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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.