Proceedings of IBIC2012, Tsukuba, Japan

MOPA06VIMOS, NEW CAPABILITIES FOR AN OPTICAL SAFETY SYSTEM
K. Thomsen, J. Devlaminck, PSI, 5232 Villigen, Switzerland

VIMOS is a dedicated safety system developed at theSpallation Neutron Source SINQ at the Paul Scherrer
Institut, PSI, in Switzerland. VIMOS very directly
monitors the correct current density distribution of the
proton beam on the target by sampling the light emitted
from a glowing mesh heated by the passing protons. The
design has been optimized for obtaining maximum
sensitivity and timely detection of beam irregularities
relying on standard well-proven components. Recently it
has been demonstrated that technical boundary conditions
like radiation level and signal strength should allow for
upgrading the system to a sensitive diagnostic device
delivering quantitative and image-resolved values for the
proton current density distribution on the SINQ target. By
determining the temperature of the glowing mesh fromthe signals in two separate wavelength bands thetemperature distribution over the mesh can be derived und
subsequently the incident proton beam current density
distribution. Work aimed at investigating the feasibility ofadding these diagnostic abilities to VIMOS shows initialpromising results.

VIMOS, SAFETY SYSTEM
One of the outstanding features of the SINQ neutron
spallation source at PSI is its vertical insertion with theproton beam impinging on the target from directly below.
Whereas this design is favourable with respect to thecirculation inside a liquid metal (LM) target, it entails thesevere risk of major facility damage in case of a leak. To
minimize the risk of burning a hole through the liquid
metal container with the beam and subsequently
perforating the lower target enclosure of the MEGAPIE
LM target, a new and additional safety system named
VIMOS has been devised and successfully installed [1,2].
VIMOS is based on a most simple and direct approach,i.e. it monitors the glowing of a tungsten mesh inserted
into the proton beam closely spaced in front of the target.
This mesh is heated by the passing protons, and VIMOS
watches for deviations from the expected visual signal. In
case that any hot spot due to unintended beam
concentration is detected, an alarm is triggered and theaccelerator is switched off in less than 100 ms. In order toreact most quickly to any unexpected increase of theglowing signal from the mesh, VIMOS has purposely
been designed with a strongly non-linear response
function [2].
VIMOS works reliable since its installation before the
MEGAPIE target irradiation started and it has proven its
value actually already in the commissioning phase. Now
it is routinely used for beam monitoring during normal
operations by the operators of the accelerator also with
SINQ’s standard solid state targets.

Aim of an Upgrade
In the light of the proven reliability and usefulness of
VIMOS, the wish for additional capabilities of the system
has emerged. One demand was to somehow calibrate thesystem for a display of beam current density. The otherrequest is for enhanced sensitivity, i.e. responding also atlower current (-densities), to provide more information
for set up, in particular at phases when the beam at low
current is newly directed onto the SINQ target.
As the original safety goal of reacting only to hot spots
imposes some limitations and inherent difficulties for
obtaining more detailed diagnostics’ information on beamprofile during normal operations, a comprehensive
upgrade of the system has been envisioned [3].

IMPLEMENTATION OF DIAGNOSTICS
One stringent boundary condition right from the start
was that the safety function of VIMOS should stay
untouched, new features should only come on top and as
far as possible without interfering with the well-proved
safety relevant installation.
Following a stepwise approach, several pre-conditions
and key issues have first been identified and scrutinized
before designing added diagnostic capabilities.

Enabling Conditions
Originally, VIMOS employed a radiation resistantcamera, which was directly mounted in the focus of thecolleting mirror and thus exposed to irradiation by backstreaming protons and high energy neutrons [4].
Epithermal neutrons in particular caused damage to thesemiconductors in the camera as, after being moderated
by scattering in the shielding in the surroundings, theychanged the doping and thus impaired proper functioning
after several months of operational exposure.
In order to increase the lifetime of the expensive
cameras and also in anticipation of possible future
upgrades, the camera position has been moved 3 meters
further away. The light is now transmitted to the newcamera position on a wall of the SINQ beam vault bymeans of a radiation resistant image guide [5].
With the improved possibility of packing substantial
graded shielding around the sensor, the original tubebased camera has been replaced by a more standard offthe shelf CCD camera [6].
A light guide and the first shielded CDD camera havebeen installed in November 2008 and work since then as
expected. The camera shows only acceptable damage inthe form of increased noise, which does not impede theproper functioning of the system. Little and not quantified
radiation damage meaning increased attenuation is seen in
the image fiber.
ISBN 978-3-95450-119-9

Beam Charge and Current Monitors

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○AbstractMOPA06

Proceedings of IBIC2012, Tsukuba, Japan

Approachc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Signal-Generation, -Transfer and -Evaluation
Following the path of the light and its detection in two
channels, the stations of the signal enlisted below havebeen identified and investigated.
x Glowing mesh: unchanged, approximately 1 per millof the beam power is deposited; cooling happens via
radiation at predominantly infra-red wavelengths
x Mirror: unchanged, almost flat reflection over thefull wavelength range in question between 400 and1100 nm (small dip at ~800 nm)
x Light guide: unchanged, determines the transmitted
wavelengths as given by the doped quartz material,
non-uniform transmission with a pronounced peak inattenuation around 950 nm
x Beamsplitter: newly inserted, neutral density, allows
light from the fiber into the new branch fortemperature determination while leaving the safety
channel ending in the now qualified CCD camera(KP-M2AP) almost untouchedx High/Low -pass filter: splits short-wavelength lightoff, while letting the longer near-IR passx Camera for visible: same as in the safety channel
x Near IR-camera: very similar to the type in the otherchannels but with enhanced near-IR response (KPM2RP)
x Image analysis software: to be written newly,comparing the relative strength of the signals in thetwo wavelength bands for current density
determination and some additional new functionality
To allow for the general adaptation of the amount of lightentering the fiber, grey filters can be inserted at the end of
the light guide close to the mirror, which is actually alsodone already now.

Beamsplitters

Fiber

Camera3

80%



Long

wavelengths

20%
Short
wavelengths


Camera1
Camera2

Figure 1: Principal sketch of optical arrangement withthree cameras for safety (camera 1) and for local
temperature / current density determination (cameras 2
and 3).

FIRST RESULTS
In a first attempt to assess the feasibility of the intended
upgrade, the light signal emitted by the glowing mesh was
determined and then folded with the transmissions of theindividual elements along the optical path. This was donein two steps, starting with deriving the calibration of local
mesh temperatures with respect to current density bymeans of numerical simulation, see Fig. 2.

Temperatures (in the Center of the Mesh)
1050
1000
950
T cen ter [°C ]With no way of changing any component along thesignal path for the maintained safety branch of an
upgraded system, the only possibility of obtaining more
information was to split off part of the light from themesh and analyse it more thoroughly than required fortriggering alarms.
In order to have a signal under normal operational
conditions, the glowing mesh had been designed to heat
up to about 900 °C at a nominal current density maximum
close to 30 μA/cm2. Thus, ample light at infra-red
wavelengths is emitted by the mesh already at lower
temperatures; this might be used for increasing the overallsensitivity. For higher temperatures, the total amount oflight increases steeply (T4) and the maximum of theemission is shifted to shorter wavelengths.
As the relative contribution of visible light to the total
increases with higher beam current densities it is possible
to determine local mesh temperatures, and thus ultimately
beam current densities, by comparing the relative signal
strengths in a short versus a long wavelength band [3]. A
principal sketch of the optical arrangement with separated
channels is depicted in Fig. 1.

900
850
800
750
700
650
10

20

30

40

Proton Beam Current Density -> SINQ [uA/cm ]2

Figure 2: Over the range of usual operational beam
conditions on the SINQ target the maximum meshtemperature changes in nice correspondence to theproton beam current density.
As the outcome of an initial very coarse approximation
was positive, calculations have been performed taking therelative transmissions all optical components for different
wavelengths into account with a binning of 20 nm.
It can be stated that the expected relative rise in the
short wavelength channel is significantly larger than for
the near-IR channel. Thus, temperature determination
based on the ratio between the signals in both channels
appears possible at a level good enough for being usefulto operators, see Fig. 3.

ISBN 978-3-95450-119-92Beam Charge and Current Monitors

Proceedings of IBIC2012, Tsukuba, JapanFIRST EXPERIMENTAL FINDINGS

7.00

6.00

KPM2RP

0,6mA/12A/cm²
0,8mA/16A/cm²

5.00

1mA/20A/cm²
Signal(a.u.)

MOPA064.00

1,2mA/24A/cm²
1,4mA/28A/cm²

3.00

Given these encouraging theoretical results concerning
the feasibility and diagnostic usefulness of the intended
VIMOS upgrade, an optical set-up has been built fromstandard components. With this, first temperatures havebeen determined simulating the glowing mesh by meansof an ordinary incandescent light bulb and simple image
analysis software, see Fig. 5.

1,6mA/32A/cm²
2.00

1,8mA/36A/cm²

KPM2AP

2mA/40A/cm²

1.00

0.00
450

550

650

750

850

950

1050

Wavelength(nm)Even in the relatively narrow window of transmission
as determined by the image light guide, temperatures in
the interesting range produce significant signals. The
diagnostic channel thus should yield a useful diagnostics’reading, and systematic errors due the changes in signaltransmission with length or irradiation appear to be oflimited impact, see Fig. 4.

0.19000

Ratio(short/long)witholdfiber

0.17000

Ratio40cm
ratio(onthedesk)

Ratio

0.15000

0.13000

Figure 5: Using a dimmed incandescent light bulb as lightsource, it was possible to derive meaningful temperatureswith a simple optical set-up and image analysis software.

Next Steps
Taking for granted that more thorough theoreticalinvestigations will further confirm the here reported
preliminary findings, effort will then be directed to therequired dedicated software.
Just to mention one additional planned feature: byintegrating the local values for the current density
distribution over the whole frame, a number for the total
proton beam current can be obtained. Comparing thisvalue to the actually measured current, a measure for theplausibility / credibility can be derived and possible errors
in the diagnostics’ display can be detected.
Assuming steady progress, an optical assembly will be
installed in the SINQ beam vault during the next service
shut down at the beginning of 2013.

0.11000

ACKNOWLEDGMENTS

0.09000

Most valuable contributions by A. Kalt for the optical
set-up are gratefully acknowledged.

0.07000

0.05000
0.6

0.8

1

1.2

1.4

1.6

1.8

2

Intensity(mA)

Figure 4: Simulated response ratios: short- / longwavelength band signals as a function of total beam
intensity, which is corresponding to central beam current
densities as indicated in the legend of Fig. 3; fiber
degradation due to exposure to irradiation can be
expected to have about the same systematic effect asswitching to a short fiber, as long wavelengths areattenuated relatively less in both conditions.

REFERENCES[1] W. Wagner, F. Gröschel, K. Thomsen, H. Heyck, Journalof Nuclear Materials 377 (2008) 12.
[2] K. Thomsen, NIMA 575 (2007) 347.
[3] K. Thomsen, “VIMOS, Beam Monitoring for SINQ”,
DIPAC 2009, Basel, May 2009, Proceedings, p. 206.
[4] Arcos 14c is produced by Opticon GmbH, Nobelstrae 2,
D-76275 Ettlingen.
[5] http://www.fujikura.co.uk/fibre_optics/
products/speciality_fibre/figr.html[6] http://www.hitachikeu.com/content/industry/kameras_sw/kp_m_serie_e.htm

ISBN 978-3-95450-119-9
Beam Charge and Current Monitors

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 3: Selecting a beam splitter, which discriminates
wavelengths at nominal 700 nm yields usable signal
strengths in both channels; the signal in the visible range
increases faster with beam current (density) than thesignal from the near-IR.