Electron Accelerator Control System Basedon Radiation–Acoustic Effects
G.F. Popov, V.A. Deryuga, A.I. Kalinichenko,
Yu.A. Kresnin, and N.G. Stervoedov
Kharkiv State University, Kharkiv, Ukraine
Abstract
The system uses ultrasonic radiation as a source of primary information about
the current status of the accelerator. The sound is generated by the electron beamand by a high-frequency electromagnetic field in their interaction with the accelerator
components and with the target. The system incorporates two CAMAC crates anda computer, and it fulfills the requirements for transporting, focusing and monitoring
the beam and follow-up phase-locking electromagnetic waves in accelerating cavities.

The system has been designed to control a traveling-wave linear accelerator with electron energyof 5 to 30 MeV, an electron pulse width of 10 to 4000 ns, a pulse repetition frequency of 1 to 300 Hz
and with an average beam power of up to 5 kW. A block diagram of the electron accelerator control
system is shown in Fig. 1. It incorporates a set of acoustical sensors with electronic preamplifiers,
two CAMAC crates and an IBM PC-compatible computer with corresponding software. Crate
A deals with phase-locking a h.f. electromagnetic field in an accelerating cavity to an electron
injector pulse and controls the transmission of a beam through a transfer line. Crate B carries out
the focusing of the beam onto a target and measuring the beam parameters. The analysis of data
and the generation of control commands are executed by the computer. The crates contain facilities
for digitizing signals from sensors, buffer memory devices, units for controlling executive devices,and microprocessor crate controllers.
The sources of primary information for system operation are ultrasonic waves σT generated by
the beam and the h.f. 1.8GHz electromagnetic σP field [1-7]. When a pulsed electron beam hits
components of the accelerator rapid heating and expansion of their materials takes place. Thus
sonic waves are generated and carry information about beam parameters and about the location of
the interaction spot.
Sonic waves are also generated by ponderomotive action of a pulsed h.f. field on waveguide
elements. The waves carry information about the form of the pulse of electromagnetic waves
and this form depends on the conditions of energy exchange between the h.f. fields and electron
bunches being accelerated [6-10].

The sonic waves σT , σP detected by broadband (∆ƒ ≤ 10 MHz) piezoceramic sensors AD-AD5 (seeFigure 1) placed on accelerator elements and on a target T. This sort of sensor has high sensitivity
and good radiation resistance and sensor construction provides protection against electromagneticinterference. The h.f. field phase-locking is executed based on signals from an acoustical sensor
AD1 placed on an absorbing load AL of the accelerating waveguide AW. The maximum transferof energy from the h.f. field to the electron bunches is achieved when the amplitude of acoustic
stress is minimal. Pulses from the sensor AD1 are led to a gated amplitude-to-digital converter incrate A. A corresponding subroutine operates through the crate controller and a relay control unit
for a klystron KL phase-shifter PS servomotor to make the amplitude of σP minimal. Thus one
strives for the maximal energy transfer.
The automatic control of beam transport is carried by multiple- cavity accelerators based on signals
from acoustical sensors AD2, AD3 placed on a diaphragm in an electron transfer line. Signals fromthe sensors are led to both gated analog-to-digital and time-to-digital converters in crate A. If thebeam is off the transfer line axis some signal amplitudes go over a preset limit. In that case thedetermination of beam position is made on the basis of the delays of two acoustical pulses relative
to the sync pulse of the accelerator. Then corrective changes of beam positioning magnets BPM
are made. Corresponding digital-to-analog converters will be used for that purpose in crate A.
Some other system of beam transport were tested on the accelerator LUE 300 in Kharkov. The
beam transmission through the system is monitored by using signals from acoustic detectors placedalong the accelerating waveguide. Both pulse amplitudes and the delays relative to the sync pulsesof the accelerator were digitized and inputed into a computer, where the beam position was determined and the control commands for changing the currents of deflecting magnets were generated.
The current correction continues until the signals from the detectors vanish and the signal from theacoustic probe AP showing beam position appears. This procedure worked in single-pulse modein a short time due to the high information content of the acoustical signals.
On high-current accelerator Fakel in Moscow these detectors serve for emergency protection of
the beam-transport pipes against damage from the high-current beam. When the amplitude of anacoustic signal goes over a preset limit the injector is disabled preventing damage to the accelerator
elements.
Focusing the beam and measuring the beam parameters are implemented on the basis of signals
from sensors AD4, AD5 placed on the target T (AD5) and on an acoustic probe AP (AD4) whichis on a metallic wire crossing the beam. The signals are digitized by a gated analog-to-digit
converter in crate B and are analyzed by a corresponding subroutine. The greater the amplitudeof the signals from AD4 and AD5 the smaller the diameter of the beam on target. The system
controls the current of a focusing lens FL by a digital-to-analog converter in crate B so as to keepa predetermined diameter of the spot. On the basis of the delay of a pulse from AD4 relative tothe sync pulse, the current position of the spot is determined. It allows a follow-up control of
the current shape of a magnetic scan deflector BCD when a scanning beam is used. This makes
it possible to keep the required dose distribution on the target. The duration and frequency ofthe electron bunches, the energy of the beam and the size and location of the electron spot on thetarget may be determined. There is a supervision program that keeps data from the experiment andpresents current information on the monitor screen. It makes it possible to view signals from thesensors, to test the system, to change parameters of the beam in response to operator instructions
and to calculate a dose profile on the target.
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Figure 1: Block diagram of the electron accelerator control system. AW — accelerating waveguide; BPM — beam positioning magnet; FL — focusing lens; BSD — beam scan deflector;
BTT — beam transport tube; KL — klystron; AL — absorbing load; PS — phase shifter; AP —acoustic probe; T — target; e− — electron beam; AD-ADC — acoustic sensors; PA — poweramplifier; A — pre-amplifier; AU — amplifier unit; SU — synchronization unit; CC — cratecontroller; AS — analog switch; DAC — digital-to-analog converter; TDC — time-to-digitalconverter; Gated ADC — gated analog-to-digital converter; RCM — relay control modulus;
PDM — peak detector modulus.