SNS Project-Wide Beam Current Monitors[^1]

M. Kesselman, BNL, Upton, NY11973, USA; R. Witkover, TechSource Inc.,
Santa Fe, NM87594, USA;L. Doolittle, LBNL, Berkeley, CA94720, USA; J.
Power, LANL, Los Alamos, NM87545, USA

Abstract

The Spallation Neutron Source^1^ (SNS) to be constructed at ORNL is a
collaboration of six US National Laboratories. Beam current monitors for
SNS will be used to monitor H-minus and H-plus beams ranging from 10mA
(tune-up in the Front End, Linac and HEBT) to over 60 A fully
accumulated in the Ring and RTBT. The time structure of the beams to be
measured range from 645nsec "mini" bunches, at the 1.05MHz Ring
revolution rate, to an overall 1ms long macro pulse. While the beam
current monitors for the SNS have requirements depending upon their
location within the system, the development of a unified approach to
satisfy requirements of the various locations with common components is
a major design objective. This paper will describe the development of
the beam current monitors. Various approaches considered for the
detectors and signal conditioning will be presented.

1  BACKGROUND & REQUIREMENTS

The SNS will consist of:

-   An H^-^ Ion Source, RFQ and MEBT (The Front End) provided by LBNL,

-   The Linac, consisting of a warm section up to 186 MeV and a
    superconducting section up to a nominal 1 GeV, provided by LANL and
    TJNAL,

-   The HEBT, Accumulator Ring and RTBT, provided by BNL

The Linac and HEBT design current will be a 57mA peak pulse of about 1
ms duration (macro-pulse) chopped at the Ring revolution frequency of
1.05MHz at a repetition rate of 60 Hz. The chopped "mini-pulses" will
consist of a 645nS long bunch and a 300nS gap. The micro-pulse structure
of 402.5MHz, which exists in the Front End, Linac, HEBT and shortly
after injection into the Ring, will not be observed with the beam
current monitors (BCMs). The Ring will accumulate this current over 1mS,
about 1000 mini-pulses, resulting in an average of about 57 amps at the
end of the cycle and a total charge of 2x10^14^ per pulse. The RTBT will
carry the current of the last turn in the Ring to the target. The
electronics in the Ring and RTBT must operate over a signal range of
1000 to 1 (60dB), however, the Ring gain must change to accommodate the
wide variation. The basic circuitry used in the Front End, Linac and
HEBT could be used with the addition of gain switching. To observe the
details of the mini-pulse, a fast response (1nS rise time with a droop
rate of about 0.1%/S) is required. For the macro-pulse current, a rise
time of about 50nS and a droop of 0.1%/mS would be sufficient.

2.  **DISCUSSION**

    1.  *BCM Sensor*

There is an estimated requirement of a total of 25 transformers^2^. The
transformers^3^ considered were passive, active-passive^4^, and fast
current transformers (FCT), but each had limitations. Instead of
utilizing two different transformers to observe the lower bandwidth
macro-pulse separately from the wide bandwidth mini-pulse, it was
decided to use an FCT with baseline restoration to compensate for the
droop and use a single transformer. An FCT produced by Bergoz
Instrumentation^5^ is being tested for use in the HEBT, Ring, and RTBT.
Similar units are under consideration in the Front End and Linac.

2.2  Signal Processing

The data required from the BCMs is current, wave shape, and mini- and
macro-pulse charge. A comfort display indicating the typical macro-pulse
wave shape and the integral of the current per micro-pulse will also be
needed. In addition, there is a requirement to provide a circular buffer
holding data from a number of past pulses for fault diagnostic purposes.
An interesting solution to these requirements would be to convert to
digital form as soon as possible, and to use digital signal processing
to accomplish much of the signal manipulation. A general block is
presented in Figure1.

![](media/image1.wmf){width="2.8930555555555557in"
height="0.7513888888888889in"}

Figure 1. General Block Diagram

As is evident from the block diagram, the turn-by-turn baseline
restoration will be done digitally. This drastically reduces the analog
signal processing and eliminates the accompanying DC drift, offset, and
non-linearities.

3.  Input Signal Conditioning

    In an attempt to minimize noise pickup, the prototype FCT was
    designed with a differential output at 78 Ohms. Initial testing
    indicate that the long (100 m) twisted pair cables required will
    cause significant distortion due to dispersion even for the highest
    quality cable. It is likely that the production units will have
    single ended output and use semi-rigid cabling to the remote
    electronics.

    The large dynamic range expected in the Ring will require gain
    switching, but switchable gain amplifiers will not settle to the
    required precision in the 300 nsec between turns. One approach being
    considered would use two amplifiers preset to different gains and
    switch between them either at their input or output. Input switching
    requires a fast switch with low on-resistance, capable of
    withholding at least 15V, and switching at the output will still be
    required. It should be possible to switch only at the output if the
    amplifier is able to withstand the peak input voltage without
    failing or saturating from an over-voltage.

    The Analog Devices AD600 has a gain range of 0 to 40 dB and a 35 MHz
    bandwidth. It is rated at 2V DC at the inputs, but can sustain the
    supply voltage (7.5V max) for 10 msec. A suitable fixed attenuator
    must precede the amplifier to match it to the expected peak current.
    As the charge in the Ring accumulates the higher gain amplifier
    nears the top of its range and the signal from the lower gain
    amplifier is used. After switching there would be no problem
    changing the higher gain amplifier to a lower gain to avoid
    saturation. This can be repeated again if a third gain range is
    desired. Fast post amplifier switching might be done digitally if an
    ADC is used for each amplifier. An Analog Devices AD6644 ADC is
    being tested at BNL for use as the digitizer. This device can
    provide 14bit conversion at up to 65MSa/S.

    The BCMs in the RTBT must handle the range from a single injected
    turn to the full accumulation of 1000 turns and thus require
    settable gain. However, since the charge will be transferred in a
    single mini-pulse, more than 15 msec would be available to switch
    gains. The same amplifiers used for the Ring could be applied to the
    RTBT.

    The Front End, Linac and HEBT BCMs do not require gain switching but
    could use the same amplifier and digitizer circuit.Turn-by-turn
    baseline restoration is required in all cases. There are several
    approaches under consideration, but digital processing is especially
    interesting. Baseline restoration can be implemented in conventional
    software at a high level in one of the various servers or
    workstations that make up a networked control system, or more
    intimately tied to the acquisition hardware, with embedded
    processing using a dedicated FPGA, DSP, or microprocessor. One
    technique would acquire multiple samples during the 300 nsec gap to
    form an average value which is subtracted from the mini-pulse to
    restore the baseline to zero. This can be done on a pulse-to-pulse
    basis using standard timing pulses during the interpulse period.
    Since the Ring, HEBT and RTBT transformers will droop less than 0.1%
    during the mini-pulse, they will require no low frequency
    compensation. Droop of the Front End and Linac BCMs will likely be
    greater since they are restricted in size, but are not expected to
    be more than a few percent during the mini-pulse. Software
    compensation for these units may be provided if deemed necessary.

    Other required processing includes calculation of the charge in each
    mini-pulse. This will be necessary in the HEBT line as well as the
    Ring to calculate injection efficiency. It will also be needed to
    compute the turn-to-turn circulating charge in the Ring and the
    final charge delivered to the RTBT line. With a 60 MSa/S, 14bit
    digitizer, sufficient samples will be available to perform a good
    integration of the current to obtain the charge per mini-pulse. The
    charge per macro-pulse will also be computed.

    For wideband viewing a 1GSa/s, 8bit digitizer will be used to
    observe the 695 nsec mini-pulse. The SNS instrumentation includes
    provision for an array of such digitizers, preceded by a wideband
    multiplexer, which would be used for acquiring other fast mini-pulse
    signals (Fast Loss Monitors, Wall Current Monitors) as well. These
    digitizers will be commercial VME or VXI units, which are
    essentially the electronics of fast digital sampling scopes. For
    precise measurements of mini-pulse and macro-pulse charge, a higher
    resolution (14 bit) 40MSa/s to 60MSa/s digitizer would be used.

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4.  *Subsystem Configuration*

A control system interface is being developed at LANL for the Linac Beam
Position Monitor (BPM) system, which will be based on a conventional
Personal Computer using the PCI standard. It offers a low cost housing
with a standard high speed bus and a very fast, easily programmed local
processor. Many commercial acquisition and control boards are available
in PCI format. The practicality of this approach as opposed to a VME or
DIN-rail environment will be evaluated.

![](media/image2.wmf){width="3.20625in" height="2.2090277777777776in"}

Figure 2: Subsystem partitioning

The general BCM electronics may be configured as shown in Figure 2, with
three possible partitions depicted. The digitizer can be included with
the digital interface, or with the analog front end electronics, or as a
separate board. The advantages and disadvantages of each approach are
being evaluated.

### 3 SUMMARY

The SNS beam current monitor system under development is focusing on a
signal processing system that converts data to digital representation as
soon as practical. Signal manipulation, calibration, filtering and noise
reduction, baseline restoration, and droop compensation will be done
digitally, as will calculation of the charge per mini-pulse and per
macro-pulse. Commercial passive wideband current transformers will act
as the basic current sensors, and the electronics will provide the
appropriate filtering to tailor the signal for digitization.

### 4 REFERENCES

\[1\] See website; [http://www.sns.gov](http://www.sns.gov/)

\[2\] M.Kesselman, et-al, "SNS Project-Wide Beam Current Monitors", to
be published in the Proceedings of the Beam Instrumentation Workshop
2000 (BIW00), May 8-11,2000, Cambridge MA.

\[3\] Bergoz Instrumentation, Espace Allondon Ouest, 01630 Saint Genis
Pouilly, France.

\[4\] K. B. Unser, "Atomkernenergie", Kerntechnik, Vol.47 (1985) No.1
(pg48)

\[5\] See ref. (3) and website; http://www.bergoz.com

[^1]: ^^ Work performed under the auspices of the U.S. Department of
    Energy.