© 1967 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE. 260 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, JUNE 1967 A COMPACT70MW, 250KV MODULATOR USING THYRATRONS C. Latham H. Menown N.S. Nicholls Vickers Ltd. English Electric Royal Radar Radiation and Nuclear Valve Co. Ltd. Establishment Engineering Division SUMMARY Pulse Repetition Rate Up to 1000 p.p.s. max. (at 0.5nsec flat-top) Pulse A compact oil immersed modulator is described Flatness *0.5% over specified suitable for driving a 30MWoutput klystron and flat-top. Pulse-Pulse using two 70KV 2,500 Amp hydrogen thyratrons as Time Jitter *5ns max. switches. The design is a development of an H.T. Stabilisation Better than f0.28 for earlier modulator using a 70KV triggered spark *3% mains variation. gapa This earlier model was limited to a maximum Response time less than repetition rate of about 250 p.p.s. but the 2 supply cycles. thyratron switches permit operation to over 1000 The modulator p.p.s. and a maximum duty cycle of .OOl. (Fig.11 is contained in a steel tank approximately Complete oil immersion in a single tank results 15ft. long by 3ft.3in. wide by 4ft.8in. high and in compactness, economy, reliability and facil- at one end supports and accomm- odates the klystron, the cathode itates the use of a high charging voltage which, bushing of which in turn, is lowered into the tank. leads to a simplification in the pulse circuit, particularly in relation to pulse flatt- Two free ening and pulse length selection. A high current standing racks housing the klystron focus supply silicon clipper is described which is used to and the klystron cooling heat exchanger are reduce the inverse voltage and which still installed in close proximity to the modulator. Other external permits rapid thyratron recovery time. items are the modulator control rack, which contains the instrumentation, INTRODUCTION trips and interlocks, and a separate motorised 1OOkVA a.c. regulator which can be installed in any convenient position. The design was commenced early in 1964, initiallyti fulfil the requirements of linear POWERSUPPLY accelerators for the Universities of Toronto and Glasgow. Although the electrical performance In the case of the TV2011 Klystron the called for new development, the physical arrange- values of pulse forming network impedance and ment was based on practice established for a pulse transformer ratio (15 ohms; 1:8) have previous generation of modulators having all been chosen such that for normal perveance tubes operating components immersed in a single oil filled tank. at 250-260kV the H.T. supply is 31-32kV. The power supply is designed Experience had shown the advantages to operate from 18kV (minimum) of this to cope with constructional form to be:- switch-on conditions, and may be run up to a maximum of 37.5kV to allow for end-of- life klystrons which 1. Excellent reliability resulting may require pulse voltages from up to 260kV. elimination of multiple high voltage bushings. It 2. Compactness. is necessary that the power supply is ad- justable over 3. Economy in manufacture. this range and that at any setting it shall be stabilised automatically. A dual 4. Simplicity of cooling and screening. system is used, one part provides some 95% of the total power and The use of a high charging is controlled by a continuously voltage, in the variable voltage regulating region of 70KV, for the transformer, while pulse forming network was the other part also retained consists of a trimming circuit from existing spark switch supplying modulator the remainder. The trimmer is fed from practice. a relatively fast acting a.c. controller employingof thyristors, the conduction angles of Modulator Specification which are determined by a circuit which compares a sample the H.T. voltage with a reference The main parameters voltage propor- are as follows:- tional to the chosen working level. Each of the Peak Power Output 70MW max. (57.514W two power supplies employs a three phase trans- normal running level former and full wave rectifier, the combining for TV2011 klystron) being done by series connection of the d.c. Mean Power Output 70kW. outputs. Flat-top Pulse Lengths 4.5 or 3.5,2.1,0.5~s selected by 3 position switch. PAC 1967 LATHAM: COMPACT 70 MW, 250 KV MODULATOR USING THYRATRONS 261 This system provides fast compensation over a thyristor control for the contactor opening coil. range covering the working level and does not The thyristor logic includes duplication for suffer from three disadvantages, associated with reliability and easy manual check out by pusn full power thyristor control:- button. a. If controlled rectifiers are used, either The Trimming System as primary a.c. controllers, or is actual rectifiers on the secondary of the trans- The trimming system and H.T. supply is shewn former, the d.c. output contains a high in Fig.2. The H.T. line is monitored by a poten- amplitude commutation spike except when tiometer immediately after the rectifiers, to the firing angles are set near to maximum avoid the phase shift and time lag of the L-C output. filter. It was found necessary to shield this potentiometer from stray fields, it consists of a In the dual arrangement the trimmer chain of resistors arranged approximately non- normally operates at half voltage output inductively in a cylindrical can 23" by 5" dia. and increases or decreases to compensate contained within the modulator tank, the output for mains variations but at no time is being brought out by a double-screened cable. the spike output sufficient to present a The d.c. sample is compared with a reference serious smoothing problem, tne L-C potential derived from a potentiometer mecnan- smoothing circuit (15ii, 2uF) being common ically ganged to the main a.c. regulator and fed to the sum d.c. of both rectifiers. from a stable d.c. source. Thus, as the regulator is raised the reference rises and the sample from b. Chopping the entire modulator input at the li.T. line should rise proportionally, Any levels in the range 50-100 kVA may lead difference constitutes an error signal which is to distortion of the supply waveform and fed via a chopper type d.c. amplifier to a three- when several modulators are run at phase thyristor controller to advance or retard slightly different conduction angles from the thyristor firing angles. The a.c. regulator a common source there is a risk of mutual runs up in about 25 set until the drive motor is interference with critical ancillary stopped by a relay circuit which also controls equipment. visual indicators on the modulator control panel and main accelerator control station. These The dual circuit has the advantage that relays are controlled by a bistable transistor because the greater part of the power in- circuit which sense both the reference potential put consists of a full cycle load the and the modulator mean current. Thus the H.T. supply waveform is relatively clean. runs up to a predetermined level irrespective of the load current imposed by widely different duty c. A tllyristor 'firing through' (due perhaps cycles. to breakdown or pick-up of spurious trigger) may lead to sudden increase in As the regulator runs up, the load current modulator output endangering the r.f. increases approximately linearly and the 11.T. tube. In the event of such a fault in volta,;e sags slightly due to power supply impe&rze the thyristor trimming system the total (mainly transformer reactance). A sample of the output voltage cannot rise more than a modulator mean current is fed to the error signal few percent and serious malfunction of amplifier to ensure that the trimming circuit the modulator is avoided. operates in the centre of its range at all levels and at all duty cycles. During modulator tests, The main d.c. supply has been designed with a rig consisting of three large wattage resistors, sufficient margin to permit full power operation shunted by a three pole contactor, is inserted in of the klystron without the trimmer if necessary. the mains supply line to produce, at will, a step of 3% in the input voltage and with the trimmer kkctification and Smoothing. working a change of less than 0.2% is seen on the klystron voltage pulse For both the main and trimring rectifiers strings of 1ijOOV silicon avalanche diodes are PULSE CIRCUIT employed. Over-current protection in the event of short circuit is provided by fast fault sens- Charging, ing to open the main contactor (common to both circuits). Also 9% reactance has been introduced The resonant charging system is connected to into the supply transformers to ensure that on the 'back end' of the p.f.n. in order to reduce short circuit the d.c. drawn from the rectifiers the stray capacitance seen by the switch tubes. does not exceed 28 Amps. The I't rating for the (Fig.3.) A single choke of 3.OH is used for all diodes permits this current for a period of repetition rates, the necessary hold-off being lOOm.sec following normal full load current, but achieved by a stack of silicon diodes consisting the circuit is arranged to clear in about 50m.sec, of 160 devices each rated at 1OOOV. Each diode is thereby giving a 2:l safety factor.