IEEE Transactions on Nuclear Science, Vol. NS-30, No. 2, April 1983

AN EXPERIMENTAL APPARATUS FOR LOW ENERGY HIGH CHARGE HEAVY IONSTO STUDY COLLISIONS ON ATOMIC HYDROGEN

C. Can, Tom J. Gray, J. M. Hall and L. N. TunnellJames R. Macdonald Laboratory

Physics DepartmentKansas State University, Manhattan, KS 66506

and

University of

S. L. VarghesePhysics DepartmentSouth Alabama, Mobile, AL 36688

Summary

The design and performance of a recoil ion sourcesystem which includes a recoil ion source, atomic hy-drogen thermal oven target and an electrostatic analy-sis system will be discussed. The recoil ion sourceproduces low velocity highly charged ions via colli-sions between heavy fast pump beams from the EN tandemaccelerator and target gases. Time-of-flight tech-niques provide initial recoil charge state separation.Collisions of the recoils with atomic hydrogen arebeing studied. The atomic hydrogen is provided by athermal oven which features long life time operationand low input power requirements. Dissociation frac-tions of 80% are achieved for 300 watts of input power.A hemispherical electrostatic analyzer allows thefinal charge states of the recoil ions to be deter-mined thereby allowing the measurement of charge ex-change processes for an energy range of 100 eV/q to5000 eV/q for the incident recoil ions.

Introduction

Recently there has been considerable interest inthe study of collisions between low energy highlycharged (LEHQ) ions with both single-electron andmultielectron targets.' One such collision resultingin the transfer of electron(s) from the target atom tothe projectile plays an important role in hot plasmas.Electron capture from hydrogen and from other impuri-ties followed by radiation from the excited states ofthe projectile ion may be an important source ofenergy loss from Tokomak plasmas.2 Collisions withatomic hydrogen provide a simple case for theoreticalinvestigation. In this paper we present an experi-mental apparatus to study electron capture by LEHQions from atomic hydrogen.

The Recoil Ion Source

The recoil ion source, shown in Fig. 1, is asmall gas cell through which a fast heavy ion beamfrom a tandem Van de Graaff accelerator passes toionize the gas. The recoil gas cell is 2.2cm in dia-meter by 1.2cm in length with entrance and exit beamapertures of lmm and 2mm in diameter, respectively.It has been shown3'4%596 that the collision betweenthe primary beam and the target atoms produces highlyionized but slowly moving target atoms which can beextracted out of the cell by application of an elec-tric field, via voltage V1, perpendicular to the beamaxis. These recoil ions, say Neq+, are then sentthrough a thermal oven to study the electron capturefrom atomic hydrogen.

Design and Operation of the Hydrogen Oven

The atomic hydrogen can be produced by dissocia-ting hydrogen molecules in a thermal oven7' 8 or in adischarge tube.9 In the case of thermal dissociation,for typical gas pressures of 1-5 mTorr and for veryhigh dissociation fractions (>90%), this requires oventemperatures as high as 2400°K. The present hydrogenoven, shown in Fig. 1, is, in some ways, similar tothe earlier designs, but is simpler and features longlifetimes and lower power requirements. The heatingelement is a tube, 7.5cm in length and 6mm in diameter,

G

< ICEM I

+V5

p

gas in

Fig. 1. Schematic of apparatus used to measure elec-tron capture cross sections by low energy highly charg-ed ions from atomic hydrogen. PB, primary beam; C,recoil ion cell; R, recoil ions; EX, extractor; G,grid; I, insulated drift tube with pump holes; S, mo-lybdenum sleeves with apertures; W, water cooling; M,machinable ceramic; H, heat shields; T, tungsten tube;P, pyrometer; E, electrodes; A, electrostatic analyzer;CEM, channel electron multiplier.

rolled from 0.025mm thick tungsten sheet. The two endsof the tube are held by a pair of molybdenum sleeveswhich also serve as entrance and exit beam aperturesand are clamped by massive, water-cooled copper elec-trodes. The tungsten tube is surrounded by four con-centric heat shields made of 0.254mm thick tantalum.A machinable ceramic cylinder concentric with the heatshields is used to support the oven assembly. The hy-drogen gas is fed into a region between the first andsecond heat shields and then enters the tungsten tube

0018-9499/83/0400- 943$01.OOC1983 IEEE

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through a 0.5mm wide slot running the length of thetungsten tube. A hole was drilled, radially throughthe heat shields and the ceramic support cylinderwhich allows the measurement of the oven temperatureinside the tungsten tube using an optical pyrometer.The oven requires about 70A AC current correspondingto 300W input power for dissociation fraction as highas 80%. Under such conditions and at full operatingtemperatures (2100-2200°K) the same oven tube can beused for more than 100 hours. There are not any obser-vable effects of the magnetic fields on the recoil ionspassing through the oven. The two voltages V2 and V3in Fig. 1 are used to prevent any positive ions andthermionic electrons streaming back into the recoilion source. A grid with V4 was also necessary to stopthe electrons'emerging from the exit aperture of theoven and being accelerated into the electrostatic ana-lyzer by the varying positive high voltage on the outeranalyzer plate. Without this grid extremely largebackgrounds composed of positive ions were encounteredas the sweep voltage was applied to the analyzer. Thegrid (V5) is used to suppress background low energypositive ions which are created in the target chamberby the passage of the high energy beam from the accel-erator. The cross sections for the production of lowenergy highly charged (LEHQ) recoil ions are large10and at chamber pressures of X 4 x 10-7 Torr, the totalproduction of LEHQ recoil ions along the beam path inthe chamber is X' 5 x 106 recoils/s. The magnitudes ofthe operating voltages for V2, V3, V4, and V5 aretypically 40V with polarities as indicated in Fig. 1.

The Analysis of Post-Collision Ions

The evaluation of oven performance was obtainedusing Ne gas in the primary gas cell. The recoil ions(Neq+) were given an energy of 400 eV/q, where q isthe recoil ion charge state.

The Neq+ recoil ions, during their passagethrough the hydrogen oven may change their chargestate to q' (q'=q-l, q-2) as a result of one or twoelectron capture from hydrogen atoms or molecules.This, of course, depends upon maintaining single colli-sion conditions within the oven target system. It isestimated that the operating pressure within the oven

at room temperature (300°K) is X 0.3m Torr. This es-timate is based upon earlier measurements of Neq+ H2+

Ne(q 1)+ H2 which were madell under controlled con-ditions for a gas cell of known length and internalworking pressure. A hemispherical electrostatic ana-

lyzer is used to study the post-collision ions. Thetime-of-flight (TOF) of an ion from its production to

its detection at the channel electron multiplier pro-vides the initial charge state q of the recoil ionsince TOF of an ion is proportional to Vci7-j , where m

is the mass of the ion. The final charge state q' isobtained from a knowledge of the value of the analyzervoltage Va at the time of detection of an ion accord-ing to the following expression

kqV = q'V

where k is a geometrical factor. The analyzer voltageis swept between k('q/q') and 3k(q/q'). A two dimen-sional LIST MODE technique recording TOF vs. Va allowsus to determine the electron capture cross sections.

The details and the results of this technique is pre-sented in another paper of this conference.

The Dissociation Measurements

The dissociation fraction of H2 is calculated12as a function of the temperature and pressure of the

gas by assuming thermodynamic equilibrium H2 t 2H.The measurement of the dissociation fraction is per-formed by utilizing a double-electron-capture processinvolving a Neq+ (q > 3) ion and the hydrogen molecule.Protons having an energy of 25 keV have been used byother investigators.13 In the present work, low

energy (400 eV/q) Neq+ (q= 3,4) recoil ions were usedwith the advantage that the measurements yield boththe dissociation fraction and the electron capturecross sections with the normalization of the cross sec-tion provided by previous measurement of Neq+ + H2 +

Ne(q ) + H + H+. Including second order terms, theNeq+ inchnng(q-2)+fraction of Ne ions changing into Ne ions forcold and hot oven conditions are given, respectively,as

Fqq-2(H2) aqq-2(H2)r(H2) + aq ql(H2)q* - , - 2

x a 1,q2(H2)[7T(H)] 2 (1)

F 2(HH2) = eq q_2(H2)- (H2) + CqYql(H2)

x Cqy q2(H2) [ (H2)2

+ Cqql(H)aq lq-2(H)[L (H)] (2)

where 7r (H2) denotes the effective target thicknessfor molecular hydrogen which is smaller than w(H2) be-cause of both dissociation and increased conductancedue to high temperatures. Assuming temperature inde-pendent flow of hydrogen and using conservation of'flow, Bavfield8 showed that, under single collisionconditions, Eqns. 1 and 2 yield

*F *1/2

q,q-2(H,H2) = (T) 1F R*) 1q,q-2(H2) T 1 + a

F2~(3)

* * *

where a =_ (H)/7 (H2), and T and T denote oven tem-peratures. This result assumes that the second orderterms in Eqns. 1 and 2 are negligible. Using Eqn. 3and the measured quantities, the values of a as afunction of the oven temperature are obtained and thenthe dissociation fractions are calculated according to

the definition

-r(H)-* 1 *7E (H2) + 2 X (H)

a

2 + a

Each value of f requires four runs; cold and hot oven

measurements and their respective background runs,without changing the needle valve setting on the hy-drogen line. The results of our measurements are

presented in Fig. 2 together with a calculated curve

for hydrogen pressure of 1 mTorr, using the equationgiven by Fay.12

Conclusions

We have presented a description of a relativelysimple atomic hydrogen thermal oven target. Thetarget is characterized by low electrical power re-

quirements, relatively long oven lifetime, and ease ofadaptability to a given experimental situation.Transport of recoil ions'through the oven shows mini-

mal interference due to stray magnetic field asso-

ciated with oven current. Measured dissociation frac-

tions agree with the general trends of calculateddissociation fractions insofar as the temperature de-

pendence is concerned.

Acknowledgment

This work is supported by the U. S. Departmentof Energy, Division of Chemical Sciences.

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1.0P =I mTorr

0.8

0.6

f

0.4 1

0.2

01400 1800 2200 2600 3000

T(°K)

Fig. 2. The calculated (solid curve) and the measureddissociation fractions of H2 as a function of tempera-ture.

References

1. See e.g., Electron Capture by Multiply ChargedIons, Proc. ICPEAC XI, Invited Papers and ProgressReports, p. 387, ed. Oda and Takayanagi, North-Holland (1980); and references cited therein.

2. D. M. Meade, Nucl. Fusion 14, 289 (1974); H.Vernickel and J. Bohdansky, Nucl. Fusion 18, 1467(1978).

3. M. D. Brown, J. R. Macdonald, P. Richard, J. R.Mowat and I. A. Sellin, Phys. Rev. A 9, 1470(1974).

4. R. L. Kauffman, C. W. Woods, K. A. Jamison, and P.Richard, ICPEAC IX, Contributed Papers, p. 939,ed. Risley and Geballe, U. Wash. Press (1975).

5. N. Stolterfoht, D. Schneider, R. Mann and F.Folkmann, J. Phys. B 10, L281 (1977).

6. C. L. Cocke, Phys. Rev. A 20, 749 (1979).7. G. W. McClure, Phys. Rev. 148, 47 (1966).8. J. E. Bayfield, Rev. Sci. Instrum. 40, 869 (1969).9. R. W. Wood, Proc. R. Soc. A 97, 455 (1920).

10. T. J. Gray, C. L. Cocke and E. Justiniano, Phys.Rev. A 22, 849 (1980).

11. S. L. Varghese, T. J. Gray, C. L. Cocke, C. Can,L. Tunnell, W. T. Waggoner, and E. Justiniano,Bull. Am. Phys. Soc. 26, 1205 (1981).

12. J. A. Fay, Molecular Thermodynamics, Addison-Wesley Publishing Company (1965).

13. G. J. Lockwood, H. F. Helbig and E. Everhart, J.Chem. Phys. 41, 3820 (1964).