Improvements to a method of externally pulsing an accelerator

NUCLEAR

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I M P R O V E M E N T S TO A M E T H O D OF EXTERNALLY PULSING AN A C C E L E R A T O R A. GIL R A M O S and P. N. C O O P E R

Department of Physics, Unirersity Of Aston in Birmingham, Gosta Green, Birmingham B4 7ET, England Received 12 February 1973 signal controls a high voltage power transistor which extinguishes the ion source rf when in the conducting state. Operating frequencies are from I Hz to 2 k H z .

A simplified method o f producing a square beam current wave-

form from a S A M E S 1 5 0 k V accelerator has been devised. Square light pulses are transmitted to the high voltage terminal by an optical link using semiconductor devices. The transmitted

that have been made to the modulation system. Since this no longer includes a modified version of the pulsing unit designed by SAMES the method is now more readily adaptable to other accelerators. The modulated light beam is still used to transmit the signal to the high voltage terminal. Since the light emitting diode type MGA 100 is now obsolete it has been replaced by the more efficient type 1A48. Also modifications have been made to the driving circuit so that the transmitted light pulse follows the shape of the driving pulse instead of producing a short light signal coincident with the leading edge. The revised circuit is given in tig. I. Driving signals are taken directly from the output of a dividing circuit constructed from SN 7490N and SN 7493N integrated circuits. When run for long periods the bistable circuit,

In a previous communication ~) a method was described tor producing a square wave modulation of the ion current from a SAMES type J accelerator. This note describes certain simplifications and improvements ,12V

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A. G I L RAMOS A N D P. N. C O O P F R

originally used to drive the switching pentode, was found to be over-sensitive to supply voltage variations and consequently did not always respond to the trigger pulse. To overcome this the bistable circuit and the switching pentode were replaced by a single high voltage npn transistor type BUI05. The base of this was driven by a square wave voltage signal derived from the transmitted light signal and the circuit for achieving this is shown in fig. 2. It was found necessary to include adequate screening and filtering against the intense radio frequency signals produced by the ionsource oscillator. One advantage of this method is that the beam pulse is in phase with the transmitted pulse whereas previously the phases could differ by 18ff~, depending on the initial state of the bistable circuit. When the base of the BUI05 is made positive the transistor conducts. In this mode it is used in one of two ways to shut off the ion source radio frequency. The first method, which was used in the original SAMES pulsing unit, is to reduce the voltage of the oscillator screen grid to near earth. This leaves the full high tension on the anode and on the coupling straps

to the ion source bottle. In consequence the plasma, under certain combinations of gas pressure and high tension voltage, may not always extinguish completely, thus giving some ion current during the off period of the cycle. An alternative method, which has proved consistently reliable, is to use the BUI05 to short the high tension supply to the whole oscillator to ground. A 2 kf2 100 W wirewound resistor was inserted into the high tension lead between the power pack and the BUI05 to act as a current limiter. This method of pulsing can be easily adapted to any desired rectangular wavcform where the on period differs from the off period, simply by applying the appropriate wavefbrm to the input of the transmitter. For an applied square wave the system has bccn found to operate reliably in the range of frequencies I Hz to 2 kHz.

Reference 1) p. N. Coopcr and L. Doukas, Nucl. Instr. and Moth. 92 (1971) 581.