ASPECTS OF THE CONTROL SYSTEM FOR THE ELECTRON LINEARACCELERATORS BUILT IN ROMANIA

M. Toma, D. I. Martin, A. Jianu, C. Oproiu, S. Marghitu

National Institute for Lasers, Plasma and Radiation Physics, Electron Accelerators Laboratory, # 111 Atomistilor St., PO Box: MG-36, R-76900 Bucharest, Romania

Abstract

The paper presents the control system for the electronlinear accelerator ALID-7 of 5.5 MeV and 0.7 kW, builtin Romania. The PC-based control methods are used inparallel with classical techniques in order to increase thepersonnel and accelerator safety.

1 INTRODUCTION

ALID-7 linac was designed and built in theAccelerator Laboratory of the National Institute forLasers, Plasma and Radiation Physics, Bucharest, to carryout the research in the radiation processes field. To developnew electron beam technologies and to provide small-scalecommercial irradiation services such as: polymericflocculants preparation for wastewater treatment,sterilization of some medical products and high powersemiconductor recovery characteristic improvement. Also,we have been attracted, during the last few years, to theconcept of microwave energy addition to electron beamenergy. In view of these arguments, the electron linearaccelerator ALID-7 was completed with a special designedfacility which permits the simultaneous electron beam andmicrowave irradiation of monomer mixtures, microbialcultures, gas mixtures containing SO2 and NOx and rubbermixtures. Our research results demonstrated that requiredabsorbed dose for micro-organisms sterilization, polymericflocculants preparation, rubber mixtures vulcanization aswell as for sulfur dioxide removing is about 2 - 10 timessmaller by simultaneous electron beam and microwaveirradiation than for electron beam irradiation only, at thesame processing efficiency. Thus, the use of simultaneouselectron beam and microwave treatment, the ionizingradiation costs could be much decreased and theapplication of low intensity electron sources, which are lessexpensive, will be extended.

2 ALID-7 CONSTRUCTION ANDPHYSICAL CHARACTERISTICS

ALID-7 is a piece of industrial equipment having arotating acceleration structure [1, 2].

Fig.1 shows the ALID-7 system configuration,including the main blocks of ALID-7 control system.ALID-7 is a travelling-wave type linac, driven by EEV M5125-type magnetrons operating in S-band and delivering2 MW power in 4 µs pulses. The electrons are injected

from a diode type gun in the acceleration structureoperating in the π/2 mode. The first part of theacceleration structure (iris-loaded circular guide) is avariable phase velocity buncher and the remainder has anuniform section, for a phase velocity equal to light speed.An axial magnetic field produced by several separatefocusing coils placed around the accelerator tube wasused to compensate the radial forces tending to dispersethe electrons, which are travelling through theacceleration structure. During and after acceleration,electrons are subjected to a succession of electromagneticdevices for beam focusing and sweeping in a horn-shapedvacuum chamber. In the output arrangement, ahead of thesweep electromagnet, the electron beam (EB) passesthrough a current monitor consisting of a ferrite ring pulsetransformer (induction monitor). The dose uniformity onthe swept surface is ensured by specific currentwaveforms of the scanning electromagnet. ALID-7 isprovided with a circular conveyor (325 cm averagediameter, 50 cm width) driven by a servomotor. Directcollection of part of the scanned electron beam is used asa monitoring method by sampling during the irradiationprocess.

Another facility, referred to here as simultaneouselectron beam and microwave irradiation facility(SEBMIF), was specially designed to be used togetherwith ALID-7, in order to permit simultaneous acceleratedelectron beam and microwave irradiation. The microwavepower is coupled to the MRC (microwave rectangularcavity) upper-end plate (as in Fig. 2) via a slottedwaveguide system (a structure based on five inclinedseries slots cut at λg/2 apart in the broad wall of aWR430-waveguide). The following beam physicalcharacteristics are important at the end of the acceleratingprocess: electron beam output power PB (the basicprocessing capability of a given electron source for a"required dose") and electron beam energy EB (whichdetermines the electron beam penetration depth). Thevalues of the maximum beam power PB and the optimumvalues of peak beam current IB and electron energy EB areas follows: PB = 670 W, EB = 5.5 MeV and IB = 130 mA.

3 ALID-7 CONTROL SYSTEM

The ALID-7 control system (ALID-7-CS) is of criticalimportance to ensure personnel and sensitive devices areprotected against dangerous events, efficient energyutilization, reliable operation and beam parameter stability.

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International Conference on Accelerator and Large Experimental Physics Control Systems, 1999, Trieste, Italy

Figure 1: ALID -7 system configuration.

In order to provide these functions, the following twomain solutions have been adopted: •The use of classical control techniques in parallel withPC-based control methods in order to increase thepersonnel and accelerator safety. •The use of the original ALID-7 triggering method forobtaining programmed beam single shots and beam pulsetrains with programmed pulse duration, number andrepetition frequency, from a diode gun linear accelerator.This is by discrete pulse, temporal position modulation ofthe gun electron pulses and magnetron microwave pulses.The method eliminates the lack of flexibility of the diodegun types. It combines the unsophisticated construction ofdiode guns and better temporal flexibility of the beam,generally available when using triode guns. It isparticularly useful for automatic control of absorbed doserate level, irradiation process control as well as in pulseradiolysis studies, single pulse or pulse train dosemeasurement and for research experiments where pulse-to-pulse reproductibiliy is required.

3.1 ALID-7-CS working principles

ALID-7-CS ensures the automatic operation of thewhole experimental assembly shown in Fig.1. An IBMcompatible PC (486, 100 MHz, 16MB RAM) controlsystem (PCCS) and manual control system (MCS)simultaneously govern the irradiation process evolutionby strict hard and soft interlocking in parallel.

Figure 2: Simultaneous electron beam and microwaveirradiation facility (SEBMIF).

Moreover, the main subassemblies may be controlledfrom their local control panel, as well as from theirdedicated modules in the central control panel. A general-purpose control and acquisition interface (Keithley DAS1600), installed in the PC, has been programmed toacquire process parameters and generate linac controlsignals. Before the start of the experiment the operatorhas to introduce by means of the PCCS keyboard, in aninteractive way, or by means display potentiometers,digital selector switches on the MCS front panel, theexperimental parameters and the operating mode(completely automated or sequential). In the completely

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automated operating mode, the operator commands onlythe start of the experiment, the system operation beingsoftware and hardware controlled afterwards. Theevolution of the experiment is continuously presented onthe PC display, every experimental phase providing theoperator with a set of commands, which he could use incase of system malfunction. In the sequential mode, theoperator from the system keyboard or MCS front panelcommands both the start of the experiment and also theevolution from one phase to the other. The systemdisplays continuously each phase of the experiment, themeasured values of the main device parameters, as well asa set of commands for operator intervention in case ofmalfunction. Both operating modes provide the operatorwith the possibility of interrupting the experiment at acertain sequence, in order to change the experimentalparameters that have previously been introduced. At theend of experiment, the operator has the possibility ofadding his observations regarding the evolution of theexperiment. He also decides whether the experiment datais to be stored on the PC as experimental data files. Themain measured and/or controlled operational parametersare represented in Fig. 1. Other parameters, such as peakforward microwave power and peak reflected microwavepower (detected at the bi-directional coupler) are alsoacquired and used to control the 2.45 GHz CW microwavepower source when SEBMIF is in operation. The speed ofconveyor travel is controlled according to the dosesrequired to be applied. The beam sweeping over thematerial to be irradiated is controlled by monitoring thesweep frequency and amplitude.

The programs have been written in the C language.They have been designed around a real-time kernel andhave a modular open structure allowing furtherdevelopment in accordance with new applicationrequirements.

3.2 ALID-7 triggering method

ALID-7 triggering method allows the beamdeliverance as single shot or pulse trains with the desiredpulses number, pulse repetition time and pulse duration.The linac beam is determined by the electron gun andmagnetron pulses overlapping. The method consists ofcontrolling the above condition in order to deliver thebeam in the desired sequence. This control isimplemented by a discrete pulse temporal positionmodulation of gun and/or magnetron pulses. A schematicdiagram of this kind of modulation applied to the gunpulses and some temporal distributions of the electronbeam are presented in Fig. 3. In order to implement thementioned triggering technique two separate modulatorsare provided: a gun modulator and a magnetronmodulator. ALID-7-CS, which synchronizes all thesystem units, delivers trigger pulses at a programmedrepetition rate (up to 250 pulses/s) to the gun modulatorand magnetron modulator via the gun and magnetronthyratron drivers, respectively. When no gun andmagnetron pulses overlap no accelerated electron beam

results at the output (Fig. 3a). The instabilities of the gunand magnetron transitory regimes are avoided byoperating the accelerator with no accelerated beam for acertain time.

Figure 3: A schematic diagram of the discrete pulsetemporal position modulation applied to the gun pulses

and some electron beam temporal distributions.

At the operator "beam start” command, ALID-7-CS,controls electron gun and magnetron pulses overlappingand the accelerated electron beam is generated (Fig. 3b).The pulse-to-pulse absorbed dose variation is thusconsiderably reduced. The proportion of filling-timeelectrons and thus electrons energy dispersion, may bereduced to some extend by arranging the electron gun totrigger at some optimum time (accelerating structurefilling time τF) after the commencement of eachmagnetron pulse (Fig. 3b). Also, an accelerated electronbeam with a controlled number of pulses (three in Fig. 3cand single pulse in Fig. 3e) or with a controlled repetitionperiod (2T in Fig. 3f) may easily be obtained by thiselectron gun and magnetron triggering method.Programmed absorbed dose, irradiation time, beam pulsenumber or other external events may interrupt thecoincidence between the gun and magnetron pulses andthe irradiation regime. The beam pulse duration may becontinuously adjusted from 0.25 µs to τM-τF, where τM isthe magnetron pulse length and τF is the accelerationstructure filling-time (Fig. 3d). The short pulse duration islimited only by the values of the gun pulse leading edgeand of the magnetron pulse trailing edge. Slow absorbeddose variation is compensated by the control of thetriggering pulse repetition frequency as well as bymagnetron and gun pulses overlapping.

4 REFERENCES

1] D. Martin et al.,” IAP linacs in applied research”,Nucl. Instr. and Meth. in Phys. B 113, p. 106-109 (1996).[2]D. Martin et al., “Low power-high energy linacs forirradiation in polymeric system”, Radiat. Phys. Chem.Vol. 45, No. 4, p. 615-621 (1995).

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