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DEVELOPMENT OF A PULSE MODULATOR TO DRIVE 6.1 MW KLYSTRON FOR 15MeV ELECTRON LINAC Kiran Thakur # , R Krishnan, S N Pethe, Rahul Patil # Society for Applied Microwave Electronics Engineering and Research (SAMEER), Research and Development Institution under Department of Electronics and Information Technology, I.I.T. Campus, Powai, Mumbai, 400 076, India Address and # [email protected] # [email protected] AbstractWe present the Design, construction, and characterization of a line-type pulse high-voltage modulator system capable of generating 140-kV, 6 µs pulses width at 150 pulses per second into a Klystron as load. The modulator is used to energize a variety of highpower microwave devices requiring voltage stability and reproducibility. Voltage ripple is less than 0.4% during the flat top, with a shot-to-shot voltage variation of less than 0.2 %. The primary circuit consists of six-stage tuneable pulse-forming networks (PFN’s). The PFN is charged by a highly stable charging power supply. The total energy stored is released through a CX-1559 E2V make thyratron into a 1: 11 pulse transformer, which generates 140 kV, 6 µs pulses. The flat-top voltage generated by the modulator is highly desirable for driving RF sources requiring high quality electron beams. Linac is one of the important fields of study in modern technology due its wide applications for Medical and Industrial purposes in addition to physics research. Keywords—Line-type Modulator, Klystron, PFN, Pulse Modulator, Pulse Transformer, Linear accelerator. INTRODUCTION A microwave tube Klystron or Magnetron requires high voltage pulsed power supplies, commonly referred as Modulator. The important parameters of the modulator are Pulse power, Pulse duration, duty ratio and impedance. The 13 MW compact pulse modulator is designed and tested with S-Band Klystron. The modulator has a careful selection and placement of components. The modulator is built with safety interlock like over current and impedance mis-match faults. The mechanism of De-Qing of charging inductor is used to regulate the pulse to pulse voltage regulation. The paper discusses development of a modulator and compares the design values with the measured. Table 1. shows the main specifications of tested modulator at SAMEER. The S-band Klystron is used of CSI make, Model-VKS-8262H. Table 2- shows the Specification of Klystron VKS-8262H. MODULATOR AND KLYSTRON SPECIFICATIONS Table 1. Shows the specifications of Modulator developed at SAMEER. Electrical Parameter Units Peak power output Average power output PFN charging voltage PFN impedance Pulse width Pulse rise time Pulse Repetition rate Pulse transformer ratio Thyratron peak current Thyratron peak voltage MW kW kV Ω µs µs Hz A kV Achieved Value 13 11.7 26 12.5 6 0.5 0 to 150 1:11 1000 13.2 Table 2- Specification of CPI make Klystron VKS8262H Electrical Parameter VKS-8262H Peak RF output power Klystron Beam voltage Klystron Beam current Pulse width (RF) Peak Drive power Duty ratio Frequency Units Value MW 6.19 kV 135 A 98.4 µs W 8.5 120 0.0026 2996-3001 MHz KEY DESIGN CONSIDERATIONS AND DESIGN DECISIONS Figure 1: Simplified diagram of Modulator The 14 kV DC supply was designed to charge the PFN capacitor. Taking into consideration the voltage and current of the klystron, which act as load, is calculated as, Impedance of Klystron = Zo = 132kV / 91A = 1450.5 Ω Pulse transformer turns ratio = N= Zo / Zin The pulse transformer used is having the turn ratio of 1:11.This transformer is having bi- filler secondary through which the klystron filament supply is fed. Commercially available tri-axial cables RG8-A of 50 Ω impedance is used to transfer energy from modulator to input of pulse transformer. The four cables are used in parallel, so the input impedance is Zin = 12.5 Ω is matching. There are many topologies that can be used to form a Pulse Forming Network (PFN). However a PFN with mutual coupling, which is called the Guillemin type E network, is used to improve the pulse characteristic. In Guillemin type E network, the value of capacitance and inductance are same in each section and there is a mutual inductance introduced between adjacent inductors. PFN with mutual coupling around 12-20 % between the adjacent inductors will improve the desired pulse shape and keeping the last inductor’s value with 25 % higher reduces the overshoot and ringing in the discharge pulse. The PFN is designed for 6 µs pulse width with 12.5 Ω impedance. The total capacitance Cn and total inductance Ln is of PFN is calculated as, Cn = T 2 Zin and Ln = B- is length of coil in inches= 24.5 N- is number of turns of coil= 69 L- is inductance in µH The PFN capacitors are charged up-to 26 kV, hence CSI make capacitor 0.047µF/35 kV are used for making the PFN. The charging choke is used to charge the PFN. Higher the inductance of charging choke better the isolation for thyratron from EHT side, but slower becomes the rate of charging PFN. So compromise is made between the charging time and isolation. The inductance of charging inductor is calculated by considering the charging time 1.5 ms. T = π LCn WhereL- Inductance of charging choke Cn- Total capacitance of PFN network The main pulse transformer primary current goes up to the order of 1000 A. So the switching device of E2V Technology, Model- CX -1559 is used. The modulator is protected from over current and mis-match faults. The klystron beam current waveform at 91A is as shown. IV. EXPERIMENTAL RESULTS AND ACHIVED PERFORMANCE Figure 2: Klystron Beam Current Waveform. TZin 2 The calculated Cn is 0.248µF and inductance is 45 µH. The PFN inductance coil is made from cupper tube of Ø 6 mm. The coil is air core single layer. The coil is designed by formula- Where – A- is mean diameter of coil in inches = 2.75 Figure.3: Charging Current waveform of charging reactor with De-Qing. are also focused interference. to reduce the electromagnetic Figure 4: Klystron beam current (Yellow) & forward power (Red) at directional coupler placed next to Klystron. Figure 5: Peak Beam voltage verses peak output power of Klystron. Figure 6: View of the Modulator CONCLUSIONS In this work, it was reported design, construction and characterization of the line-type pulse modulator. The simulated results showed a good agreement with experimental results. The modulator is capable of driving Klystron to deliver 6.19 MW peak power with 6 µs pulse width at 150 Hz repetition rate. The input peak drive power fed to Klystron is 130 watt. Hence, the modulator design is suitable for driving high power Klystron. Efforts REFERENCES [1] www.ga-esi.com [2] G.N. Glasoe and J.V. Lebascqz, Pulse Generator, New York: McGraw-Hill,1948. [3] Medical Electron Accelerator by C.J. Karzmark