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DESIGN AND DEVELOPMENT OF COMPACT ELECTRON GUN AND ITS PERFORMANCE WITH COMPACT LINAC OPERATION D. Bhattacharjee*, R. Tiwari, D. Jayaprakash, R.L. Mishra, Shiv Chandan, A.R.Tillu, R.B. Chavan, B. Nayak, V.Yadav, S.R. Ghodke, N. Chaudhary, A. Waghmare, H. Sarukte, K.C. Mittal and L.M. Gantayet Electron Beam Centre, Accelerator & Pulse Power Division BARC, Mumbai – 400085 e-mail: [email protected] Abstract The electron gun for the compact linac was designed using the CST Particle Studio Software. It is a triode gun Pierce focusing electrode of ID 23 mm and angle ~ 67.5 degrees, to give ~ 1 A of beam current at 85 kV and having a diameter of less than 3.0 mm at a distance of 100 mm from the cathode. The cathode-focusing electrode distance is 8 mm. The electron gun was fabricated, assembled and tested on test bench. It was then connected to the compact linac and beam commissioning was done. By knowing the change in rf reflected power from no beam condition to full beam load condition, the beam current was estimated. Also, injection voltage was varied to get different output radiation dose from the target. The paper presents the compact gun design and development, gun testing on test bench and its performance with compact linac. INTRODUCTION Based on the demonstration of 9 MeV LINAC at ECIL, in order to make the cargo scanning system more compact, a 6 MeV, 700 W compact linac based on magnetron as microwave generator, was developed by APPD, BARC. The 6 MeV pencil beam electrons are made incident on a tungsten target to produce x-rays with a dose rate of 8 Gy/min/m. The specifications of the linac are given in Table 1. The detailed description of the compact linac system is given in ref. 1. Table 1: Specifications of 6 MeV Compact Linac Beam Energy : 6 MeV Peak beam current : 160 mA Avg. beam power : 700 W (max) X-ray beam focal size : < 2 mm X-ray dose : 8 Gy/min/m X-ray field size : Std 30º cone Pulse width : 3.4 µs Pulse rep. rate : 250 Hz (max) Length of linac : 0.6 m RF Frequency : 2856 ± 2 MHz Injection voltage : 85 kV (max) DESIGN ASPECTS The initial 3 buncher RF structure cells are same for both the 9 MeV ECIL linac and the 6 MeV compact linac. Hence injector can be same. An injector between 50 to 85 keV can be used with a beam current of 1 A. Also, it is known that a converging or collimating beam of smaller beam diameter at the linac input will improve the capture efficiency and hence the transmission of the beam. For the compact linac, the output beam from the linac should be of diameter ≤ 2 mm at the Tantalum target. To achieve this, a Pierce gun design approach was attempted. Considering all above, we had decided to design and implement an injector of 85 keV, 1 A configuration with a beam size less than 3.0 mm at a distance of 100 mm from the cathode. To design the electron gun, simulations were conducted using the CST Particle Studio. It is a triode type thermionic gun. It incorporates a Pierce geometry grid, also known as focusing electrode. Studies were conducted to obtain good beam quality, beam size and beam current by varying (a) anode inner diameter, (b) Pierce angle of the grid, (c) cathode-anode gap, (d) cathode-grid gap, (e) anode shapes. The simulations resulted in the required 85 kV triode electron gun with Pierce focusing electrode (grid) of ID 23 mm and angle ~ 67.5 , to give ~ 1 A of beam current and having a diameter of less than 3 mm at a distance of 100 mm from the cathode. The grid biasing is -1 kV. The anode is also shaped with aperture of 10 mm. The cathode-grid distance is 8 mm and the anode-cathode distance is 30 mm. The gun is operated in the space charge region. Figure 1. CST Simulation of the 85 kV, 1080 mA Pierce triode gun with -1 kV negative bias. CATHODE AND CATHODE ASSEMBLY The cathode is a LaB6 Pellet of diameter 10 mm and thickness 1 mm with beam emission region of diameter 8 mm. The Lab6 pellet is indirectly heated by a coil comprising of tantalum heat shields, tungsten heater assembled along with lanthanum hexaboride (LaB6) pellet (of size 10 mm diameter x 1 mm thickness) as an electron emitter, a Pierce grid electrode and an anode. Cathode unit and Pierce grid electrode are supported on feedthrough flange. Anode is supported on an intermediate adaptor flange. High voltage insulation between cathode and anode is provided by an alumina ceramic (99.9 %) insulator tube. An Aluminum wire of diameter 0.5 mm was used for vacuum sealing of ceramic isolator with metal parts and it was separately leak tested with a leak rate of 2x10-9 mbar.l/s. Also asbestos based SUPER 54 gaskets were used in place of Teflon gaskets as cushion between the collar of the ceramic insulator tube and metal split flange. The adaptor flange is introduced in between the ceramic isolator and gun vacuum chamber for easier maintenance of electron gun. The whole assembly was made compact with reduced component sizes and weight. A filament connector assembly was designed, fabricated and connected to the electron gun during its integration with the linac system. It consists of delrin holder to firmly hold the filament and grid cables, and copper connectors which connect to the feedthroughs of the gun. filament made out of Tungsten wire. A filament power of 270 - 290 W raises the temperature of the LaB6 pellet enough to emit the required current of 1 A. LaB6 cathode is chosen because it has low thermionic work function 2.66 eV, high melting point 2715 C, high current density, it can work in vacuum of the order of 10 6 – 10-5 mbar. LaB6 offers the capability of long life and orders of magnitude less sensitivity to air exposure than conventional dispenser cathodes. In the case of mild cathode poisoning resulting in lower beam emission, it is observed that after conditioning the cathode to higher filament power a few times, there is a an improvement in the output beam current. The cathode pellet is housed in a 0.05 mm thk. rhenium cup which is then housed in an outer cylindrical tantalum cup of 25 mm diameter and 0.15 mm thickness. The pellet is held in place by two thin strips of rhenium which are spot welded to the tantalum cup. The opening of the rhenium and tantalum cups decides the beam emission area of the cathode to be of diameter 8 mm or 6 mm. The Lab6 pellet is indirectly heated by a coil filament made out of tungsten wire of diameter 0.5 mm and consisting of 9 turns with coil diameter of 5 mm ‘figure 2’. The gap between the tungsten wire and the pellet is about 1.5 mm. The radial heat shields consist of two cylindrical heat shields of tantalum (0.15 mm thk.) and having diameters 20 mm and 25 mm. The inner cylindrical Ta heat shield also has two Tantalum discs as heat shields in the axial direction. The inner and outer cylindrical heat shields are connected by Ta strips (2-3 mm wide, 0.15 mm thk.). The outer cylindrical heat shield is connected to the ceramic base by 4 Ta strips (2-3 mm wide, 0.25 mm thk.) and a SS 304 cup. The filament is spot welded to two tantalum rods (diameter 2.5 mm) which are rigidly supported on a ceramic base. These Ta rods have ceramic sleeves on them to electrically isolate them from the heat shields. The ceramic base sits on the SS 304 cup. One of the Ta rods is shorted to the outer cylindrical heat shield to maintain same potential of the filament and the cathode. Figure 3. Compact electron gun. Figure 4. Comparison of electron gun sizes. The right one is the compact electron gun for the 6 MeV compact linac. Figure 2. Schematic of cathode assembly, fabricated cathode assembly and filament with heat shields. GUN CHARACTERIZATION GUN FABRICATION AND ASSEMBLY The electron gun test bench consists of DN-100 CF six way chamber on which the electron gun is vertically assembled. Side ports of the chamber are used for TMP connection and a view port was also incorporated in the The components of the electron gun were fabricated which consists of the SS parts and the ceramic insulator tube. The electron gun assembly consists of a cathode unit 2 system. A Faraday cup assembly was designed and fabricated. It consists of a collector made of copper and isolated from the body. Beam current can be measured through a feedthrough on a DN 35 CF flange. The faraday cup can be mounted on CF-100 flange and connected to one of the port for beam collection and measurement. The filament transformer was rated for 70 kV and also the ceramic insulator tube withstood 72 kV insulation test. Hence the cathode-anode gap and the cathode-grid gap was changed to get a beam current of 1 A. Thus the electron gun was assembled with cathode-anode distance of 25 mm and cathode-grid distance of 9 mm. After the assembly and vacuum leak testing, the electron gun was put on test bench for emission testing and characterization. The gun was conditioned to 300 Watts of AC filament power. The I-V characteristics of the gun were measured using an in-house solid state modulator. A maximum beam current of 800 mA was extracted from gun at 66 kV anode voltage and at 296 Watts of filament power which was found repeatable. Further increase in the anode voltage resulted in inter-electrode high voltage arching inside gun so the anode voltage was limited to 66 kV. The reasons for the arching have been identified and the improvements will be incorporated in future gun assemblies. In all the above beam experiments cathode and grid were shorted to each other. With cathode-grid shorted we also have the values (a) 60 kV, 564 mA, 259 W and (b) 65 kV, 560 mA, 230 W. Grid biasing experiments resulted in the following outputs: (a) 60 kV, 232 mA, -2.8 kV, 259 W; (b) 60 kV, 872 mA, +2.8 kV, 259 W. From the VSWR value we can back calculate the output beam current to be around 100 mA. Inorganic scintillation detector, especially Cadmium tungstate (CdWO4) in form of a linear array, is used for high energy x-ray imaging applications. Therefore some beam trial experiments with CdWO4 pixels were carried out. The detector pixel was placed in beam line at 4.2 m from target and the dose rate of 0.08 Gy/min was measured at exit of primary collimator at parameters of 2.5 MW, 25Hz, 50 kV. Detector responses were measured by using oscilloscope. Dose measurement was done at exit of primary collimator at various linac parameters. The maximum dose rate of 2 Gy/min at exit of primary collimator (around 0.41 mm from target) was measured under the operating parameters of 3 MW RF peak power, 25 Hz PRF, 56 kV injection voltage & 175 A solenoid current. On extrapolation at 250 Hz, this dose rate will be 20 Gy/min. Corresponding dose rate at 1 meter from target will be 4 Gy/min which is sufficient for cargo imaging applications. Dose measurement was done at fixed linac parameters at different angles at the exit of primary collimator. Dose at +3cm, +6cm, +9 cm & +11 cm are 94%, 60%, 37% & 16% respectively compared to dose at the centre. Dose at -3cm, -6cm, -9 cm & -11 cm are 85%, 70%, 44% & 13% respectively compared to dose at the centre. Total RF ON time during this period was 80 hours, EGun Filament ON time was 100 hours & total beam ON time was 60 hours. collimator focusing coil RF Window linac cavity E-gun BEAM COMMISSIONING The gun was assembled with the linac system and after He leak testing (leak rate: 2 x 10-9 mbar.l/s), the system was evacuated to a base vacuum of 2.0 x 10 -6 mbar and then a total of 120 hours of continuous baking was done at uniform temperature of 100 0C. Vacuum of 4.0 x 10 -7 mbar is achieved in the linac. E-gun filament conditioning was carried out for 50 hours, with filament power being increased gradually up to 300 W. With this, the linac was ready for beam trials. First beam trials were carried out with the aim to determine the dose rates produced by the linac. At RF input power of 1.65 MW, 20 Hz PRF and injection voltage of ~20 kV, dose rate measured at the tungsten target was 3.1 Gy/min. Experiments were conducted to study the effect of various linac parameters on the measured dose rate. The parameters included pulse repetition rate, injection voltage and RF Power. Dose rate increase was linear with all the three parameters one at a time. Effect of Reflected Power with E-Gun Injection Voltage was also seen. At 2.0 MW RF power reflected power without beam was 45% (VSWR 5.0), which reduced to 28% (VSWR 3.2) at 57 kV injection voltage. Figure 6: Linac under vacuum & RF conditioning CONCLUSION The compact electron gun was designed for 85 kV and performed up to 66 kV, 800 mA. Integration with the linac system resulted in an output dose sufficient for cargo scanning applications. REFERENCES [1] Shiv Chandan et al, “Commissioning and beam trials of 6 MeV RF electronlLinac for cargo scanning”, this conference. 3