Nuclear Instruments and Methods in Physics Research A244 (1986) 166-169 North-Holland, Amsterdam
N E U T R A L P A R T I C L E S IN T H E B E A M O F A S I N G L E E N D E D VAN D E G R A A F F A C C E L E R A T O R L. B A R T H A , ,A.Z. KISS, E. K O L T A Y , Gy. S Z A B 0 a n d L. Z O L N A I Institute of Nuclear Research, Hungarian Academy of Sciences, P.O. Box 51 H-4001, Debrecen, Hungary
I. N Y I L A S Bessenyei Teachers" College, P.O. Box 166, H-4401 Nyiregyhi~za, Hungary
Neutral particles have been observed in the extension to the entrance channel of the magnetic analyzer. They may originate from charge exchange processes in different parts of the trajectories of accelerated ions. On the basis of /0//analyzed /0//Iotal curves and secondary electron emission coefficients, conclusions are drawn on the presence of neutral particles in the acceleration tube. 1. Introduction
The presence and motion of neutral particles in acceleration tubes have been investigated in earlier papers from the point of view of their origin, and their contribution to the electron load of the tubes as well as the possibility of the practical applications of neutral beams has been discussed. The charge exchange process in ion-atom collisions partly resulting in energetic neutral atoms was first investigated as early as 1939 . Recently, this field became equally interesting for basic atomic physics  and because of its important role in generating multicharged ions to be accelerated in tandem accelerators [3-5]. In the case of practical applications, charge exchange appears in the stripper units (stripper foil of gas channels) situated in drift elements between acceleration sections of the tube. Gas leakages from the ion source and from the gas stripper tube into the acceleration tube may result in a more or less widely distributed local increase of the rest gas pressure giving rise to distributed stripping near the ion source  and along the whole tube [71, respectively. Special measures can be taken to cut the trajectories of neutral particles near the ion source , on the other hand inclined field tubes or straight tubes with shortened sections with electron trapping effect will effectively suppress the secondary electrons released by the charged and neutral particles impacting tube electrodes. Interesting investigations aiming at the practical use of neutral beams in neutral-injected three-stage tandem configurations were described in refs. [8,9]. Recently, the generation of neutral accelerated beams for application in basic ion beam collision physics has been discussed [101. The application of high current neutral beams becomes increasingly important in fusion experiments. 0168-9002/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
In our earlier work neutral particles have been obServed in the tube extension to the entrance channel of the magnetic analyzer by measuring secondary electrons released from a target here while running the accelerator with the magnetic analyzer switched on . The neutral particles may originate from charge exchange processes in different parts of the trajectories of charged accelerated particles. In order to learn more about the origin of the neutral component, in addition to its intensity in the tube extension we measured the relative neutral beam intensity obtained on an equilibrium thickness gas stripper target as well as the secondary electron emission coefficient both for charged and neutral beams. These data together with energy values of the neutral particles obtained from elastic scattering measurements yield information on the most probable places of origin of the neutral component and its role in secondary electron loading of the acceleration tube. 2. Experimental method In the first part of the present work the intensity and energy of the neutral component in the direct beam have been measured. As the second step, the equilibrium fraction of the neutral component in hydrogen, helium and nitrogen beams penetrating through an air stripper channel has been determined together with secondary electron emission coefficients for total charged and neutral fractions of the beams. The combined experimental arrangement consists of a stripper chamber, a deflection chamber and a measuring chamber as shown in fig. 1. The measuring chamber alone was attached to the extension tube of the entrance of the magnetic analyzer when the intensity and energy of neutrals in the direct
L Bartha et al. / Single ended Van de Graaffaccelerator FARADAY CUP
FOIL LAODER GAS STRIPPER TUBE
SORFACE .ARR,ER OETEC.TOR
D,AP,RA6M IBl '
Vcup Vdi~hragm VfoiI ~ ~ ~
FO,LFo;ME EME'S it ~-
,/ DIFFERENTIAL PUMPING
PNEUMATICALLY OPERATED OISC.
Fig. 1. Experimental arrangement used in detecting intensity and energy of neutral and charged beam components.
beam of the accelerator was measured, while the whole arrangement was joined to one of the beam lines when neutrals were generated through charge exchange in a gas stripper. In the measuring chamber the intensity of the neutral and charged components were measured with the target assembly  composed of electrodes referred to as diaphragm, Faraday cup and target in fig. 1. For neutral impact on the target, the intensity of incoming particles is detected through the i c current of secondary electrons collected on the positively biased Faraday cup. In the case of a beam containing both neutral particles and ions, i c is identical to the sum of electron currents released by the separate beam components, while target current i t amounts to the sum of incoming and outgoing currents of ions and electrons, respectively. The field of the diaphragm is to prevent secondary electrons originating on surfaces other than target, from interfering with current measurement. A thin scatterer foil fixed to a target ladder and a surface barrier particle detector situated at 135 ° with respect to the beam direction are used to determine the energy of incoming charged a n d / o r neutral particles from elastic scattering. For the definition of the scattering angle both beam and detection directions are defined by sets of collimators. A second foil attached to the ladder is used as an equilibrium foil stripper in measuring intensity ratios
I'he deflection chamber contains a pair of electrostatic deflection plates 50 cm in length. With + 2.5 kV power switched on and off, the target will be bombarded with pure neutral ( I °) and mixed charged-and-neutral ( I tot~J = I 0 +/chars*d) beams, respectively. The stripper chamber contains a gas stripper tube and is differentially pumped with a high speed diffusion pump stack. To simulate rest gas conditions in the
acceleration tube, air is let into stripper tube through a needle valve. The intensity ratios l ° / l t°tal, deduced from electron currents measured with the deflection field switched on and off may be influenced by the fact, that the secondary electron emission coefficient varies with the charge of the bombarding particle, i.e. the response of our beam detector is charge dependent. This difficulty can be overcome by equilibrating the beam charge in a thin foil just before the beam impinges on the secondary-electron-emitting surface . Then, independent of the charge of beam components, the same charge mixture is incident on the target. This is why the foil stripper was used in I ° / I t°tal measurements before the target assembly. The secondary electrons released from the foil are retained with + 5 kV on the stripper foil. On the other hand, secondary electron emission coefficients T° and y+ for neutral and charged beams, respectively, can be determined from I ° / I t°tal ratios and currents i measured on target and Faraday cup with deflection field switched on and off. They are given by the expressions .yO
tc.on C ( It,off - ic,off )
and y+ _ ic.ofl -- ]c.on I t.off -- ic.off '
C - - - - 1 .
i o / i t°t~
i c and i t denote current on the Faraday cup and target, respectively, and the subscripts on and off refer to the state of deflection field. lIl. ION SOURCES
L Bartha et aL / Single ended Van de Graaffaccelerator
J___2 ~ /to1 ....-~N N
. . . .
i , 1
. . . .
Fig. 2. Neutral beam intensity relative to the intensity of analysed beam as a function of charged particle energy. The 7 data are extremely sensitive to the surface conditions of the target on which the secondary emission appears. In our case the process is investigated with respect to the electron loading conditions in acceleration tubes, the electrodes of which (normally made of stainless steel) are only technically clean. Therefore our target plate which plays the role of secondary-electron-emitter surface is made of stainless steel and is only cleaned with the procedure normal in vacuum technique.
The intensity of the neutral component of the direct accelerated beam was measured in the measuring cham-
-. . . . .
H I : j~ i
. . . . ~-1--
i • iT'l:l II il_ .
~_-~ ~---~ IT,l'=J. J l :~
.-..... [ - . ~ -¢ w
MeV Fig. 3. /0//total curves compared with published data as a function of beam energy.
Fig. 4. Secondary electron emission coefficients as a function of beam energy.
L Bartha et al. / Single ended Van de Graaffaccelerator ber joined to the extension to entrance channel of the magnetic analyzer. To remove charged components the magnetic field was switched on and adjusted to the value corresponding to the ion species of given energy. For the sake of a consequent comparison beam intensities were expressed relative to the intensity of the analysed ion beam. The results obtained for H, He and N beams are shown in fig. 2 as the function of terminal voltage. Because of the different efficiency of transporting neutral and charged beams to their respective places of detection it is rather the shape of energy dependence and its vertical shift for the different ion species than the actual value of I ° / I anal which reflects the conditions typical for the direct beam. Energy measurements performed on the scattered neutral particles led us to the conclusion, that the overwhelming majority of the neutral particles arrives at the chamber with the total energy corresponding to the terminal voltage. Lower energy peaks belonging to molecular components are sitting on a weak tail appearing on the low energy side of the main peak. The beam neutralisation process was investigated on an analysed beam transferred through a gas stripper channel by measuring the equilibrium fraction /0/itotal for air as stripper gas. Present data together with those taken from Allison's paper  for H and He (E < 0.5 MeV) and from the work of Stier et al.  for N (E ~<0.2 MeV) are shown in fig. 3, as a function of the beam energy. Fig. 4 presents secondary electron emission coefficients 7 °, y+ and their ratio 7°/-t + for the same cases. For a comparison, data taken from the papers of Stier et al.  (E ~ 0.2 MeV), Dettmann  ( E < 1.5 MeV), Barnett and Reynolds  (E < 1 MeV), Jamba  ( E < 0 . 3 MeV) and Kiss et al.  (E~<4 MeV) are also shown.
The effect of ion beam neutralisation in distributed stripping seems to contribute to the secondary electron loading of the acceleration tube. From the experimental data obtained in this work the following conclusions can be drawn: -The intensity of the total neutral component I ° / I an~ in the direct beam at the tube exit decreases with increasing terminal voltage. This behaviour is fully explained by a similar tendency observed in the energy dependence of the equilibrium fraction I ° / I t°ua after a gas stripper channel, - The strong decrease of the 10/Itotal curve with increasing ion energy shows that the neutralisation at a given terminal voltage takes place mainly in the low energy end of the acceleration tube. In the case of single ended machines the high rest gas pressure here will enhance the process, - Measured energy distribution of the neutral p a r -
ticles shows that the predominant part of neutrals observed in the direct beam is generated in the ion charge exchange process near the high energy tube end. This fact combined with the strong energy dependence of 10/]total makes the conclusion plausible that orders of magnitude higher neutral intensity is present at the low energy tube end. These particles, however, will not be transported along the whole tube, their trajectory will end up on the electrode surfaces in the tube, - Neutral particles will release here secondary electrons with the high probability expressed by the measured secondary electron emission coefficient ~,0, - The concentration of neutrals to the low energy region in the tube is less expressed for heavier ions. - The electron loading caused by neutrals can be decreased by more effective vacuum pumping at the low energy tube end. The proper adjustment of minimum gas leak rate at the ion source and an improvement on electron trapping generally weak at low ion energy region can be useful, as well. - Because of the fact that the secondary electron effect caused by neutral particles is mainly concentrated on the low energy tube end, the bremsstrahlung emitted in electron impact will be of low end point energy. In situ dose rate measurements could be performed with thermoluminescent dosemeters  and would make an additional tool in investigations of this field.
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