BALKAN PHYSICS LETTERS Bogazici University Press 15 November 2016 BPL, 24, , pp , (2016)

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1 139 BALKAN PHYSICS LETTERS Bogazici University Press 15 November 2016 BPL, 24, , pp , (2016) VACUUM STUDIES ON ELECTRON GUN AND INJECTOR LINE OF TARLA FACILITY E. COŞGUN, E.P. DEMİRCİ, Ö. YAVAŞ Physics Engineering Department, Ankara University, Ankara, TURKEY. A. AKSOY, Ç. KAYA, E. KAZANCI, B. KOÇ Institute of Accelerator Technologies, Ankara University Ankara, TURKEY. Abstract. TARLA Facility (Turkish Accelerator and Radiation Laboratory in Ankara) is a superconducting electron linac based on free electron laser and bremsstrahlung facility that is under construction as the first facility of Turkish Accelerator Center (TAC). Main parts of TARLA facility are e-gun, injector line, beam transport lines, RF cavities, optical cavities, and experimental stations to use IR FEL and Bremsstrahlung radiation in research. All components must be kept under ultrahigh vacuum (UHV) to prevent electron-beam scattering. To reach UHV, three IP s (Ion Pumps) will be installed on injector line. In order to reach HV (High Voltage) level inside the e-gun one must go down to mbar regimes. For pre-vacuum two portable TMP (Turbo Molecular Pump) will be used. Cleaning and degassing procedures will be applied, leaks will be measured by a He leak detector and the vacuum pressure will be measured by gauges. In this study, details of design, installation and tests of vacuum system of e- gun and injector line of TARLA facility are explained. *Corresponding author: e-cosgun@hotmail.com Work supported by Republic of Turkey Ministry of Development (Grant No: DPT2006K ) 1. INTRODUCTION TARLA is an electron accelerator based on free electron laser facility with 1mA current in Continuous Wave (CW) operation and has an electron beam up to 40MeV energy. The FEL generation hall will consist of two independent optical resonator systems housing planar undulators with periods of λ U90 = 90 mm and λ U25 =25 mm. Electron beam will be used to produce bremsstrahlung radiation for fixed target experiments, also. Injector line of TARLA facility consists of the all components from electron gun to the first accelerator module, including the electron gun. The schematic view of the facility is given in Figure 1.

2 140 BALKAN PHYSICS LETTERS Figure1: Schematic view of TARLA Facility The injector line is based on totally normal conducting technology and consists of a 250 kev thermionic DC electron gun, two buncher cavities that operate at 260 MHz and 1.3GHz, five solenoid magnets and several steerer magnets. All components must be kept under ultrahigh vacuum (UHV) to prevent electron-beam scattering and reach HV level up to 300kV. Schematic view of vacuum design of injector line is shown at Figure 2. Figure 2: Vacuum design of the injector line

3 E. COSGUN et al. : VACUUM STUDIES ON ELECTRON GUN MATERIALS AND METHODS Vacuum Definition, Ranges A vacuum is a space from which air or other gas has been removed. The amount we need to remove depends on the application, and we do this for many reasons. At atmospheric pressure, surfaces are constantly bombarded by molecules. The vacuum environment plays a basic and indispensable role in present day technology and is used by a wide variety of scientists including physicists, chemists, biologists and engineers who work in research, development and industrial production. The standard international (SI) unit of pressure is pascal (Pa) which is equivalent to Newton (N) per square meter (m 2 ) but it is still quite common to use milibar (mbar) which is not SI unit. The milibar is quite convenient because, in the majority of applications of vacuum technology, the concept of force per unit area is not relevant and we are more likely to be interested in molecular density. Among other units the most common one is Torr. The relationship between these units and atmosphere, that is a natural unit of pressure, is: 1 atmosphere=760 torr = 1013 mbar = 1.013x10 5 Pascal. The variety of applications of vacuum technology demands that the range of pressures should extend over more than fourteen orders of magnitude and it is useful to divide the total pressure range into four regions. The Table 1 shows vacuum ranges. Table 1: Vacuum ranges Vacuum Ranges Pressure Range (mbar) Pressure Range (Pa) Low vacuum 1.0x x10 1.0x x10 3 Medium vacuum 3.3x10-1.0x x x10-1 High vacuum 1.0x x x x10-4 Very High vacuum 1.0x x x x10-7 Ultra high vacuum 1.0x x x x10-10 Extreme Ultra High vacuum 1.0x x10-10

4 142 BALKAN PHYSICS LETTERS Vacuum Procedure of Injector Line In order to provide a comfortable path for the particle beam (to increase the beam lifetime and also the beam quality) and in order to provide a clean environment for the critical components (to keep their high performance) the vacuum is applied to many of the components of accelerator like facility. For example the less gas molecule density on beam path, the less interaction between accelerated particles and gas molecules, the less beam loss and ionizing radiation. In another example to reach high gradient acceleration in a DC accelerator such as TARLA gun, the environment, where the applied voltage, must be very clean. Design of vacuum system is quite complex work which has close connection with beam dynamics. The vacuum procedure of injector line is given in below; Mechanical Pump: Used from atmospheric pressure (10 +3 mbar) to 10-3 mbar. Turbo Molecular Pump (TMP): Used in the range between mbar. In addition to TMP, there is a mechanic pump station on it. In order to pump down the injector line, there are two portable TMPs. Ion Pump: Used in the range between mbar. There are three 150 l/s and one 55 l/s pumps on the injector line. Cold and Hot Cathode Gauges: Used for monitoring the level of vacuum to compare the vacuum pumps. Pneumatic Valves: Used for divide the injector line. Main aim of this partition is to be able to interfere one part which keeps the other parts in vacuum at injector line. In addition, in TARLA, leak detector is used with minimum detectable helium leak rate <5x10-12 mbarl/s. Leak detection is used to determine if where a leak has occurred in systems which contain liquids and gases. Clean Room Procedure Typically used in manufacturing or scientific research, a cleanroom is a controlled environment that has a low level of pollutants such as dust, airborne microbes, aerosol particles and chemical vapors. To obtain ultrahigh vacuum, a clean room is necessary. In TARLA Facility, installation of clean room is in progress. In addition to, there is also a portable cleanroom. All components of vacuum chamber have to be clean. Cleaning procedure is given in below;

5 E. COSGUN et al. : VACUUM STUDIES ON ELECTRON GUN 143 To perform cleaning, water get from water purification system resistivity of pure watervapor (DIWater) should be 18.2 Mohm.cm. The ultrasonic cleaner is used for 15 minutes at 65 0 C for the contaminations on components, for oil based contamination an industrial detergent is also used. Pure water is sent to ultrasonic cleaner and vacuum materials are cleaned with it. After ultrasonic cleaning, an oven is used to remove the water vapor for 60 minutes at 85 0 C. Clean rooms are classified according to the number and size of particles permitted per unit volume of air. Table 2 shows clean room standards. Table 2: Cleanroom standards Class Maximum particles/ft 3 ISO 0,1µm 0,2µm 0,3µm 0,5µm 5µm equivalent , ,007 ISO ,07 ISO ,7 ISO ISO ISO ISO 8 To detect the amount of particles per ft 3, particle counter is used. In order to reach UHV level the clean room should be higher than ISO 5. When we performed the count, we obtained the results which are given in Figure 3. It is seen that the quantities of size of particles are nearly in the range of ISO 6.

(UHV region). Therefore, this clean room is an acceptable environment to reach the UHV region.

6 144 BALKAN PHYSICS LETTERS Figure 3: Particle counter at clean room On the other hand we designed a vacuum test setup in the clean room and the vacuum level reached to 1x10-11 mbar (UHV region). Therefore, this clean room is an acceptable environment to reach the UHV region. Figure 4 the shows vacuum level from ion pump controller. Figure 4: Vacuum Test Setup at clean room

7 E. COSGUN et al. : VACUUM STUDIES ON ELECTRON GUN CONCLUSION TARLA beamline should be kept under UHV conditions ( mbar) for two main reasons. Any interaction between electrons and gas molecules along the beam path causes the electrons scatter, resulting beam loss. This effect should be taken into account especially at the low energy region. Another main reason is to avoid voltage sparks between high and low potential regions produced in electron gun and cavities to generate accelerating gradient. It is expected that the vacuum level will be around mbar in injector line. We expect that this level will decrease to the 10-9 mbar when the electron beam is passing through the injector line. REFERENCES [1] P. Chiggiato, Vacuum Technology for Ion Sources, CERN, Geneva, Switzerland. [2] G. F. Weston, Ultra High Vacuum Practice (Butterworth's, London, 1985). [3] D.Y. Tripenyuk, Vacuum Technology, General Physics Institute RAS, Valilova 38, , Moscow, Russia. [4] N. Marquardt, Introduction to the Principles of Vacuum Physics, Institute for Accelerator Physics and Synchrotron Radiation, University of Dortmund, Dortmund, Germany. [5] A. Aksoy et. Al, TARLA Design Report, IAT(2015). [6] [7] [8]

ION Pumps for UHV Systems, Synchrotrons & Particle Accelerators. Mauro Audi, Academic, Government & Research Marketing Manager

ION Pumps for UHV Systems, Synchrotrons & Particle Accelerators Mauro Audi, Academic, Government & Research Marketing Manager ION Pumps Agilent Technologies 1957-59 Varian Associates invents the first