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Development of High Voltage Surge Limiting Resistor for Protection of HV Multiplier of 3MeV DC Accelerator S. Dewangan*, D.K. Sharma, R.I. Bakhtsingh, R.N. Rajan, V. Sharma, R Barnwal, S. R. Ghodke, S.K. Srivastava, R. Patel, S. Banerjee, Mahendra Kumar, S. Nayak, S. Gond, N.B. Thakur, A. Waghmare, K.C. Mittal and L.M. Gantayet Electron Beam Centre, Accelerator and Pulse Power Division, Bhabha Atomic Research Centre, Sector-7, CBD Belapur, Navi Mumbai *[email protected] Abstract A 3MeV, 10mA DC electron beam accelerator is in commissioning stages at EBC, Kharghar, Navi Mumbai. The accelerating potential of -3MV is generated by a Parallel Coupled Voltage Multiplier (PCVM) scheme using 74 stages of HV rectifier stacks in the 6kg/cm2 SF6 gas environment. The HV surges of order of 600kV, 42kA, 10ns is estimated across the rectifier stacks during sparking in the multiplier column. To limit the surge current and protect the rectifier diodes, a non inductive thick film surge limiting resistor (SLR) and protective spark gap is designed and developed. The rectifier stacks with surge limiting resistors at both the ends and protective spark gap in parallel has been successfully tested in simulated surge condition at an impulse voltage of 212kVp, 150ns FWHM and surge energy of 200J, 10ms, 20kV at 6kg/cm2 SF6 gas environment and found satisfactorily. Subsequently the HV multiplier was installed with this surge protection scheme and is being tested at 1.2MeV level. This paper describes the design features and test results of the non-inductive surge limiting resistor. rectifier diodes. For a 3MeV dc accelerator, the HV surges of the order of 600kV, 42kA, 10ns is estimated across the rectifier stacks during sparking in the multiplier column. To limit the surge current in the rectifier diodes, a thick film non inductive surge limiting resistor (SLR) rated for 3kΩ, 10W, 150J at 100ms has been designed and developed. Rectifier stacks are connected across the corona rings of a high Voltage multiplier. To protect the rectifier diodes, spark gap operates across corona rings whenever applied voltage exceeds 120 kVdc or 600kV pulse within 10nsec time. ANALYSIS OF HIGH VOLTAGE SURGE The block schematic of the 3MV Multiplier column [1] is shown in Fig.1. When there is a sparking between 3MV terminal and ground, the voltage appearing across the rectifier stack is of the order of 600kV with 10ns time. INTRODUCTION An accelerating potential in 3MeV dc accelerator is generated by parallel feeding of radio frequency power to a series connected rectifier stacks. The RF power is provided by a 50 kW, 120 kHz triode tube based power oscillator which is powered from three-phase 50 Hz mains. The stabilized and variable 3Φ input is stepped up, rectified and filtered to achieve variable 10kVDC, which is inverted to 120kHz by a colpitts oscillator. An air-core RF Transformer steps-up this voltage at 300kVp to feed to a pair of semi cylindrical RF Electrodes surrounding the HV multiplier column. The multiplier column is configured in a cylindrical-geometry in SF6 gas medium at 6kg/cm2 SF6 gas environment. The multiplier column having 74 nos. of the rectifier stacks cascaded through 40 pairs of the corona guards generates -3MV DC. Each rectifier stacks are rated for 304kV PIV, 500mA forward current and 50ns reverse recovery time. The high voltage dc accelerators are prone to HV surges of the order of few kA, few nanosecond rise time while conditioning & load changes. This can damage the HV Fig.1: Schematic of 3MV Multiplier Column The Spark channel resistance in Ohms is given by kl -------- (1) R Q where k = toepler constant (0.8x10-3 for air), l = spark length between HV terminal and ground, in cm and Q = charge transferred through spark channel, A.sec. The Spark Channel Inductance in µH is given by 2l 3 --------- (2) L 0.002l ln 4 Where is the spark channel diameter in cm. For 3MeV system, l = 33cm, dome to ground capacitance =100pF and R= 88Ω. As a thumb rule, a spark channel inductance of 15nH/cm gives 0.5µH and surge frequency of about 22MHz with peak current of 42kA. Design Strategy of Surge Protection Scheme The surge protection scheme for the rectifier stacks of 3MV multiplier should be able to limit the surge current to 20A @ 8.33ms and absorb the surge energy of 150J for 100ms. Due to conditional sparking between two consecutive rectifier stack, 0.7A current flows through stack, these causes the dip in the HV column voltage and system will trip within 100ms. The resistor has to withstand a surge voltage of 600kV for 10ns time. Considering this, the resistor is to be incorporated in series with rectifier stacks at both the ends with suitable terminals. The design involves optimization of various electrical and thermal parameters such as self inductance ls, Self capacitance Cs etc for the dimensions of resistor to achieve minimum temperature rise and adequate high voltage insulation. The equivalent circuit of the HV multiplier column with rectifier diode is shown in Fig.1. From the given RF input of 300kV, capacitive coupling factor coupling factor, beam current and diode peak current rating of 20A @ 8.33ms, the resistance value have been finalised to 3kΩ for rms current of 31.4mA. From these considerations, the design parameters have been finalised as given in table 1. Table 1. Design parameters of Surge Limiting Resistor Parameter Resistance Value Resistance Tolerance Type of resistor Self Capacitance Self Inductance Current through resistors Power Dissipation Surge rating Voltage Withstanding Ambient Temperature Shape & Size Working environment 45C (Max) Cylinder; Length: ≤ 50mm; Both the ends to have brass cap as terminal. I.D = 20mm +/- 1/0mm; O.D = ≤ 45mm SF6 gas at 6kg/cm2 pressure CONSTRUCTION DETAILS At a surge frequency of 1- 100MHz, thick film based resistor with uniform coating over the alumina base cylinder are suitable to make non-inductive resistor. The cylindrical geometry is decided for efficient heat dissipation in compact size. The resistor outer surface is coated with Silicone anti tracking coating for high voltage insulation. The resistor is modelled in ANSYS for thermal analysis. The model is subjected to the heat load impulse for a short duration of 100ms. Transient analysis is carried out to visualise the temperature changes with respect to time as shown in Fig 2. Value 3kΩ 5% Ceramic, high voltage, high surge capability ≤ 1pF <100nH 31.6mA-rms@100kHz (for a load current of 10mAdc) 20A for 8.33msec (for transient Sparking condition) 10W (continuous) 150 Joules for 100msec. 300Vdc continuous 120kV pulse( For transient Sparking condition 5us) Fig.2.Impulse power Temperature(K) profile (Watts) Vs Maximum The base material of the resistor has been made of high alumina ceramic bushes. Uniform coating of thick film has been done over the alumina base to achieve required resistance for better heat transfer and high voltage insulation. The end terminal made with brass has been soldered with film resistor. The resistor is connected in the rectifier stack at both ends with spring touch contact. The assembly photo of the rectifier stack has been shown in Fig 3. Charging time Discharging Time Fig. 5 Waveform of energy pulse applied to the resistor Without SLR With SLR Fig. 6 Waveform of Impulse voltage generated by 300kV UWB Marx Generator applied to the resistor Fig.3. Rectifier stack assembled with surge limiting Resistor at both the ends PERFORMANCE EVALUATION The surge limiting resistor has been fabricated as per the design detail and the parameters were analyzed using the parameters like R, self inductance using programmable LCR meter 4284A-Agilent make. At 120kHz, the measured resistance is 3kΩ with a tolerance of < 5%. Self capacitance is measured to be < 2pF. A continuous 10 W power is applied on resistor and kept it for long time till steady state temperature is reached. Fig 4 shows the variation of temperature with time at 10W power. The resistor has successfully tested at a surge energy of 150J, 25ns rise time within 15ms as shown in the Fig.5. Fig. 6 shows the waveform of the energy pulse applied to the resistor. The resistor has been tested at an impulse voltage of 110kV/5ns rise time and 150ns FWHM in open air, which is generated by 300kV, UWB Marx generator. The rectifier stacks with surge limiting resistors at both the ends and protective spark gap in parallel has been successfully tested in simulated surge condition at an impulse voltage of 212kVp, 150ns FWHM and surge energy of 200J, 10ms, 20kV at 6kg/cm2 SF6 gas environment and found satisfactorily. CONCLUSION The surge protection scheme for the protection of the rectifier stacks of 3MV multiplier has been implemented in the 3MeV dc accelerator and tested upto 1.5MeV in SF6 gas at 6kg/cm2. The maximum beam power is 5kW. The accelerator is functioning satisfactorily. The accelerator is being prepared for the flue gas experiments at 1MeV and 5mA electron beam. ACKNOWLEDGEMENTS Authors are very thankful to Dr. J. Bhattacharya, ECIL, and his team for constant support and successful fabrication of this high energy high voltage non-inductive surge limiting resistor. REFERENCES Fig.4 Graph showing temperature on resistor at 10W [1] K. Nanu, et al, “Design of 3MV/10mA DC power source for E-Beam Accelerator” Proc. Indian Particle Accelerator Conference, Centre for Advanced Technology, Indore, Feb. 3–6 (2003), p-246. [2] R. I. Bakhtsingh, et al, “High Voltage Surge Protection System for Gun Power Supplies of 3MeV, 30kW DC Electron Beam Accelerator” Proc. Indian Particle Accelerator Conference, Centre for Advanced Technology, New Delhi, 2011.