Atomic and Molecular Physics

1017

IEEE Transactions on Nuclear Science, Vol. NS-28, No. 2, April 1981

GENERATION OF THE HIGH-Q IONS BY THE CRYEBIS METHOD.

Joel ARIANER

Institute of Nuclear Physics, Orsay, France.

Summary

Very high ionization degrees may be rea-

ched by a stepwise ionization process. The CRy-

ogenic Electron Beam Ion Sources are linear

containment devices which mainly use a magneto-

focused electron beam.

We give herewith the operating principles

of this machine and descriptions of some tech-

nological elements. The performances of the

operating devices are described.

Introduction

The idea of an Electron Beam Ion Source

(EBIS) has been initiated by Donetz' after the

work of Redhead2 on the production of high

charge states by electron bombardment of elec-

trostatically confined ions. Production of H-

like light ions and Au19+ has stimulated re-

searches to obtain a suitable ion source for

cyclotrons or synchrotrons. A new generation

of cryogenic devices was firstly tested in 1971

by Donetz3) and pr-omptly produced Xe29+. Now

Xe5O+ has been performed with CRION II 4).Constant progresses make successful the

use of such machines as ion sources for high-

energy as well as atomic physics.

Operating principles of a cryogenic electron

beam ion source.

The configuration is shown on Fig.1Into an ultra-high vacuum vessel, a very

dense electron beam is fired from an electron

gun on the axis of a solenoid, though a series

of cylindrical electrodes. This beam blows up

in the decreasing fringe field of the solenoid

on a cooled electrode named electron collector.

An electron repeller prevents primary electrons

disturbing the ion-beam line.

Three different potential distributions

are successively applied on the successive drift

tubes

1) Distribution PDI, injection phase.A continuous gas flow (some 10-7 torr.l.

s-1) is fed on the tube n°2 which is at the li-

quid nitrogen temperature (770K). The singlycharged ions due to the interaction with the

C. solnodgun cathode

,JT1 T2 T3 T4 _T/

elctron

Fig.] - Basic configuration of an EBIS andlongitudinal potential distributions

fast electrons diffuse towards the containment

region of the source.

The radial motion of ions is limited by

the potentiel "well" due to the negative space

charge of the primary electrons.

2) When the number of singly charged ions

achieves a certain value, the distribution PD2

is switched on : this is the containment phasis.

The tubes T3 to Tn are at the liquid Helium

temperature (4.2°K) so as to minimise the neu-

trals inside the containment volume , the ions

which are produced inside the injection tube

are then expelled towards the gun. The confined

ions increase their charge states by sequentialionization. The negative space charge is more

and more neutralized until the balance between

charge densities of fast electrons and ions is

reached; this occurs at the "neutralization "

time. The quantity of singly charged ions which

are injected is adjusted so that the desired

charge state is attained just before the neu-

tralization time.

3) The ions are ejected from the source byrising the bottom of the PD2 distribution up to

0018-9499/81/0400-1018$00.75© 1981 IEEE1018

the PD3 one, in a variable time Z exp. A new

cycle of the source may then begin again.

Ionization theory

Experiments have shown that the step-by-step ionization is the main process occuringin such sources. The different charge state

populations Q , normalized to the initial num-

ber of neutral atoms , obey a system of equa-

tions of the type :

dQi = - Qi(T)cri .i+1 + Qi-I(T)cyi_i 2.1dT

T is the ionization factor-product of the

electron flux J (el.cm-2.s1')by the interac-

tion time t (sec), and ci * i+l the cross sec-

tion of the transition i + i+1.

This system is rather simple as the elec-

tron-ion recombination is negligible at such

low densities. This is the result of a fast

heating by Coulomb interaction of the slow electrons coming from the ionization. Charge exchan-ges between ions and neutral atoms are avoidedby a very low background pressure in the confi-nement tubes and a pulsed neutral injection. Atypical solution of the system 2.1 is drawn onFig.2 for Argon at 10 keV electron energy. Wehave also plotted charge state distributionsat various ionization factors to show clearlythe equivalence of an EBIS and a variable thickness-stripping foil.

It is possible to calculate for any ele-

ment, the product Jt to get any charge to mass

ratio with the maximum relative abundance, at

a given energy (Fig.3). Then, theoretically,itis possible, with this device, to reach any

ionization degree if the required Jt is prati-cally attainable. The two terms of the productare limited, less and less with the technologi-cal progresses.

Ne Ca Kr Xe Ta Ze

Fig.3 - Chart ofat 10 KeV

the ionization factors

The containment time is limited by the

natural neutralization time LN due to the back-

ground gas ions. If PT is the average pressureinside the tubes at 300 K and if hydrogen is

considered as the ultimate component of an ul-

trahigh vacuum vessel then one has :

Fig.2 - Evolution of the charge state popu-lations and spectra versus the ioni-zation factor.

t # 3.Itij Ve1-0 L (3Ve)

N

PyT Ln(3Ve)2.2

1019

For example, if a pollution of 10% of

the source is allowed, the maximum containment

time is 100 ms at 10 9 torr. In the next para-

graph, we discuss the J production and limita-

tion.

Electron beam focusing.

The electrons have two functions : io-

nization and radial trapping. These actions are

possible if the beam is cylindrical along the

source volume, this is achieved by the magne-

tic focusing of a soleno;d. The electron flow

is generated from an electron gun which is ex-

ternal or immersed in the fringe field of the

solenoid. The three parameters : induction B(T)

electronic current I(A) and accelerating volta-

ge Ve(V), are adjusted to get the smoothest

flow as possible, called the "Brillouin flow"

then :

J = 46.B2.Ve1/2(1-k2)-1/2 2.3

where k is the ratio of the fluxes

threading the cathode and the cross-section of

the beam at the considered abscissa. Then, to

get high values of electronic density, high

fields are required (the Brillouin flow is ide-

al, the practical ones are more B consuming,

for example at 10 kV, 5.103 A.cm-2 is a realis-

tic value of density for B = 3T.)

beam

tive

is:

The trapping effect of the electron

is due to the potential well of the nega-

space charge. The depth of the beam well

AU = 1.5 10 .I.Ve / 2.4

with respect to the tube voltage, this

depth becomes :

£U' = AU (1 + 2 Ln rT/rF) 2.5

where rT is the tube radius. Obviously

A U' has to be smaller than Ve to allow beam

transmission and then the tube radius is limi-

ted to a maximum value, for instance to solve

the problem of pumping. It has been previously

said that the well is levelled in proportion as

ions are produced. This point is not so clear

the slow electrons coming from'the ionization

or from the injection region (directly accele-

rated by the first trap during the containment

phasis) contribute to maintain the well. Never-

1020

theless experiments yield a limit for the sum

of the positive charges Q+ close to the theo-

retical capacity of the well Q- This may just

be a limit of focusing, considering the separa-

ting effect of the expulsion distribution PD3

on the ionic and electronic populations.

Ionic yields5)

If we consider the classical limitation

Q+ < Q-

the yield for the charge state E , ex-

pressed in terms of number of particles per

pulse n ,pp is :

n ,pp 101.I.Ve_ . l.K 2.6

where K is the abundance of the charge

statee in the charge spectrum and 1s the sour-

ce length in cm. From the chart of Fig.3, it

is possible to deduce at a given density, the

necessary time ZC G to get the desired charge

state t.Thus, if we suppose that the injection

and expulsion times are negligible, then the

number of source cycles per second may be:

Z-cand the yield per second:

N = 1011 I.Ve / ls. K

I, pss2.7

In the case of a high duty-cycle D of

the source, where the expulsion time ?exp is

not negligible, the source yield is directly

multiplied by (1-D). Fig.4 is the theoretical

yield of Saturne II CRYEBIS at 10 keV-2 A -

103 A. cm-2.

Technological description and hints.

The very special criteria to make this

source work imply a technological complexity

and the adjustement of a prototype is a tedious

work. To produce a high-density beam and to

keep it stable for a long time (up to 100 ms)

on a long distance (1 meter) are the two ma-

jor problems.Generally the electron gun from

which the flow is fired is of the Pierce type

with an impregnated tungsten cathode. The elec-

trostatic convergence of the gun and the magne-

Fig.4 : Theoretical and experimentalyields of CRYEBIS.

tic compression up to inductions higher than

2 T give densities in the range 102 104 A.crm2

provided that the coaxiality between the gun

axis , the tube axis and the magnetic axis is

better than 0.05 mm, in other words, the elec-

tron beam cross-section must encircle the geo-

metric axis. This is made easier by using a

superconducting winding of the soleno;d for

high inductions and high azimuthal homogeneity

of the field.

The second problem is the achievement

of a pressure lower than 10-9 torr in a series

of stainless steel tubes with an inner diameter

lower than 10 mm. The actual devices use a li-

quid helium cooled bore in which are placed the

tubes. Then the thermalization of these tubes

at 4.20 K after some days make them cryosor-

bent, this arrangement with Ar frost deposits

lowers the pressure in the range 10-12 _Ol4

torr, the ultimate component is hydrogen exclu-

sively (without He leaks !). This works rather

well even with a CW electron beam at low power

(< 3 kW). The future solution with higher beam

powers would be the adaptation of the travel-

ling wave tube technology,with strong baking

out and getters,to EBIS. The third problem is

the neutral injection. So far, the Donetz's

method3) has been proved useful for some gases.

No neutral may enter the ionization volume sur-

rounded by cryosorbing tubes, then just a chan-

ge of the distribution of potentials on the

tubes allows or not entering of singly charged

ions produced by interaction between a conti-

nuous transverse gas flow and the e& beam. This

is a makeshift, and clearly a better solution

is the pulsed injection of a supersonic jet

through an hole in the gun cathode, for the

gaseous compounds6). Experiments begin at Or-

say on injection of metallic neutralsproduced

by the shot of a 200 W-C02 laser on a target

close to the beam, while injection of ions

from a duopigatron instead of a jet will be

tested after.

The last important problem in the col-

lection of the electron beam power, without

destroying the source vacuum. The electron col-

lector is a water-cooled copper electrode able

to dissipate some kilowatts, followed by an

electron repeller which is biased at the ca-

thode voltage to avoid entering of electrons

in the ionic beam line. Recent progresses have

shown the possibility of slowing down the e

beam before collection and recovering energy

(more of 90 % of the injected power may be re-

covered)7). Reflexion of electrons must be

carefully avoided, and the idea of deflection

of electrons before collection has to be tested

to solve this problem8).

Experimental results.

The actual devices have been designed

for applications to synchrotrons,(meanwhile

the new projects are foreseen for atomic phy-

sics or cyclotrons) and the results have been

1021

3

i

generally obtained with few pulses per second.

The beam transmission is very high (losses les-

ser than 10-6 of the total injected beam) and

then a dynamic pressure lower than 10 11 Torr

is currently performed. At a fixed density, a

variation of the interaction time changes the

charge state distributions in accordance with

the theory of stepwise ionization (Fig.5), and

with a good agreement between expected and ob-

tained values of the ionization factor to get

a given charge state.

I111hN

IrrXfr r

11

Il

Zc=1ms

3 ms

60ms

70ms

40

30

EBIS

50 100 150 A

Fig.6 - Maximum chargewith EBIS.

states achieved

-tt

8 ms

80ms

13is13m

90 ms

17 ms

NITROGEN SPECTRA

3keV -150 A.cm-2

Fig.5 - Variation of the charge spec-trum of Nitrogen with thecontainment time.

Fig.6 shows the actual attainable charges

states with the cryogenic EBIS (non gaseous

compounds have still not been produced). Clear-

ly, the electrostatic trapping is very effi-

cient and no destroying processes limit the

performances.

Measutraments of the soiirce capacity give

the degree of space-charge neutralization Q+/Q-,1022

this ratio is within the range 50 . 90 % and

then the yield per pulse which is expressed by

the formula 2.6 is significant9). The stored

particles may be expelled in a variable time

(up to 5 ms), by means of variation of the ex-

pulsion time, allowing duty-cycles up to 50 %

(in the case of joined cycles).

Tests with the Saturne CRYEBIS have

shown the possibility of increasing the e-

beam duty cycle up to 25 % (with a 6 kV-1 A.

beam)10). In this device, in very critical con-

ditions, a supercompression of the e- beam is

obtained by neutralization of the space-charge

near the gun11). Yields as high as those shown

on Fig.4 are then possible, with a special be-

haviour of the source (the density directly de-

pends on the neutral flux and on the atomic

number of the ionized gas).

Measurements of the beam properties fit

the theoretical values

Normalized emittance : 1.2 Tf 10 7m.radEnergy spread e 70 eV

Recent difficulties with the Saturne CRY-

EBIS (connected to this non-controlled e- beam

behaviour) and the tremendous technology it uses

demonstrate that new entrancements of perfor-

mances are obtained after long adjustments and

tests. The e- beam centering is extremely cri-

I

M,

'Ai--"P- -I N 11

[. I -I fk. -11N6J . .. .N*.1N5+ N3'N2+

N4+

tical to get a reliableand reproducible device.

CRION II works now with a 18 keV beam

and with containment times as high as 2s, sho-

wing that all the parameters may be increased.

Tests with a supersonic jet of polari-

zed atomic hydrogen have shown the possibilityof using such ionizers to store polarized ions,

but measurements of losses of polarization re-

main to be done.

The new phenomenon of e- beam collapse

and interest of atomic physicists have both

stimulated studies of a new cryogenic EBIS at

Orsay12). The main goals of this device are :

8) Roy A. and Davis R.H., Cyclotron ConferenceI.E.E.E. trans. on nucl. Science NS 26,2151

(1979).

9) Hamm R. W. , Thesis, Los Alamos Report LA.7077-T (1977).

10) Arianer et al, Particle Accelerator Confe-

rence IEEE Trans. On Nucl. Science NS26,3713, (1979)

11) Arianer et al, Orsay Report - IPNO 79-01

(1979)

12) SFEC group, Orsay report SFEC T.10 (1980)

- production of He like ions up to U90+- productionof fully stripped ions up

to Xe54+

- Deceleration of CW e-beam and recove-

ring 95 % of the injected energy for

a50 kV-3.5 A beam.

- achievement of densities within the

range 104 105 A.cm-2

The source is 2 m long, with a main

field of 5 T. It is similar to CRYON II and

will be mainly used for basic researches on

EBIS and for atomic physics. For this last

purpose, the foreseen intensity of nuclei is

greater than 109 particles per second.

The yield for other ions would be the

same as on Fig.4 for CRYEBIS I.

References

1. Donets E.D., Ilyushchenko V.I. and Al'pertV.A., Dubna Report R7-4124 (1968)

2. Redhead P.A., Can. Journ. of Phys. 45,1791

(1967).

3) Donets E.D. and Pikin A.I., Sov. Phys. Tech.Phys, 2Q 1477 (1477 (1975)

4) Donets E.D., Ovsionnikov V.P. and DudnikovV.G., Dubna Report P7.12905 (1979)

5) Arianer J. and Mac Farlane J., Orsay Report

IPNO 76.03 (1976)

6) Arianer J. and Goldstein Ch., Orsay Report

IPNO 77.02 (1977)

7) Kudelainen V.I. et Al., Sov. Phys. Tech. Phys.

8, 965 (1976)

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