View of The System for Control and Stabilization of the Beam Position in the Microtron MT-25 in Prague

1 Introduction

The electron beam transmission system in the Prague MT-25 microtron requires accurate alignment of the beam with all deflecting or focusing magnetic elements. Without knowledge of the beam position at crucial points of the trans- port system, the alignment is a troublesome operation even for an experienced operator. Therefore it is desirable to have actual knowledge about the beam position, which however must be obtained without any deterioration of the electron beam quality. Once the alignment is obtained, the main duty of the operator is to hold the beam in the center of the output window. This function, requiring permanent attention from the side of the operator, can be automated as well.

There exist several types of beam non-disturbing position detection systems. In our case we decided to apply the system based on secondary electron emission from impact of acceler- ated electrons on thin metallic wire probe. The amount of energy dissipated in a wire probe is negligible and no supple- mentary cooling of the wire is necessary. The deterioration of the beam by scattering on a single wire is very small; never- theless it must be taken into account, if several wires in series are placed in the beam path. Therefore the wire probes can be used only in the period of the alignment of the beam and should be removed from the beam path after the alignment has been accomplished. Only probes situated at the beam periphery can stay at place permanently.

2 Experimental part

2.1 Stabilization of the beam position at the output window

The wire probe method was tested in connection with the stabilization of the beam position at the output. To be sure,

that the method will work in vacuum as well as in air, an elec- trically isolated copper wiref0.4 mm was placed in the axis of a thin wall cylindrical aluminum chamber of 28 mm ID, which could be evacuated. The chamber wall was grounded and a load resistor was inserted between the wire and the ground.

The potential on the resistor was measured by a low noise, low drift instrumental amplifier. The chamber was exposed to the electron beam with the wire in the beam axis and evacuated.

The portion of the 0.8mA, 22 MeV electron beam passing through the wire was estimated to 0.15mA. The total beam in- tensity was monitored by an induction pick up system [1] at the end of the electron transmission line. The wire potentials, measured on load resistors going from 10⁵to 10⁶ohms, were in the mV range. Currents calculated from potentials and re- sistor values were in all cases 0.08mA, independent on the re- sistor values. The loss of intensity of the primary beam at pas- sage through the wire being negligible, the current is due ex- clusively to secondary electrons leaving the wire and reaching the wall of the chamber. The wire is charged positively and be- haves as a constant current source.

The situation changed, when the same measurements were made at atmospheric air pressure in the chamber. In this case several effects must be taken into account. Most of the secondary electrons, with an energy spectrum from fraction of eV and more, cannot reach the wall of the chamber. Their energy is dissipated in collisions with the nitrogen and oxygen atoms or molecules and finally, captured by them, they form negative atomic or molecular ions. A negative space charge barrier built up in the space between the wire and the wall of the chamber prevents low energy secondary electrons to escape from the region in vicinity of the wire. The diffusion of the space charge both toward the wire and the chamber represents additional currents. At atmospheric pressure, the backward current of negative ions toward the central wire overweighs the current toward the wall of the chamber. Con-

© Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 59

Czech Technical University in Prague Acta Polytechnica Vol. 44 No. 1/2004

The System for Control and Stabilization of the Beam Position in the Microtron MT-25 in Prague

Č. Šimáně, M. Vognar, D. Chvátil

A method of control the beam position at crucial points of the transport system and for the stabilization of its output position has been proposed and preliminary tested. The method is based on secondary electron emission from a thin metallic wire probe induced by electrons from the 25 MeV microtron. It was demonstrated, that magnetic field of the order of 0.2 T and parallel to the wire probe in front of the orifice of the extraction channel in the acceleration space, does not prevent the functioning of the method. A strong parasitic effect of secondary electron emission from the material of the channel and its support construction was found, leading to the inversion of the electron current polarity from the wire. This effect can be to great extent eliminated by negative electric potential bias relative to the channel. At the electron output current of 1mA the secondary emission current from the wire probe of 0.3 mm diameter is of the order of several nA. Two electromechanical systems were designed for the removal of the probes from the beam path, to avoid the deterioration of the electron beam quality by scattering.

Electronic schemes used for remote measurement of small probe currents, suppressing the influence of strong electromagnetic noise, are described. For stabilization of the output beam position two wire probes situated in air close to the Al output window were used. These probes having been placed at the periphery of the beam did not deteriorate the beam quality. The difference of their emission currents was used as an error signal to control the magnetic field of the last dipole, which kept the beam in the center of the output window.

Keywords: microtron, electron transport, beam detection, beam position stabilization, secondary electron emission.

tact potentials between the Al chamber and Cu wire as well as thermoelectric potentials may also in some way modify the load resistor current. In detailed analysis, the pulse character of the microtron electron beam should be also taken into ac- count. Each 2.5ms beam pulse is followed by a 2.5 ms pause, during which the space charge diffuses without being nour- ished by secondary electrons. Some role may also play the ion- ization of air primary beam. The observed final result of all these rather complicated effects is the reduction of the cur- rent in the load resistor as compared with the current in the evacuated chamber. When keeping the load resistor below 100 kW, the observed reduction represents about 50 %.

Based on these experimental results, a system for stabiliz- ing the beam in the horizontal plane was developed [2]. In its simplest version, two parallel 0.4 mm Cu wire probes (E1 and E2) were placed in air vertically in front of the exit window of the electron transport line, symmetrically to its center. The 6 mm distance between them was selected as suitable for normally adjusted 3 mm beam half-width. For this probe con- figuration, using measured values from Fig. 1, the difference U2-U1 of potentials on the load resistors (error signal) as function of the beam departure from the central position was calculated (Fig. 2).

The probes were connected to the inputs of a differential instrument amplifier (Fig. 3) through a frequency filter sup- pressing the pulse character of the secondary electron emis- sion currents and environmental electromagnetic noise. The

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current of the probe was measured as function of the channel position relative to the electron orbit.

The experiment furnished two important results. First, it was confirmed, that magnetic field existing in the accelera- tion space of the order of 0.2 T and parallel to the wire probe, does not prevent its functioning, which was not at all evident in advance. Secondly, a strong parasitic effect, leading to the inversion of electron current direction from the wire probe, was observed, the origin of which was the secondary electron emission from the channel and its support material hit by the beam. The current of the probe is thus an algebraic sum of its own secondary emission current and of current of secondary electrons emitted from the support material and collected by the probe. As seen from Fig. 5, the setting of the wire probe on negative bremspotentials-9,-18 and-27 V against the channel potential inhibited progressively the collection of sec- ondary electrons from the construction material. At a bremspotential of-27 V the maximum of the current peak was clearly identifiable and current inversion to great extent suppressed.

In next experiments, the possibility of using the system was verified for finding the correct channel position at pas- sage from an electron orbit to the next (to the previous) one.

During the passage, the effect of secondary emission from the channel and its support material was very strong and could not be fully suppressed even if the bremspotential was used.

From the same reason the determination of the correct chan- nel position by differential wire probes, which has been tried too, gave not satisfactory results. Nevertheless, as can be seen from Fig. 6, using only a single wire probe the correct channel position on two neighbor orbits can be found unambiguously.

The experiments gave also a crude estimate of the order of magnitude of the secondary emission current. The f0.3 mm wire probe was situated in front of the orifice of the iron channel, where the horizontal width of the beam is about 6 mm. Setting the accelerated electron output current to 1mA, the maximum of the secondary electron current was of the or- der of 20 nA. Because of the unknown distribution of the cur- rent density in the beam the coefficient of electron secondary emission could not be calculated and compared with the ex-

perimental re also the th idvalue218.3(2)2.1(-218.3(id.2(n)-)-21908(i)2.1(nformive)-.252(alsoA252(1(brlowTJ0 -1.2 TD-.0143 Tc[(Dunoi)-221(8(brlowT33.1(d)-.75(r)3if-231.3(and)alo-21630me)plied)r233.1(hit.5(r)as221(8(bren)-283.3(o)buil-231.3(an-3.5(irork3.6(ir).6(n)--21630mi-283.3(o)t-3.5(iroTJT*-.0139 7c[(w)-3.2(i)2.1(2)35.2(e)2.95.7(1ur)-21.4(r)35.2(e)2)-191.6(Sel)2(a)oop191.6(w)-3.2(i)2.1(2))-297.6(the)-725.7(1um1.4(0)sur)3g)-190.1(maitructm2)-191.6(Se)-191.8(the)-725.7(1up7(f)al)]TJT*-.0139 Tc[(Duin194.5(coe)-725.2(r)34.8(e)3mot-725.2(r)1.7(onfi)34.6(o)-l192.1(r)34.6(o)-o)-18897(onT.1(f))-725.2(r)e)plied)r231.6(3)-3.5(a)-s725.2(p)-lac)-191.5(coin194.5(coviennTJ25.6301 .0002 3D0 Tc(-)Tj-25.6301 -1.2002 3D-.0143 Tc[(Duy)-38449(o)-30(f)-24347(t)-1.7(he)-26346(o)celera)]t)-345.9(atis345.1(a).445.6(p).1(r)le)-21346(o)-3.5(ir).6(n))-293.9(th34.6(e)1.7(nttive)-l-38449(o)lowT3449(r)-561(d)adiion)-2J0 -1.2 TD-.0143 Tc[(0.lert-219.4(t)-1.5(huo220.5(6)-.1(lgvoid221.5(9ts)-23218(p)d1.3(e)3rialorion)-245.5(9t)-21421(d)radiion)-.219.2(paI-245.5(9t)-)-21818(p)-3.3(ir-.1(lg-21421(d)-1.5(he)-218.3(inhiTJ25.6301 .0002 3D0 Tc(-)Tj-25.6301 -1.2002 4D-.0142 Tc[(tofluen)-215.7(t)-.1(f)-245.8(x)1.7(eet34.7(om))-195.6(wa1.8(suctr)34.6(t)-.1(f)mnetic)-365.7(t)noi)-225.7(t)in195.9(x)e)-725.7(t)m1.9(heic34.6(t)-.1(f)t34.7(om))TJ0 -1.2 TD-.014 Tc[(do34.9(obe)-275.0(Se)-190.1(thasur)34.9(ed)mt)-274.0(id.2(f)-24570)-1.4(he)-23769(thsml)-27168).4(r)34.9(e).2(f)b-23769(thr)-22(r)34.9(ent)--269.w)-3.2(as)(f)-269.w)2(viect)TJ25.63 .0002 4D0 Tc(-)Tj-25.63 -1.2003 4D-.0144 Tc[(tion)-l-38261(suppor34.5(essed)-]TJ1.4919 -1.3791 TD-.0138 Tc[(toe)-29327(s)1.7(amportd)-31230(p)d1.3(4)3rialorion)-24230(p))-30130(the)-21327(s)2.2(e)- oiruaty ee

be oriented perpendicularly to the transmission line axis and will be made extractable from the beam. For this pur- pose a special mechanism was designed securing the linear transport of the probe by external magnetic fields (Fig. 7).

Additional differential wire probes will be situated in the beam transmission line at the periphery of the beam, serving for determination of the sign of deviation of the beam from the axis. Being situated at the periphery of the beam, they will perturb but insignificantly the beam and can be fixed perma- nently in the transmission line. The wire probe in front of the orifice of the extraction channel is fixed on a coil with two sta- ble positions. In one stable position the wire is in front of the channel orifice, in the other position the probe is removed from the beam path (Fig. 8). Current pulses of appropriate polarity actuate the flipping of the coil from one to the other position when magnetic field in the acceleration space is on.

The system was preliminary tested inside the acceleration space of the microtron with satisfactory results.

3 Conclusion

The use of the secondary electron emission from wire probes represents an advantageous way of electron beam position control when setting the parameters of magnetic deflecting and focusing elements of the electron transport system. The differential wire probes can be used for automatic stabilization of the output beam position. Introduction of

62 © Czech Technical University Publishing House http://ctn.cvut.cz/ap/

Acta Polytechnica Vol. 44 No. 1/2004 Czech Technical University in Prague

Output connector Anchor in working position

39 wire probe

Wire probe out of the beam path R2

5

Electromagnets

Wire probe in the beam path

Electron-guide

173

Fig. 7: Linear shift mechanism for setting the wire probe in and out of the electron beam path

Fig. 8: Flipping coil mechanism for setting the wire probe in front of the electron extraction channel and to withdraw it:

1-extraction channel input orifice, 2-wire probe, 3-coil, 4-holder of the extraction channel, 5-shaft of the flipping coil holder, 6-pole pieces of the microtron electromagnet

these probes in the transport system in the way described does not deteriorate the beam quality. No additional cooling sys- tem is required, what represents a very important feature of the wire probes. The incorporating of the beam position con- trol in the existing transport system is expected to result in optimization of its parameters, improvement of the out- put beam quality and simplification of the overall microtron control.

References

[1] Šimáně Č., Vognar M.: Induction Pick Up System For Microtron MT25 Current Measuring and Position Indication.

Šimáně, Č., Vognar, M.: Czech Technical University in Prague, Workshop 98, Prague, 1998.

[2] Šimáně Č., Vognar M., Němec V.:Stabilization of micro- tron electron beam position. Czech Technical University in Prague, Workshop 2000, Part B, Prague, 2000, p. 540.

Prof. Ing. Čestmír Šimáně, DrSc.

Ing. Miroslav Vognar Ing. David Chvátil

Department of Dosimetry and Application of Ionizing Ra- diation

Czech Technical University in Prague

Faculty of Nuclear Sciences and Physical Engineering Břehová 7

115 19 Praha 1, Czech Republic

63

Czech Technical University in Prague Acta Polytechnica Vol. 44 No. 1/2004