# Spallation Neutron Source Ring Diagnostics\*

P. Cameron, J. Brodowski, P. Cerniglia, R. Connolly, J. Cupolo, C.
Dawson, C. Degen, A. DellaPenna, D. Gassner, R. Gonzalez, M. Grau, J.
Gullotta, L. Hoff, A. Huhn, M. Kesselman, C. Liaw, J. Mead, R. Sikora,
G. Smith, K.Vetter, M. Wilinski, BNL, Upton, NY 11973, USA\
S. Assadi, W. Blokland, C. Diebele, D. Purcell, T. Shea, ORNL, Oak
Ridge, TN , USA M. Plum, LANL, Los Alamos, NM , USA R. Witkover,
TechSource, Santa Fe, NM, USA

Abstract

Brookhaven is providing the Ring and Transfer Lines Beam Diagnostics for
the Spallation Neutron Source (SNS), to be installed at Oak Ridge
National Laboratory. The customary diagnostics that will be present
include Beam Position Monitors (BPM), Ionization Profile Monitors (IPM),
Beam Loss Monitors (BLM), Beam Current Monitors (BCM), Coherent Tune
Measurement, and Wire Scanners. An overview of these systems is
presented, along with brief discussions of SNS-specific problems that
must be addressed, including unprecedented beam power, large dynamic
range, a stringent loss budget, space charge, beam halo, and electron
cloud. We also present an overview of systems more specifically tailored
to address these problems, including Beam-in-Gap measurement and
cleaning, two types of incoherent tune measurement, halo monitor, and
video monitors for stripping foils and the electron catcher.

## Overview

An overall description of the SNS project and status is available in
these proceedings\[1\], as well as an update on the Ring and Transfer
Lines\[2\]. Additional background information on SNS diagnostics is
available in earlier conference\[3\] and Beam Instrumentation
Workshop\[4,5\] proceedings. An overall layout of SNS Diagnostics is
shown in Figure 1.

The SNS Diagnostics team is taking full advantage of the cost and
performance advantages offered by the ubiquitous PC platform. With a few
exceptions, a typical SNS diagnostic is a rack-mount PC running embedded
Windows 2000 or XP, with control of data acquisition and accomplishment
of data analysis handled by LabVIEW\[6\]. Each PC also runs EPICS IOC
core, so the PCs appear to be Input-Output Controllers to the EPICS
Control System. Whenever possible standard off-the-shelf PCI analog
front end/digitizer boards have been used. In the more demanding
applications (BPM, BCM, IPM, tune systems) a custom PCI board is used.
Timing signals are decoded by a standard SNS timing module embedded in a
PCI-interfaced FPGA. For systems using the custom PCI AFE/digitizer
boards, this gate array is the PCI interface, and also offers the
possibility of fast
pre-processing.![](media/image1.wmf){width="6.593055555555556in"
height="3.7395833333333335in"}

![](media/image2.png){width="3.022222222222222in"
height="1.9381944444444446in"}![](media/image3.png){width="3.035416666666667in"
height="1.9090277777777778in"}

## Beam Position Monitors

Dual plane 50ohm 250mm long stripline electrodes are located at all
quadrupoles. The transfer line striplines are shorted to take advantage
of the superior mechanical accuracy offered by that construction. The
Ring striplines are open to permit biasing for electron clearing.
Apertures vary from 12cm in HEBT to 36cm in the downstream portion of
RTBT.

Verification of signal path and calibration of electronics will be
accomplished via S21 measurements. Beam-based offset determination
(BBOD) will be employed to find BPM electrical centers relative to quad
magnetic centers\[7\]. Log-ratio processing in combination with BBOD
opens the possibility of further calibration of the electronics. All
Ring and RTBT electronics will be dual-band, with both baseband and
402.5MHz processing.

## Ionization Profile Monitor

The SNS IPM is an improved version of the IPMs installed\[8\] in the
RHIC ring. Modifications were necessary due to rf coupling to the beam,
susceptibility to beam loss, and interference from electrons.

To deal with rf coupling, the microchannel plates were moved outside the
beam image-current path and 100dB of rf shielding was added.

Beam loss can cause particle showers to pass through the micro-channel
plate and the collector board, generating large background signals.
Again, moving all components outside the aperture minimized this
problem.

Interference from electrons is handled by extending the IPM's electric
and magnetic fields beyond the active volume to prevent electrons
created outside from entering. Additionally, the beam pipe in the ring
is TiN-coated to suppress electron creation. Finally, the IPM's strong
electric field will prevent electron multipacting.

The modified RHIC IPM was tested by scraping beam upstream on the beam
pipe wall. As shown in Fig. 2, the profile using the improved IPM is
much cleaner than the profile measurement using the unmodified unit.

## Beam loss monitors

The high average beam power requires that uncontrolled losses be kept
very low to allow machine maintenance. The BLM system is designed to
detect an uncontrolled loss of 1 part in 10^4^ at 1 MW over the full
ring and provide a signal to inhibit further injection. Ion chambers
(IC) will be used as the detectors to provide the interlock function and
multi-turn loss data. Several photo-multipliers will provide observation
of losses within the bunch. The ICs will be at each quadrupole, the
collimators, and key injection and extraction loss points. Several
movable units may be placed at local hot-spots, for a total of 70
channels.

A new design IC has been developed for the SNS\[9\], resulting in
significantly faster ion transit times and higher dose rate capability.
Nominal sensitivity is 70 nC/rad. The system is designed to handle a
10^4^ dynamic range. Each BLM signal will be acquired by an individual
channel of a 24-bit ADC sampling at 100 KS/sec\[10\].

The fast BLMs consist of bare photomultipliers, eliminating the problems
of radiation darkening and disposal of mixed wastes. Unit-to-unit
variation and radiation effects on the window of the tube will require
periodic re-calibration and individual HV power supplies. The signals
will be integrated over the bunch length and read through digitizer
channels of a multi-channel fast digitizer to observe losses within the
bunch.

## beam current Monitor

The BCM system is designed to handle the 2x10^14^ circulating protons
required for a 2MW beam. Peak current can reach 100A, and care must be
taken to prevent transformer core saturation. To achieve 1% accuracy and
0.1% resolution, droop is specified to be less than 0.1% during the
1msec accumulation period, requiring a decay time constant of about 1
second. Rise time is specified at 50ns. A Bergoz current transformer
with rise time \<1ns and droop of 0.1%/μs is used. Software\[11\]
compensates for transformer droop and provides the 1 second time
constant. The signal is processed by a switched gain amplifier followed
by a 7MHz Gaussian Filter. A 5-pole 17MHz Chebyshev filter provides
anti-aliasing before digitization at 68MS/s. Charge is calculated for
each turn as well as the macro-pulse total. A 100MHz BW output goes to a
fast digitizer capable of capturing the edge information.

## incoherent tune

Incoherent tune will be determined by both dipole and quadrupole beam
transfer function measurements. Both systems will employ resonant
kickers and pickups. Kicker control and data acquisition and analysis
will be accomplished with modified BPM electronics. The objective of the
BTF systems is to map the tune footprint, which is large in the SNS Ring
as a result of chromaticity and space charge tune depression. Both
systems will be similar to an existing system in RHIC\[12\].

## big/halo Monitor

Gap beam will be vertically kicked onto the collimators for the last 60
to 100 turns of the accumulation cycle by 4.5m long 50 ohm striplines
driven by 7KV pulsers, and observed with a gated FBLM. Kick strength and
number was determined utilizing particle-in-cell tracking code\[13\].
Scrapers for BIG tuning and halo observation will be installed adjacent
to the IPMs. Kicker control and data acquisition and analysis will be
accomplished with modified BPM electronics.

## coherent tune

The BIG/Halo kicker will also be utilized in the coherent tune system.
Position signals will come from a dedicated BPM. Kicker control and data
acquisition and analysis will be accomplished with independent modified
BPM electronics. Both FFT and fit algorithms will be employed. In
addition to this independent system, the capability to calculate tune
using all ring BPMs will be a part of the Orbit software.

## wire scanners

The wire scanner mechanisms and electronics are provided by LANL, and
are described in detail in ref \[14\[

## video foil Monitor

Normal operating temperature of the primary stripping foil will be
\~2000K, permitting use of a vacuum tube radiation hard video camera,
located in a cubby near the injection region. This camera will also be
used to confirm the position of the primary foil and monitor its
integrity. A second similar camera will be similarly installed to
monitor beam profiles at the secondary stripping foil when a phosphor
screen is positioned in the beam path. During tuning, a similar phosphor
screen can be positioned at the primary stripping foil location. The
position and angle of the injected beam can be measured, and beam
profile data can be obtained. A third system will monitor the electron
catcher, which is located just below the primary injection stripping
foil. Each system will have a multi-position neutral density filter
assembly mounted between camera and lens to avoid saturation. The analog
video signals will be processed using the NI-PCI-1409 Image Acquisition
board, and live video will be available via the network using a
video-to-Ethernet converter.

## fast daq systems

Fast signals from the BLM, BCM, and electron detector systems will be
processed by 'PC scopes', utilizing standard commercially available
PCI-based digitizers.

## Conclusion

A full complement of sophisticated diagnostics systems is essential to
provide the information needed to commission and operate the SNS. The
challenge of integrating these systems and bringing them on-line with
limited manpower is being met by utilizing commonality between systems,
in both the inter- and intra-laboratory arenas.

## References

\[1\] N. Holtkamp, "Status of the Spallation Neutron Source", these
proceedings.

\[2\] J. Wei, "Spallation Neutron Source Ring -- Status, Challenges,
Issues, and Perspectives", these proceedings.

\[3\] R. Witkover et al, "Beam Instrumentation for the SNS Ring", proc.
PAC1999, New York.

\[4\] T. Shea et al, "SNS Diagnostics", BIW2000, Boston.

\[5\] M. Plum et al, "Diagnostics Challenges at SNS", DIPAC2003,
Frankfurt.

\[6\] W. Blokland et al., "Network Attached Devices at SNS", these
proceedings.

\[7\] R. Talman and N. Malitsky, "Beam-based BPM Alignment," these
proceedings.

\[8\] R. Connolly et al, "Performance of the RHIC IPMs", PAC2001,
Chicago.

\[9\] R. Witkover and D. Gassner, "Design and Testing of the New Ion
Chamber Loss Monitor for SNS", these proceedings.

\[10\] D. Gassner, et al, "Spallation Neutron Source Beam Loss Monitor
System", these proceedings.

\[11\] M. Kesselman, "SNS Beam Current Monitor Electronics", PAC2001,
Chicago.

\[12\] P.Cameron et al, "RHIC Third Generation PLL tune System," these
proceedings.

\[13\] S. Cousineau et al, "SNS Beam-in-Gap Cleaning and Collimation,"
EPAC2002, Paris.

\[14\] R. Hardekopf et al, "Wire Scanner Design for the SNS
Superconducting-RF Linac," PAC2001, Chicago.

\*supported by US Department of Energy