Monitoring the Energy of Electrons in Industrial Linacs
V.N.Boriskin*, V.A.Gurin, A.N.Savchenko, V.I.Tatanov, A.E.Tolstoy, V.L.Uvarov, V.A.Shendrik
NSC, Kharkov Institute of Physics and Technology, Ukraine
Abstract
Recently, technological linear electron accelerators withthe energy up to 10 and 25 MeV and pulsed current up to1A have been developed and put into operation in theScientific Research Complex (SRC) "Accelerator" of theNational Science Center, Kharkov Institute of Physics andTechnology (KIPT). These linacs are equipped withcontrol systems which allow for the control of the electronenergy at the exit of the linacs.

thickness. Such a construction appeared to be quite
acceptable for the assumed scanning frequency and themanufacturing technology. The excitation winding consistsof two coils with 60 turns each which are locatedsymmetrically with respect to the working clearance andwinded with copper wire of 2.36x5.00 cross section. The
cooling is natural.

1 The output beam scanning and forming
The zone of the technological object irradiation by theaccelerated electrons is created by the magnetic scanningsystem [2]. This system represents an electromagnet withthe spatially homogeneous magnetic field and the normalboundaries on the beam input and output. The magnet is
excited by the generator of a two-polar sawtooth current[6]. The scanning system provides the required value anduniformity of the scanning as well as measurement andoperative control of the electron beam energy characteristics [4].
The basic characteristics of the accelerator beam and theirradiation zone, which determine the type and constructionof the scanner, are presented in the following table:
Parameter

Value

The electron energy

Å=8-25 MeV

The beam diameter at theinlet of the scanning device∅ ≈ 1cm( r0 = z0 =± 0. 5 cm)

The input beam angulardivergencer '0 = z'0 ≈ 5 ⋅ 10−3radn

Energy dispersion∆E / E ≈ ±5 ⋅ 10−2

The maximum angle of thebeam scanningϕ = ±20o

The effective length ofthe magnet field

Ln ≅ 16. 3 cm

The scanning frequency(controlled)

f = 1... 3 Hz

Structurally the magnet system of the scanning device
represents an electromagnet with spatially homogeneous
magnetic field and normal boundaries on the beam input
and output. Fig. 1 shows schematically the scanning zone
from the accelerating section outlet to the output window
with real boundaries of the edge field on the inlet andoutlet of the scanner magnet clearance.
The magnet core and the working clearance poles areassembled from punched isolated steel plates with 2mmFig. 1 The zone of beam scanning and output: 1- theaccelerator axis; 2 and 6- the effective field boundaries on
the inlet and outlet of the scanning magnet; 3 and 5 -themagnet boundaries (by iron); 7- the energy control plane;
8- the slot hole Faraday cylinder (FC).
2 The operative control of the electron energy
In addition to the main task the electromagnet of theoutput device can be used for the operative control of theelectron energy. The technique is based on the relationshipbetween the total energy of the charged particle E n and"rigidity" of the magnetic field H ( with the account ofrelativism ) [3]:1Hρ =E n 2 − E 02 ,(1)ecwhere c- is the light speed, e and E 0 - are the electron
charge and energy, respectively, ρ - is the radius of theparticle trajectory in the magnetic field with intensity H .
The following expression for the kinetic energy of theelectron can be obtained by a simple transformation of (1)taking into account dimensions and units of measurement:Ek =E 0 2 + (3 ⋅ 10−42H ρ)− E 0,(2)where E k - MeV, H - Oe, ρ - cm. One can see that tomeasure E k it is necessary to determine two values: theintensity of the deflecting magnetic field H and the radiusof the electron curvature in this field ρ. However, whileworking with an accelerated beam the direct measurementof the specified characteristics, especially ρ, is connectedwith almost insuperable difficulties. That is why thetechnique is based on the measurement of the linear andangular parameters - the effective length of the deflecting
magnetic field Ln (taking into account the scattered fields
on the inlet and outlet of the magnet) and the geometry of
the scanning zone (Fig. 1). These parameters are measured
in advance and do not change their values during theexperiment. The excitation current force of the scanningelectromagnet is obtained from the curve of magnetization
H = f (I ) . Such a characteristic for one of the magnets
is depicted on Fig. 2. Noticing that for the small angles ofthe beam deflection
relationship is valid

ρ=( ϕ ≤ 20o)theLn,
sin ϕfollowing(3)

Finally we arrive at the practical formula for E k :Fig. 2 The magnetization characteristic

Ek =

E0 +2

k2I2− E0 ,sin 2 ϕ(4)where k is the constant accounting for the relationshipbetween H , I and Ln for a specific magnet and isdetermined by stand investigations. From Fig. 1 one cansee that if the value of the electron beam deflection fromthe axis ( R ) on the plane located at distance h from themagnet is found by using any appropriate method and thevalue of I is known, the value of E k .can be obtained. Inthis case

ϕ = arctg( R / h )(5)

For the technological accelerators at SRC "Accelerator"
two techniques are mainly used for the operative control ofthe most probable average electron energy.
3 The method of photometry
A piece of glass is located on the energy control plane(Fig.1). During several scanning periods this glass isilluminated by the electron beam with a fixed value of I .
As a result, a blackened print of the beam trace is formed.
Then the distance R is measured and the most probableaverage beam energy is determined by using thedependences E ( R , I ) calculated beforehand on thebasis of (3,4). The error of such a measurement is 5-10%.
In this case the control time and, correspondingly, the lostbeam time of the accelerator is about half an hour.
4 Express method
The paper of Ref. [4] describes the technique of energy
control by using the special slot-hole Faraday cylinder(FC)and automatic control system [5]. Taking into account themaximum power of the scanning magnet feeder for eachcontrolled energy range the distance R FC (Fig. 1) ischosen so that with the working value of the amplitude ofa sawtooth feeding voltage of a magnet (U ) at the extreme
working location of the beam center ( R w) electrons will
not hit a Faraday cylinder; at the extreme control location( R max ) the beam center will go behind the FC. Then byusing the relationship E = f (U , R ) the value of E
is found. This process of control does not require the beamswitching-off and takes a few seconds.
The operation experience showed that this technique did
not allow to control the electron energy over a wide energy
range, as with the sawtooth voltage (U ) the current in themagnet has a sinusoidal form and, correspondingly,
characteristic E = f (U , R ) is nonlinear. And only fora narrow energy range, within the limits of ±1 MeV, foreach energy range the estimations of E can be obtainedwith an error less than 5%. Besides the inhomogeneity on
the edges of irradiated objects reached 15%. To eliminate
these flaws some work on improvement of the scanner
feeder has been done [6]. As a result the strictly sawtoothform of the excitation current of a magnet has beenobtained. It allows to consider that the movement of abeam center for each accelerator pulse satisfies thefollowing condition: ∆ϕ = const.
At the accelerator operator's command the control systemincreases the amplitude of a halfscope of a sawtooth
feeding current of the magnet ( I ) from a working value tothe maximum one. The beam deflection angle is increased
up to ϕ max. The measuring system registers (Fig. 3) values
I (t), pulse beam current I AC and a (FC) pulse current
I FC and determines the scanning frequency ( F SCAN) andthe frequency of the accelerator work ( F AC ). Then the
program of filtration and image recognition processes thegraphics of I (t), I AC and I FC. If the beam center crossed
the FC than numbers of pulses n1, n2 (Fig. 3) arecalculated as well as a number of beam pulses ∆n , withtrajectories between ϕ FC and ϕ max .

∆n = n 2 − n1 .
(6)
The total quantity of accelerator pulses during the quarterof a scanning period is given by the following expression:
1F AC / F SCAN
4
ϕ max= ϕ FC+ ∆ϕ ⋅ ∆n / 2 .
n=2 2

Ek =E 02 +k I
2sin ( arctg ( RFC / h )(2n2n − ∆n− E 0,(9)))

Currently this expression is used to obtain the mostprobable average value of the electron energy. In the future
it is supposed to use the proposed technique to estimate thebeam energy spectrum.

(7)

(8)
By simple substitutions taking into account therelationships (5,6,7, and 8) expression (9) valid under thecondition ϕ w < ϕ FC < ϕ max is obtained from the formula(4).

Fig. 3 The videogramme of the accelerator energy control.

References[1] A.N.Dovbnya et al., "Electron Linacs Based Radiation Facilities of Ukrainian National Science Center",
KIPT / Bulletin of the American Physical Society,
May 1997, V.42, No.3, p.1391.
[2] A.N.Dovbnya, et al. , "The Output Beam Scanningand Forming in the Multipurpose Electron Accelerators of KIPT", VANT, Series: Nucleic Physics, 1997,
vol 1(28). p.114-121.
[3] J.Livingud, "The Principle of Operation of CyclicAccelerators", IL, Moscow, 1963
[4] V.N.Boriskin, A.E.Tolstoy, V.L.Uvarov et al., "Automatic Control of the Electron Energy in the Technological Linear Electron Accelerators", Digest of theXIV Meeting on the Accelerated Particles,
Protvino,Russia, 1994, vol.2, pp.97-98[5] V.N.Boriskin et al. ,"Control System for a Linear
Resonance accelerator of Intense Electron Beams",
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Tatanov, "The Linear Accelerator Beam Scanning bythe Correction of the Irradiation Field", VANT.
Series: Nucleic Physics, 1997, vol. 1(28), p.141