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BNL-67457
2000 Beam Instrumentation Workshop
Cambridge, MA, May 8-11, 2000
SNS Project-Wide Beam Current Monitors
M. Kesselman*, R. Witkover**, L. Doolittle **, J. Power****
*BNL, Upton, NY 11973, ** TechSource, Inc., Santa Fe, NM 87594, *** LBNL
Berkeley, CA 94720 , **** LANL Los Alamos, NM 87545
Abstract. A consortium of national laboratories is constructing the Spallation Neutron Source'
(SNS) to be installed at Oak Ridge National Laboratory. There are signal similarities that exist
in the beam diagnostic instrumentation that could permit common designs. This paper will focus
on the beam current monitoring requirements, and the methods under consideration to measure
beam current in various locations throughout the SNS facility.
REQUIREMENTS
The Front End, Linac and HEBT will have a 57mA peak current pulse of about 1
ms duration (macro-pulse) chopped at the Ring revolution frequency of 1.05MHz
(mini-pulse). Mini-pulses consist of approximately a 645nS long bunch and a 300nS
gap. The micro-pulse structure of 402.5MHz, which exists in the Front End, Linac,
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. 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
same basic circuitry used in the Front End, Linac and HEBT could be used with the
addition of gain switching.
Transformer droop during the macro-pulse, which will cause a baseline shift, is the
result of finite inductance and load resistance limiting the IUR time constant. If more
turns are added to increase inductance the sensitivity goes down, and resonances with
winding capacitance and delay line effects will limit the high frequency response.
Additionally, the Front End and Linac have little available space, and the transformers
must be mounted in close proximity to DC magnetic fields. Large, shielded
transformers, needed to obtain high inductance and exclude stray fields, will not fit.
To observe the details of the mini-pulse, a fast response (inS rise time with a droop
rate of about 0.1%/uS ) is required. For the macro-pulse current, a rise time of about
SOnS and a droop of 0.1%/mS is required. The distribution of transformers is shown
in Table 1.
t Work performed under the auspices of the U.S. Department of Energy.
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