Download INTERLOCK SYSTEM WITH FAST RESPONSE AGAINST

INTERLOCK SYSTEM WITH FAST RESPONSE AGAINST SUDDEN
FOIL RUPTURE AND VACUUM FAILURE IN LINAC STRUCTURE
V.C. Petwal#, Ajay Kumar, A.K. Jain, R.S. Choudhary, M. Seema, A. Kasliwal, Y. Sheth,
R.Sridhar, J. Dwivedi, Raja Ramanna Centre for Advanced Technology, Indore, 452013.
Abstract
A 10 MeV electron Linac is operational at RRCAT,
Indore. To extract the intense electron beam from vacuum
to atmosphere, it is scanned inside a vacuum chamber
sealed with titanium foil at the exit. If scanning locks or
fails during high power operation at 200-300 Hz pulse
repetition rate, the un-scanned beam will burn a hole in the
window, leading to sudden failure of linac vacuum. Under
such accidental situation, if subsequent RF pulse is
delivered to the linac, the structure will get damaged. The
interlocks used for vacuum failure detection are generally
taken from the head of cold cathode gauge, which has large
response time, hence not effective to protect linac under
such accidental condition.
We have carried out a series of experiments to
simulate the process of accidental vacuum failure and
investigated various methods for fast detection of vacuum
failure and subsequent inhibition of RF pulsess. The
response time of the interlock from the vacuum gauge was
measured to be 160 ms. Response of the analog output
signal of gauge controller corresponding to change in
vacuum is relatively fast but takes 20-30 ms to reflect
measurable change in analog value. The ion current of the
SIP is found very sensitive towards vacuum and responds
very fast for any change in the vacuum level. An interlock
circuit has been designed which senses the ion current
signal of SIP and stops the master trigger system of the
linac in less than 3 ms after rupture of the foil. Results of
the experiments and design details of the interlock system
are presented in this paper.
INTRODUCTION
A 10 MeV industrial electron linac is operational at
RRCAT. Diameter of the electron beam coming out from
accelerator window is 10-20 mm. In order to extract the
electron beam into the atmosphere for industrial
applications, it is scanned inside a vacuum chamber sealed
with a titanium foil mounted at the exit. It is realized that
during high power operation at 200-300 Hz pulse repetition
rate, if scanning locks or fails, the un-scanned beam will
burn a hole in the window, leading to sudden failure of
linac vacuum.
Moreover, there may be a sudden
mechanical rupture due to weakening or embrittlement of
the foil. Under such accidental situation, if subsequent RF
pulse is delivered to the linac, the structure will get
damaged. The interlocks used for vacuum failure detection
are generally taken from the cold cathode gauge, which has
#
Email: [email protected]
large response time, hence not effective to protect linac
under such accidental condition. Similar accidental event of
sudden vacuum failure may arise in synchrotron beam lines
or storage rings leading to severe damage to the facility.
Hence a fast responsive interlocking system to detect
sudden vacuum failure is highly required to protect the
accelerators against such catastrophic eventualities. A series
of the experiments have been carried out to simulate the
event of sudden vacuum failure and response time of the
vacuum monitoring devices installed on the linac is
analyzed and functionality of the fast interlocking system
developed for the purpose is demonstrated.
LINAC VACUUM SYSTEM
Figure 1 shows the vacuum system of the linac which
is about 3 m long starting from electron gun to the end of
scanning horn sealed with titanium foil of thickness 50 μm
at the exit. Vacuum better than 1x 10-6 Torr is required
inside the Linac for its proper operation. There are 4 Nos.
of SIP’s installed on the linac. Two SIPs are installed
nearby the electron gun, one at the exit end of linac
structure and one on the scanning horn. Two cold cathode
gauges (Pfeiffer make, model No PKR 251) are installed to
monitor vacuum level inside the linac. The power supplies
of SIPs and the controllers of the cold cathode gauges are
placed in the equipment room located ~20 m away from the
accelerator. During normal operation vacuum inside the
linac as shown by the gauge-1 and gauge-2 is 2 x 10-8 Torr
and 3 x 10-8 Torr respectively.
DESCRIPTION OF THE EXPERIMENT
Figure 2 shows the experimental setup prepared to simulate
the sudden rupture of the foil and subsequent measurement
of the response time of various devices used for linac
vacuum monitoring. A vacuum chamber having five ports,
suitable for mounting different devices was fabricated. The
first port is connected to a TMP (Varian, Turbo VG301) for
initial evacuation of the chamber to a vacuum level of 10 -2
Torr. The second port (bottom side) is connected to a SIP
(70 ltr capacity). The required electric field between anode
and cathode is supplied through a 6 kV dc power supply.
Vacuum level of 10-7 Torr is achievable inside the chamber.
Third port is connected to a cold cathode gauge (Pfeiffer
make) and the vacuum level is displayed on gauge
controller (Pfeiffer make, TPG 261). A needle valve is
connected to the fourth port for controlled venting of the
chamber. A ceramic chamber top of which is sealed with 50
μm thick Ti foil is connected to the fifth port.
Figure 1: Vacuum system of the Linac
Figure 2: Experimental set-up for response time measurement and testing of interlock circuit
The ceramic chamber isolates the Ti foil from the
remaining system. A sharp metallic needle (javelin) is
used to rupture the Ti foil suddenly. A CRO (Lecroy
make) is connected to measure and record the various
events/signals.
Schematic diagram of the experimental setup is
shown in Figure 3. The controller provides two output
signals; an analog out voltage (0-10 V) corresponding to
the vacuum level sensed by the gauge head and a relay
contact (NO). The relay contact changes status as the
vacuum level falls below the set value of 1x 10 -6 Torr.
Both the signals are connected to the CRO. SIP
generates penning discharge ionization current when gas
molecules present inside the chamber are ionized and
these ions are trapped on the surface of the cathode kept
at high potential 6kV. The ionization current, which is
inversely proportional to the vacuum level inside the
chamber, is passed through a 500 Ω resistor and the
voltage is measured by the CRO. The puncturing javelin
is also connected to the CRO through a 15 V dc supply
and generates a trigger signal as it touches the titanium
foil. During the experiments the javelin rapidly pierces
the titanium foil and generates the trigger signal for the
scope to start capturing the events on other channels.
RESULTS AND DISCUSSION
Vacuum level of 1x 10-7 Torr was achieved inside
the experimental chamber. The titanium foil is
forcefully punctured by sharp javelin leading to rapid air
rush inside the chamber causing increase in the
ionization current of the SIP. As the rupturing event is
performed very forcefully delay between foil touching
and rupturing is negligible. Figure 4 shows the screen
shot of the scope recorded when foil is punctured. The
event of foil touching/ rupturing is recorded in ch-2, and
variation in SIP ion current (in voltage form) is recorded
in Ch-1. Analog signal and relay contact status of gauge
controller is recorded at Ch-3 and Ch-4 respectively.
Measurements shows that the response time of the relay
contact is very large and takes about 160 ms to change
Figure 3: Schematic diagram of the experimental system
the status after the foil is ruptured. The analog output
voltage of the controller changes slowly and takes about
20-30 ms to reflect measurable change in the voltage
signal. Hence the response time of the both output
signals, which are conventionally used for vacuum
interlocking are very high as compared to the RF pulses
coming at an interval of ~ 3- 4 ms. Thus both these
signals are not favorable choice for fast interlocking
system. However, the ion current of SIP (Ch-1) is found
to be very sensitive and fast towards any deterioration in
vacuum level and provides an excellent opportunity to
gauge the instantaneous vacuum of the system. The SIP
ion current converted to the voltage signal is further
explored for fast interlocking system.
In the subsequent experiment, voltage signal
corresponding to SIP ion current is taped to energize an
interlock circuit. The reference voltage of the
comparator used in the interlocking circuit is set to
1.35V corresponding to the vacuum level of 1 x 10 -6
Torr. When vacuum level deteriorates and crosses the
pre-set value, output of the interlocking circuit flips and
latches to detect and record the unhealthy condition.
This switching event fires a solid state relay (turn on
time 150 μs) to disable the master trigger generator.
Figure 5 shows the screen shot of the scope as
recorded during the second experiment. The
measurements show that the SIP ion current remains
constant (indicating good vacuum condition) for about
1.5 ms after the foil is ruptured and then start
deteriorating very quickly. The output of the interlock
circuit changes status within 2.2 ms after foil rupture.
In the third experiment output of the interlock
circuit is connected to the solid state relay (SSR) of the
master trigger generator used for triggering the
microwave system at 1-300 Hz pulse repetition rate.
When the titanium foil is ruptured again, the interlock
system detected the sudden vacuum failure and acted to
disable the master trigger generator within 2.5 ms.
Figure 4: Various events captured by CRO (Ch-1: SIP
ion current, Ch-2: Foil rupture event, Ch-3: Analog
output of controller, Ch-4: controller relay output.
Figure 5 Various events captured by CRO (Ch-1: SIP
ion current, Ch-2: Foil rupture event, Ch-3: Output of
the interlock circuit, Ch-4: Controller relay output.
CONCLUSION
Experiments have been carried out to measure the
response time of gauge controller output signals,
conventionally used for vacuum monitoring and
interlocking. The measured value of the relay output
and analog signal of the gauge controller is 160 ms and
25 ms respectively. The ion current of the SIP is found
very sensitive and fast to detect the instantaneous
change in the vacuum level. A fast interlocking circuit
has been designed and tested to capture the sudden
vacuum degradation by sensing SIP ion current. The
time taken by the interlock circuit to detect the poor
vacuum condition (<1x10 -6 Torr) and disable the master
trigger generator is less than 2.5 ms, after rupture of the
titanium foil. Hence the interlock system is suitable to
inhibit the RF pulses coming at an interval of 3-4 ms
Based on the results of the experiments the fast acting
interlocking system has been developed and installed on
the SIP-4 of linac, which is working satisfactorily.