generated if the integrated loss for
> any BLM over a 1 second period exceeds a preset limit. This shall be
> done in software at the IOC and executed through an MPS line in the
> Utility Module.
>
> **[Gain setting and readback for each BLM.]{.underline}** Each BLM
> electronics channel will have 3 jumper selectable gain settings to
> compensate for high radiation locations, such as at dumps, injection,
> extraction, and the collimators, or lower loss locations such as lower
> energy regions and well shielded areas. The "Viewing Gain" for the
> wideband-output can be changed (x1, x10) via the control system
> without affecting the fast loss trip or the 1 W/m level sensing. Gain
> setting may be required pulse-to-pulse to allow for different mode
> cycles, but not during the macro-pulse. Read back of the
> jumper-selectable gain and Viewing Gain states will be provided.
>
> **[Bias voltage control.]{.underline}** During studies periods, it may
> be necessary to vary the detector bias voltage. Readback of the
> voltage and current will be needed. Each rack location will contain at
> least 2 HV Bias Supplies, powering alternate BLM detectors. The BLM
> System can be tested by switching the HV bias power supplies off and
> on again. The resulting transient capacitively couples between the
> high voltage and signal electrode and through the BLM electronics,
> checking all components of the system, including the bias and signal
> cable continuity.
>
> [**System calibration**.]{.underline} Each BLM detector will be
> calibrated in a laboratory test setup using a static radiation source.
> Data will be taken throughout the operating voltage range for each
> detector. Testing will be done at BNL until the production is stable,
> with subsequent BLMs tested at ORNL. Insitu tests of the system
> through the installed cabling will be performed using a calibrated 1
> G-Ohm resistor and voltage source for use in the calibration database.

**Expected Losses**

Experience at LAMPF and the PSR, and studies for APT at LANL[^8] have
determined that losses of 1 W/m will allow hands-on accelerator
maintenance (\~100 mR/hr at 1 foot), with transverse losses primarily at
quadrupoles where the beta-max occurs[^9]. The beam-on dose rate can be
estimated from the beam-off dose rate by a "rule-of-thumb": multipliers
of 500-1000 are typically used. Taking 100 mR/hr at 1 foot as the
beam-off activation, the beam-on dose rate may be estimated at 100 R/hr,
or 0.46 mR/pulse, which is equivalent to 0.46 R/sec during the 1 msec
beam.

The upper end limit has been specified as 1% local loss based on the
expected losses in the Ring collimators[^10]. The BLM system must
generate a beam abort signal for the MPS in less than 10 μsec. This will
be done by integrating the beam loss during the macropulse and comparing
the result to a programmable reference. Longer term losses at lower
level can also do damage to beamline components. The numerically
integrated output of each BLM will be compared to settable thresholds in
the local processor. Excessive loss will generate an abort signal to the
MPS through a VME utility module in the IOC crate. Still lower level
losses, corresponding to the 100 mR/hr, 1 W/m loss will be similarly
monitored and a software warning provided to the operators.

# Detector

SNS will use 255 (Linac 88, HEBT 52, Ring 75, RTBT 40) argon-filled ion
chambers as the primary detectors for monitoring beam losses. Argon has
the advantage of fast electron transit time compared to slower air
filled detectors[^11]. The initial choice was an ion chamber designed by
Shafer[^12] at FNAL in 1982 for the Tevatron, and modified to improve
radiation hardness and reduce noise for RHIC at BNL[^13], but there was
concern about saturation at high dose rate and long ion transit time (\~
700 μsec at 2 kV bias). The ions, constituting 25% of the signal, would
appear as a continued rise during the 1 msec pulse followed by a long
ion tail after. However, processing could unfold the pulse shape. A new
ion chamber, designed to overcome these limitations[^14], utilizes a
larger inner diameter electrode to significantly decrease the ion
transit time and raise the collection efficiency for a 1% local loss.

Figure 1 shows the measured response at the equivalent if a 0.05% local
SNS loss in the 200 MeV BNL Linac to Booster beamline, for the FNAL and
a new prototype detector with nitrogen fill . The slow rise during the
beam pulse, and long tail afterwards due to the ions is clearly seen for
the FNAL detector. The "noise" is chopper modulation of the beam.

![](media/image1.wmf){width="2.825in"
height="2.4319444444444445in"}![](media/image2.wmf){width="2.7527777777777778in"
height="2.4715277777777778in"}

**FIGURE 1**. Beam Tests of the FNAL and New Prototype Ion Chambers

## Installation

The detectors may be mounted directly to the beam pipe, magnet, or moved
away from the radiation source to reduce signal output, allow a wider
view over a larger area without intervening shielding. A typical SNS
installation might raise the BLM 30 cm above the beam line on the
upstream side of a quad. BLM's will be placed so that successive units
can have overlapping views to allow limited coverage in the event a
channel fails. The system will be packaged so that adjacent detectors
will not be placed on consecutive electronics channels in the same
circuit module, where possible.

Some BLM's may be exposed to X-rays from RF cavities. Measurements at
BNL indicate that this may reach 50 R/hr during beam time. A study is
needed to determine the type and amount of shielding required that would
reduce the X-ray contribution to an acceptable level, and its effect on
the beam loss measurement.

## Cabling

Low tribo-electric RG-59 type cable such as Belden 9054 or 9224 will be
used for the signals. Tribo-electric noise comes from friction between
the conductors and insulation due to movement, such as vibration, but a
low friction coating between insulator and conductors will reduce this
significantly. Two HV cables (RG-59), from separate HV power supplies
will connect to alternate BLM's from each rack to provide some coverage
in the event of a high voltage short or power supply failure.

# ELECTRONICS

The BLM system will be distributed throughout the SNS. The controls
electronics will be in VME, with analog signal conditioning housed in a
non-VME crate in the same rack. The analog crate will use linear rather
than switching power supplies because of the very small signal currents.
Figure 2 shows the system block diagram.

![](media/image3.wmf){width="6.013194444444444in"
height="2.8506944444444446in"}

**Figure 2.** BLM System Block Diagram

**System Dynamic Range Requirements**

The BLM System must measure beam losses from a 1% local loss down to 1%
resolution of a 1 W/m loss, a dynamic range of 126dB , or at least 21
bits. The design exploits the difference in bandwidth between these
limits to allow additional resolution at the low end. During the 1 msec
beam pulse the 1 W/m dose rate is estimated as 0.46 R/sec at 1 ft. For a
typical BLM sensitivity of 70 nA/R/sec, this corresponds to 32.4 nA
signal current during the pulse. The upper end loss, allowing for future
improvements to 2 MW beam power, corresponds to 20 kW. Since the signal
must inhibit the beam within 10 sec, only the electron signal will be
considered, reducing the BLM sensitivity to 35 nA/R/sec. Then I ~Max~=
16.2 x 10^-9^ x 2 x 10^4^ = 0.324 mA for a loss of 1% uniformly during
the 1 msec pulse.

## Analog Front End Electronics

Figure 3 shows the AFE electronics schematic. The circuit provides 3
outputs to meet the fast trip, wideband-wide range data, and 1 W/m
sensing requirements.

#### Wideband Signal Circuit

Since the ion chamber is a current source, the input resistor doesn't
affect the signal gain but does determine the voltage noise gain and
input signal risetime. A 100 m cable and 470 Ω resistor gives about a 5
μsec risetime. A 6.2 kΩ feedback resistor puts the signal mid-range for
5 V ADC for a 1% beam loss. For the 35 kHz SNS BW, the 10 pA at 10 Hz BW
noise observed in RHIC would correspond to 3.7 μV. Three jumper
selectable gains with readbacks are provided. "Viewing Gain" can be set
and read back remotely without affecting the beam interrupt or 1 W/m
outputs. The signals of Figure 1 were made using this circuit.

![](media/image4.wmf){width="5.797916666666667in"
height="3.4298611111111112in"}**Figure 3.** Analog front-end electronics
schematic

**The 1 W/m loss Circuit**

A 1 W/m loss will produce 200 μV out of the input stage, roughly an LSB
for a 5V, 16-bit (15-bit plus sign) ADC. Another 7-bits would be needed
for 1 % resolution of a 1 W/m loss. Using a 1 kHz low pass filter, the
noise would be reduced by the equivalent of almost 3-bits. Having an
analog filter precede the output amplifier reduces the peak of short
duration losses, allowing the additional gain of 10. A lower cut-off
might not allow sufficient time to measure the baseline for offset
subtraction. The output goes to a 24 bit, 100 kSa/sec ADC. Tests of one
ADC (ICS-110B) indicate it can achieve 18-19 bit resolution. This is not
quite enough for the 1% resolution, but sampling over 10 seconds or
longer should provide the required sensitivity. Detailed testing of this
and other ADCs, including thermal drift of offset and gain, is in
process. Using measurements prior to the pulse, offsets of the AFE as
well as the ADC, which are likely to be in excess of tens of microvolts,
can be measured and subtracted. Dedication of one ADC channel to read a
reference voltage may be required to compensate for gain thermal drift.

#### The Pulse Integrated Dose Circuit

Experience at LANSCE[^15] has shown that a beam inhibit signal should be
based on integrated dose rather than dose rate. Thus 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 (see Figure 2). A gated
integrator with a triggered reset would give a precise integral of the
loss, but the gating, reset, and charge injection compensation circuitry
add considerable complexity to the design. A "leaky integrator", using a
large value resistor to bleed the charge, is simple and provides an
adequate representation of the pulse dose. Simulations indicate that for
an RC time constant of 1/3 the pulse period, the output will reach
equilibrium in 3 beam pulses with an error of less than 10% while
decaying to under 5% by the next beam pulse. This is a reasonable
trade-off to the reduction of complexity and reliability of the
triggered integrator.

Since the 695 nsec wide RTBT beam pulse would be too narrow for the 100
kSa/s ADCs to acquire through the wideband output, the signal from this
integrator will be jumper selected for input to the Viewing Gain stage.

# Acknowledgments

The authors would like to R. Shafer and M. Plum of LANL for providing
valuable insight and discussions regarding the topics in this work. The
BNL staff includes Chaofeng Mi who built and tested the AFE circuit.
Yongbin Leng evaluated the ADC's, John Smith has been invaluable in
supporting the Controls aspect of this system. Tony Curcio, Paul
Ziminski and members of the Instrumentation Group provided enthusiastic
cooperation in building, testing and installation of the prototype
detectors and circuits.

# References

[^1]: ^^ Work performed under the auspices of the U. S. Dept. of Energy

[^2]: Alonso, J., \"The Spallation Neutron Source Project\", Proceedings
    of the 1999 PAC, NY, 1999, p574. For current status see
    [http://www.sns.gov](http://www.sns.gov/)

[^3]: Witkover. R. L. ,\"Microprocessor Based Beam Loss Monitor System
    for the AGS\", IEEE Trans. Nucl. Sci **26** 3313 (1979)

[^4]: . M. A. Clarke-Gayther, et al. "Global Beam loss Monitoring Using
    Long Ionisation Chambers at ISIS" , 4^th^ European Particle
    Accelerator Conference, London, England, 1994, p1634

[^5]: Witkover, R. L., Michnoff, R. J. and Geller, J. M., \"RHIC Beam
    Loss Monitor System Initial Operation\", Proceedings of the 1999
    PAC, NY, 1999, 2247

[^6]: Bergoz Inc, http://www.bergoz.com

[^7]: Bai, M., et al., "RHIC Beam Loss Monitor System Commissioning in
    the Year 2000 Run", To be published these proceedings.

[^8]: Hardekopf, R. A., \"Beam Loss and Activation at LANSCE and SNS\",
    LA-UR-99-6825, Los Alamos National Laboratory, Los Alamos, NM. Sept.
    1999

[^9]: Loc. Cit. Reference 7

[^10]: Y. Y. Lee, BNL, Private Communication, Oct 2001

[^11]: Plum, M., and Brown, D., \"Response of Air-Filled Ion Chambers to
    High Intensity Radiation Pulses\", Proceeding of the 1993 PAC, p2181

[^12]: . Shafer, R. E., et al., "The Tevatron Beam Position and Beam
    Loss Monitoring Systems", Proc. The 12^th^ Int'l Conf. High Energy
    Accel., p609, 1983

[^13]: Loc. Cit. reference 4.

[^14]: Witkover, R. L. and Gassner, D. "Design of an Improved Ion
    Chamber for the SNS", These proceedings

[^15]: Plum, M., and Shafer, R., Private communications.