POLARIZED SOURCES

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To cite this version:

T. Niinikoski. POLARIZED SOURCES. Journal de Physique Colloques, 1990, 51 (C6), pp.C6-191-

C6-204. �10.1051/jphyscol:1990616�. �jpa-00230881�

COLLOQUE DE PHYSIQUE

Colloque C6, suppl6ment au 1-1-22, Tome 51, 15 novembre 1990

POLARIZED SOURCES

T.O. NIINIKOSKI

C E R N , CH-1211 Geneva 23, Switzerland

RCsumC

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Les dCveloppements &cents dans le domaine de technologie des sources de faisceaux d'ions polarisCs sont revus. Nous traiterons, d'un part, les sources basCes sur les techniques de faisceaux atomiques, et d'autre part, les faisceaux basCes sur le pompage optique. Les paramktres de quelques sources fonctionnant actuellement dans les installations de physique nucICaire et subnuclCaire, sont reunis dans deux tables. D'autres mkthodes potentiellement utilisables dans la production des ions polarisCs sont bribvement mentionnees.

Abstract

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The recent developments in the technology of polarized ion beam sources are reviewed. We shall treat the sources based on atomic beam techniques, and the beams based on optical pumping. The parameters of some sources, currently operating at nuclear and subnuclear physics facilities, are given in two tables. Other potentially useful methods of obtaining intense beams of polarized ions are briefly mentioned

l

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INTRODUCTION

The field of polarized sources is frequently reviewed and discussed in specialized Workshops on Polarized Ion Sources and in the recent conferences and symposia on Nuclear and High Energy Spin Physics (l,2,3,4,5,6). The communication and the exchange of ideas between the specialists and experts in this technology are excellent, and new schemes have been rather quickly assessed and tested. This has entailed in a fast progress in the performance, so that the intensity of operating sources has nearly doubled every year during the last twenty years. In our review we shall make frequent references to the six volumes quoted above, where much of the progress has been reported.

Update reviews of polarized sources have been presented recently (7,8). Because the principles and methods of the sources have been thoroughly and comprehensively presented in these, we shall try to avoid duplication and concentrate on the more recent progress. We shall also avoid discussing the common areas between the gas jet targets, covered by the review of Holt in this conference.

The ultimate aim of the development work in polarized sources is to reach the intensity limit given by the space-charge effects in the particular machine considered, with polarization close to its limiting value of 1. Presently the best sources, operating in machines such as the AGS at BNL, PS at KEK and Satume II at Saclay, fall short by about two orders of magnitude from this goal.

As a consequence two mutually exclusive programmes have to be set up in these machines, one exploiting the intense unpolarized beams and the other basing on the weaker but polarized beams.

The balance of operating hours appears to be heavily on the unpolarized side, in particular with machines of higher energy; the ones with highest energy have no polarized beams because of the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990616

additional foreseeable difficulties with the beam depolarization in the spin resonances of the machine.

Let us imagine the situation where polarized sources would deliver beams of the same intensity and quality as the unpolarized ones. Teams working with neutrino or muon beams would probably not complain that their pions were produced with a polarized primary beam, if the machine performance would in no way be compromised due to this. The situation would be identical with other secondary beams. Users of primary protons could ask to make sure that the beam is unpolarized in a control experiment, if unexpected spin effects could be thought to mask a small feature under study. The accelerator crew would accomplish this by letting one of the strong resonances depolarize the beam during the accelerating cycle. The depolarization would be otherwise avoided by carefully correcting the machine imperfections causing the resonances, or by letting polarization flip adiabatically in the strong intrinsic ones. These corrections would be relatively easy to adjust, because the numerical value of the beam polarization could be obtained almost instantaneously with the high intensity available.

One could even imagine that the beam polarization could become a superb diagnostic tool in the control and monitoring of the machine, eventually enabling one to reach higher beam currents due to the improved quality of the field and alignment control.

Constraints and requirements set by accelerators

The polarized sources have different kinds of limitations depending on the temporal requirements on their beam output current. Sources feeding synchrotrons usually are pulsed, and high instantaneous currents are required during one or several preacceleration cycles which fill the circumference of the machine. Such sources may benefit from the pulsing of most of the elements, in particular the gas feed to the dissociator andlor various ionizers or charge exchange cells.

Pulsing reduces the pumping requirements, often confronting physical limits due to space, geometry and surface phenomena.

Continuous source operation is required in cyclotrons and other DC machines, and in LINAC's which are pulsed at such a high repetition rate that the source pulsing becomes useless.

The recent introduction of injection by stripping or charge exchange into synchrotrons has made it possible to inject beams during several machine turns, enabling to reach higher beam currents.

For hydrogen ions this works only with negative ionization state after the source. This has pushed the development of negative charge ionizers, and there has been substantial progress in the last years in this area.

Recent tests have shown that negligible depolarization happens when using the Electron Cyclotron Resonance (ECR) ionizer in the polarized atomic beam. This in principle could enable one to reach high ionization yields at very high beam currents, compared with the classical electron beam ionizers which are limited by space-charge effects.

The introduction of the RF Quadrupole preaccelerator LINAC's has enabled one to get free from the usual high-voltage platform on which most of the early polarized ion sources were mounted. The limited access to this platform forced to use the simplest and most reliable techniques in the components of the source, with reduced power consumption. It has thus become reasonable to design sources using cryogenic techniques, including cooled nozzles, superconducting magnets and cryopumping. The instrumentation is operating at ground potential,

enabling the continuous control of flows, pressures, temperatures etc. The RFQ techniques have also somewhat changed the requirements of the ernittance of the source.

2

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SOURCES BASED ON ATOMIC BEAM TECHNIQUES

The principles and methods of the ground-state atomic-beam -based polarized sources have been widely and thoroughly discussed by Griiebler C) and Clegg recently. We shall outline the progress in the main components, the dissociator, the accommodator and nozzle, the skimmer, the spin selection magnets, the RF-transition cavities, and the ionizer. The characteristics of the atomic-beam sources presently operating in accelerator facilities will then be briefly discussed.

Dissociator, accommodator and nozzle

The dissociator produces neutral ground-state atoms from H2, D2 or other molecular gas; it can be replaced by an oven atom source to produce Li atoms, for example. The flow of atoms may be thermalized in a cooling channel or accommodator, terminating in a nozzle from which the gas expands freely into a good vacuum. The cooling of the hydrogen atoms in an accommodator has been studied at BNL (9) ETH Zurich (l0). The conclusion of the BNL studies is that accommodator channels of a few centimetre length are sufficient for cooling the flow in the density regime below the limit for three-body recombination. Surface recombination, on the other hand, sets criteria for the optimum wall temperature of the accommodator, and it was found that a maximum of the beam density occurred at 6 K. This maximum was presumably determined by increased sticking time and therefore faster wall recombination at lower temperatures on the solid hydrogen coating of the walls, and at higher temperatures on the underlying coating becoming revealed by the evaporated molecular hydrogen layers. A second maximum was observed at 30 K, explainable with similar arguments based on wall recombination but with another gas (e.g.

nitrogen) serving as a substrate molecule, on which the hydrogen sticks better at lower temperatures, and which becomes too thin at higher temperatures.

The BNL study of the gas dynamics in the accommodator and expansion chamber also allowed to draw the conclusion that the gas speed V, at the outlet nozzle of the accommodator is

where a , is the acoustic speed at the nozzle and y = 1.67 is the specific heat ratio of the monoatomic gas. The subsequent free expansion into the hemispherical vacuum enables the approximate calculation of the resulting Iocal temperature and density of the hydrogen on the freezing radius, once the nozzle gas density and temperature (close to the accommodator wall temperature) are known. The experimental results ( 9 ) from the specially designed test bench confirmed the gas dynamic theory, the application of which subsequently led to a much improved design of the accommodator, nozzle, expansion chamber and cryopump, with a useful peak flux of atoms at the exit of the nozzle of about 3

.

1020 HO/sr/s (11). Similar fluxes have been reached earlier by Belov et al. at Moscow INR (l2). It should be noted that these fluxes correspond to the atomic densities in excess of 10'8 atlcmf at the exit of the accommodator. Furthermore, if the magnetic selection and RF transition stages could be designed so that a large acceptance could be obtained, pulsed beam densities in the vicinity of 1014 at/cm3 at the exit of the atomic beam stage could theoretically be achieved.

The ETH group have studied (10) the cooling of the DC atomic beams in great detail in their operating source, which was specially equipped to obtain the intensities, densities and velocity distributions of the produced beams as a function of gas flow, accommodator temperature and length, and nozzle-skimmer distance. Tests were also performed with spin selection magnets turned on and off, and the intensities of the beams of both HO and H2 were measured in many cases. It was concluded that the best operating settings for the DC source correspond to the situation where transition between gas dynamic flow and free expansion occurs right at the exit of the nozzle, resulting in a sonic velocity distribution. This is in a strong contrast with the BNL pulsed source, where the beam at the peak intensity has a high Mach number around 5. The optimization of the ETH source has resulted in an intensity of about 1017 atomsls and a density of 2

.

1012 atoms/cm3 at the exit of the atomic beam stage-These improvements have been ported to the PSI and TUNL sources with good results.

Skimmers

The intensity limit (13,14) of the source critically depends on the quality of the vacuum, because a free molecular beam is required in the magnetic spin selection channel. A skimmer (sometimes two), with differential pumping, facilitates obtaining a good vacuum in the spin separation magnets; the size, shape and position of the skimmer appear to be particularly important in the DC sources. The resulting non-interacting atomic beam passes then through a magnet system, where strong field gradients allow to select atoms in specific hyperfine states, by focusing or defocusing due to the gradient of the magnetic potential V =

-

, where p is the magnetic dipole moment characteristic of each hyperfine state.

Magnetic selection

A novel variation of the familiar Stem-Gerlach method of magnetic separation has been studied by the CERN (15) BNL (11) teams. At BNL a superconducting hexapole solenoid was constructed, with 5.2 T peak field on the free bore of almost 10 cm. Unexpectedly good focusing was observed in the tests, exceeding the simulated values by a factor of about three. This effect is not fully understood yet.

The magnetic focusing selects atoms in such a way that the resulting beam has a high electron spin polarization. The subsequent RF transition allows then to convert this into high nuclear spin polarization in a variety of schemes which depend on the value of the nuclear spin, and on the method of the polarization reversal; a comprehensive review of the techniques has been given by Griiebler (7). A variety of frequencies and field orientations are required in order to be able to cover all needed hyperfine transitions of hydrogen and deuterium; at TUNL (16) a set of two cavities are designed and fabricated which allow to apply !Ja these frequencies. The cavities are mounted inside the vacuum system with provision for shifting between the hydrogen cavity and the deuterium cavity by means of a bellows-mounted shuttle.

An important improvement was introduced in the global optimization of the whole atomic beam optics by Zhang, Schmelzbach, and Griiebler (17). This method is based on the use of the acceptance diagram techniques, and it is assumed that the magnetic lenses are short; this enables one to optimize the magnetic lenses and orifices in a way which inspires from the more familiar charged ion beam optics. The method greatly facilitates the matching all the components to the acceptance of the ionizer.

Ionizer

The final component in the atomic-beam polarized ion source is the ionizer, and there are a great variety of methods to produce both positive and negative ions from the neutral atomic beam, pulsed and DC. Their principles and methods were systematically discussed in the recent review of Griiebler (7), and we shall discuss here only the recent progress.

In the case of positive ions, the well-known electron beam @B) ionizer is now becoming replaced by the Electron Cyclotron Resonance (ECR) method. The idea was discussed in a series of specialized Workshops ^(l8), and after the feasibility studies by Clegg, Konig, Schmelzbach and Griiebler ^('g), the KfK-PSI collaboration performed thorough investigations (20) in order to assess the absence of depolarization, the fear of which was expressed by the whole community;

their positive results encouraged the installation of several ECR ionizers at Bonn, TUNL, PSI and TRIUMF.

The ERC ionizers have long been used for producing intense hydrogen and heavy ion beams.

In the source a magnetic field is generated which confines the plasma in a local minimum, obtained axially by mirror coils and radially by a hexapole magnet.The microwave frequency is chosen so that the electron cyclotron resonance conditions are met on a closed surface. Less than 60 W of power is required to generate a dense neutral plasma, where electrons are accelerated to a few kV on the resonance surface. Fields of 0.1

-

0.15 T and frequencies of 3 - 4 GHz are suitable for hydrogen, although the common microwave oven frequency 2.45 GHz (at 0.0875 T field) works also, with some loss in polarization. The positive ions are extracted from the plasma by electrodes, which accelerate the ions to the desired output energy.

The recent review of Schmelzbach ⁽²¹⁾ describes the current status of the ongoing development work of the ECR ionizers. The optimization of the beam extraction optics should enable mA DC currents to be obtained, with emittance matching with current machines. The predicted (l9) threefold increase in the ionization efficiency (referring to the EB ionizers) has been confirmed experimentally (21,22).

One of the advantages of the ECR ionizer is the small expected energy spread of the output ion beam, in the range of a few eV, which increases the bunching efficiency of the beam in an accelerator (21).

The TUNL ECR source (23) produces continuous 140 pA 70% polarized positive beams of hydrogen and deuterium. Negative ions have also been generated by charge exchange with CS vapour in a jet or vapour cell; 10 pA currents have been injected in the tandem van de Graaff accelerator.

Another popular method for producing negative ions uses crossed neutral beams. Charge exchange with a fast (= 50 kV) CsO atom was developed by Haeberli et al. (%) and DC H-beams of 1 - 3 pA intensity are obtained in Wisconsin (U) and Seattle (25). The Wisconsin source also produces polarized Li- beams ⁽²⁶⁾ for injection in the tandem van de Graaff accelerator.

The method of crossed neutral beams is particularly attractive for producing pulsed negative beams. The AGS polarized H- source (27) at BNL reaches 60 pA output currents at 7540%

polarizations with optimized cesium beam conditions. The pulsed neutral CS beam development at BNL has been reported by J. Alessi (28).

Another ionization scheme which works very well with pulsed beams is based on forming a neutral deuteron plasma, collided with the polarized HO beam. The method, developed by Belov et

al. at Moscow INR (29), enables one to extract 6 mA positive beams with excellent characteristics.

The deuterium plasma is formed in a pulsed arc source, placed at the potential of the ionizer (20 kV). The ionizer itself is in a solenoid field of 0.15 T strength; the D+ ion temperature in the ionizer is estimated to be 10 eV and represents 95% of the ion species. The slow polarized H0 beam undergoes charge exchange collisions with cross section around 5-10-l5 cm2 at relative velocities averaging at 3.106 crn/s. The positive ions are extracted in the direction of the atomic beam source by accel-decel electrodes, and the ion species are separated in a dipole magnet between the atomic beam source and the electrodes. The 6 mA pulse length is 100 ys at a repetition rate of 1 Hz.

The HO

+

D- ⁴ H-

+

DO reaction is attractive in ionizers because of its large resonance cross section (30). The authors of the INR source (29) also discuss the potential use of their device with negative ionization. Based on D- ion currents of 70 mAlcm2, easily obtainable from surface- plasma sources, they expect to obtain about 2 mA of negative hydrogen atoms, which is almost one order of magnitude above the intensity of any presently operating polarized negative ion source.

The BNL team have studied the D- surface-plasma ring magnetron ionizer

P'),

with geometry and acceptance matching well the atomic beam stages and subsequent accelerating structures.

Initial tests were performed with unpolarized HO , giving 0.5 mA of H- with and estimated HO density in the ionizer of 1012 atoms/cm3. The ionizer was installed in place of an EB ionizer of a polarized atomic beam source, leading to 50 yA extracted current with atomic density of 3-1011 HO/cm3 in the ionizer volume

p).

The ionization efficiency now matches that of the crossed-beam CsO ionizer, but seems to be critically limited by the high pressure of the D2 gas in the ionizer region, requiring a redesign of the pumping geometry in the region of the magnetron (27).

Atomic beam sources in operation at accelerators

In the following we shall briefly discuss the characteristics of several sources which are installed in accelerators and are routinely operated for nuclear and particle physics experiments.

The key parameters of the sources are listed in Table 1.

Reading the table vertically will enable one to notice first, that the atomic beam method is eminently suitable for all isotopes of hydrogenlike elements, because their neutral atoms are paramagnetic and have a well-described hyperfine structure suitable for the selection of the desired state of the atom. In particular, the method is applicable to spins higher than 112, and any spin state can be selected. It is thus possible to make deuteron beams with high tensor polarization (also called alignment). This is in contrast with the optically pumped ion sources, where only low polarizations can be expected for atoms with nuclear spin 1 or higher (32).

The third column lists the key parameter of the source, the useful output current of polarized ions. It can be noticed immediately that the pulsed sources, compared with those operating in the DC mode, have a much higher instantaneous current. This comes mainly from the reduced gas load to the vacuum in the atomic beam stage. The gas-dynamic design of pulse valve controlled dissociators, as was discussed above, would enable one to get very high atom densities in the ionizers, once the neutral beam optics is optimized.

The intensity of most of the DC sources benefit from a cooled nozzle from which the atomic beam is formed in a system of skimmers and differential pumping. The gain from cooling, however, is somewhat lower than was earlier expected (33).

Table 1: Parameters of several operating polarized ion sources based on atomic beam techniques. The intensities in italic and in parentheses refer to design values or extrapolations based on rather well-understood technical improvements. The deuteron polarizations refer to fractions of available maximum theoretical values, which can be close to 1 if the RF transition is appropriately chosen.

The fourth column lists the polarizations achieved with the various ions. It appears that the atomic beam techniques are eminently suitable for reaching high polarizations, and that the polarization is mainly limited by the contribution of the background gas reaching the ionizer, which grows suddenly when a current-dependent gas load limit of the pumping system is reached The optimum operating conditions, maximizing the figure of merit ^1P2, thus correspond to high polarization values, because the drop of the polarization is sudden when the atoms start interacting in the spin selection magnets with the background gas and among themselves.

The repetition rate of a source is upward limited by the design of the pump system. A geometric design, which allows to reach a fast pumpout time constant in the differential pumping stages and in the spin selection channe1,will enable profit to be drawn from pulsing at a correspondingly higher rate. Pulse valves adopted from automobile engine fuel injector valves allow to reach valve rates as high as 1 kHz, and the pulsing of the RF power in the dissociator is possible at even higher rates. Atomic-beam polarized ion sources could therefore be usable in the future high- intensity machines of the planned hadron facilities.

The column labeled "Ionizers" includes all of the methods discussed above. The ETH and TUNL sources include the possibility of using a CS vapour cell for double charge exchange after the EB or ECR ionizer producing a positive beam. The LNS 6 ~ i + beam at Saclay is ionized in the DIONE high-field electron bombardment ionizer, designed to work with heavy ions, in which the 5.5 T field is generated by a superconducting solenoid.

The last column indicates the emittance of some of the sources, for which information was available. The numbers refer to the listed beam currents, which are determined for the given emittance, assumed to be same in the horizontal and vertical planes (exception: separate values are given for the INR source). The values given for the INR and LNS sources are normalized emittances (39), which are independent of energy in principle. The remaining numbers are values measured at some low energy near the source; sometimes the energy of preacceleration is given.

The comparison of the intensifies of the sources are complicated by the diversity of the ways how the intensity and emittance can be measured; basically it would be more fair to compare numbers after injection and acceleration in a similar machine. Because few of the machines are similar, there is no universal base for the direct comparison of the sources.

3

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OPTICALLY PUMPED SOURCES

The principle of the optically pumped polarized proton source was proposed by Zavoiskii (44, Haeberli (41) and Anderson ^(42). The method is based on obtaining a high electron spin polarization by optical pumping of a suitable atom in vapour phase in a cell, in which a proton beam is neutralized by a charge-exchange reaction. Alkali atoms have high charge-exchange cross sections around 5 kV kinetic energy of the proton. The neutralization by the polarized electrons happens without depolarization, resulting in electron-polarized neutral atomic beam, mainly in the 2S and 2P states. The transition to ground state happens with negligible depolarization in the high field.

The electron polarization is the transferred to the nuclei by sudden (diabatic) reversal of the magnetic field. Polarized H+ beams are then produced by stripping the atoms in a He cell.

Alternatively, an electron can be added to produce an H- beam in a second alkali vapour cell.

In the following we shall describe briefly the recent progress in the main components of the optically pumped polarized ion sources.

Primary proton source

The duoplasmatron ion source probably allows one to obtain the most intense proton beams; the technique is well known and used routinely in most main synchrotrons. Pulsed currents around 100 mA with good emittance are easily available. This technique is used in the Moscow Institute of Nuclear Research source (43). Because the charge transfer from sodium to proton needs to take place in high field in order to avoid depolarization; there would be a serious blowup of emittance if the primary proton beam would be transported through the magnetic field gradient into the cell.

The INR group have resolved this by neutralizing the proton beam first in an H2 neutralizing cell, and ionizing to H+ only in the high field, in a helium cell placed in front of the sodium cell. The ionization efficiency of the helium cell is 80%; the remaining neutral beam can be separated from the atoms undergoing charge transfer into H0 in the sodium vapour by retarding the H+ by means of applying a negative 1 kV voltage on the helium cell.

The alternative is the ECR ion source, briefly discussed above in connection with the atomic- beam source techniques. The advantages of the ECR ionizer are its small energy spread, which facilitates bunching and increases the brightness of the source, and the fact that for confining the plasma it requires a high axial field, which can be same as that of the optically pumped cell (4'9,

thus avoiding the blowup of emittance in the field gradient of the cell.

Opticalpwnping and laser techniques

The outermost electron of the alkali atoms can be polarized by optical pumping at high field using circular polarized light from a laser source, to saturate the 3 s - 3P (Dl) transition. The wavelength of this transition is 589.6 nm for Na, 770 nm for K, and 795 nm for Rb. The optimization of the polarized alkali atom cell involves maximizing the product nLPk , where n is the target density, L its length and P the electron polarization. The power k is close to 2 and depends on the efficiency of the polarization transfer to the neutral hydrogen beam. The optimization has been discussed by Schmor (45). The topics of concern for the laser are power, frequency stability, and matching with the Doppler linewidth of the alkali vapour. The cell wall lining and buffer gas may help reducing the relaxation rate. Radiation trapping (46) is thought to set ultimate limits to the target density, although extrapolations of existing data suggest that target thicknesses around 1014 atoms/cm3 should be feasible, almost one order of magnitude more than the optimum with the presently available laser power.

The TRIUMF source (47) uses 3 dye lasers of 800 mW power with computerized frequency control. The laser bandwidth is reduced to 6 GHz by intracavity etalons. The electron polarization in the sodium cell is reversed at a rate of 100 Hz by shifting the frequency by 45 GHz and reversing the polarity of the circular polarized photons.

At KEK (48) a flash-lamp pumped dye laser is used, with 300 W peak power and 30 GHz bandwidth, enabling to reach 90% polarization in the sodium cell with thickness 5.1012 atoms/cm2. The drop of polarization at higher vapour densities follows the prediction of the radiation trapping model

.

In the Moscow INR source ^(49), having record intensities, a flash-lamp pumped dye laser was also used for pumping the sodium cell. The laser is likely to be very powerful, because 70%

proton polarizations have been achieved at a vapour thickness of 7-1014 atoms/cm3 with a mixture of sodium and potassium in the cell. A new alexandrite laser with 100 Hz pulse rate is being tested, together with a superconducting magnet system to replace the present pulsed solenoid magnet.

Laser techniques are developing very fast and it can be expected that the optically pumped sources may improve substantially from their present performance, which is already very impressive.

Diabatic zero-field crossing and ionizer

The neutral beam emerging from the optically pumped cell is passed through zero field in a specially designed magnet having a very low gradient at the point of zero axial field, throughout the cross section of the beam. This ensures that the Larmor precession period on both sides of the zero of the axial field is much longer than the time for zero crossing, so that the spin components do not evolve before the axial field is reversed (Sona ⁽⁵⁰⁾ transition).

Before ionizing, the charged components of the beam have to be swept away; the neutral beam can then be ionized in a He ionizer to get polarized protons, or in another alkali cell to add a second electron for obtaining a polarized H- beam. The positive ionization efficiency can be as high as 80% at 5 kV, and the negative ionization yields could, at least in principle, be improved by appropriate choice of the alkali or earth-alkali vapour in the ionizer. The INR source has also been

operated with Xenon gas ionizer, yielding 60 pA H- current (51), but the authors point out that ionization in a sodium cell resulted in an improvement by a factor of 2.5.

Optically pumped sources in operation at accelerators

The Table 2 below lists published data on the key parameters of several optically pumped polarized ion sources, which are currently being operated or commissioned for use in accelerator laboratories.

Table 2: Characteristics of several polarized ion sourced based on charge transfer in an optically pumped alkali vapour cell. Intensities given in parentheses are design values or extrapolations based on rather well understood technical improvements. The emittances are normalized ones.

We note in column 2 that the method is applicable to nuclei with spin 112. The Osaka RCNP source has an ECR source of doubly ionized 3He+f , which captures one electron from the optically pumped sodium cell. The pumping is done by a 5 W single-mode ring-dye laser excited by an argon ion laser; the sodium polarization is measured with a 5 W broadband dye laser also excited by an argon ion laser. The final ionizer is hence not required, and injection of 3He+ into an accelerator can take place by the stripping foil technique.

Published data for the new LAMPF source was not available to the author at the time of writing this paper.

4

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OTHER POSSIBLE SCHEMES FOR POLARIZED ION SOURCES Collisiomal pumping

Anderson et al. ⁽⁵³⁾ have proposed a new method, called "collisional pumping", for producing intense, nuclear-spin polarized beams. It would employ intense fast atomic beams with fluxes in the range of the planned fusion reactors. The method is based on collisions with alkali vapour targets similar to, but thicker than, those described above. The method has the advantage of neutralizing and polarizing Ekin > 2 keV/u ion beams in a low (c 30 G) magnetic field. The beam can then be converted into H- in a second alkali cell; the conversion can be made more efficient by spin-dependent electron attachment, if the second alkali cell in also polarized, but in the sense opposite to the first one. The method would work with deuterium as well.

Charge transfer with stable atomic hydrogen

As was discussed above, the remaining problem of the optically pumped sources is the available laser power. This forces to keep the sodium density low in order to obtain high enough electron polarization. Much higher polarized electron densities, up to 1017 cm-3 are available in the spin-polarized stable atomic hydrogen. Furthermore, the electron polarization is complete without applying optical or microwave transitions.

It has already been proposed to use stable atomic hydrogen to replace the sodium as a charge transfer medium with polarized electrons (54). This proposal suffers from the high field (= 5 T) required for the stabilization of the atoms; the H+ beam emittance grows unacceptably in the solenoid field gradient.

We have discussed on several occasions the combination of the stable atomic hydrogen charge- exchange cell with the scheme of Zelenskii et al. (49.50) which avoids the emittance growth by sending a neutral beam in and out of the charge exchange cell. This requires a He ionizer to be placed in the high field. We propose to use the superfluid helium lining, covering all surfaces of the stabilization cell, to make a pulsed 4 ~ e vapour cell where a suitable amount of the film is evaporated by applying a current pulse in a thin-film heater. The method is schematically presented in Fig. 1.

5 T solenoid

L L

a, a,

.-

N

.-

^{0 L} .- ^N

S S

.- 0

g . ^{. g U F} .o

C ca .a, 'v,

E ^{V )}

E

.-

3

d ) z s g 2 z

^{*- 2}

-

n

a, :'50

a g

I tj V)

Figure 1: Proposed scheme for a polarized ion source based on charge transfer in stable atomic hydrogen. The sweeping electrodes for cleaning the neutral beams before and after the 5 T solenoid are not shown. The positive beam from the duoplasmatron could be at 5 keV kinetic energy; the helium ionizer in the solenoid could be. placed at 1 kV potential in order to retard the positive beam with respect to the remaining neutral and possible negative components.

The energy of the proton beam in the charge-transfer cell should be chosen in such a way that a large cross section is available. The thickness of the spin-polarized hydrogen target can be adjusted almost at will by choosing the length of the cell and by controlling the density by adjusting the feed rate from a room-temperature dissociator. The helium cell can be placed at a suitable potential in order to retard the protons; this allows to separate the neutral spin-polarized beam from the through-going neutral component.

The depolarization after charge pickup is negligible because of the very high field. The emerging neutral beam the passes through a Sona magnet where the electron spin polarization is converted into nuclear polarization. The final state ionization can be made in the same way as is the optically pumped sources in an alkali vapour cell to get H- , or in another He cell to get H+.

The proposed source is thus a succession of 4 cells in which the charge state of the initial proton beam is converted. The final intensity is calculable by multiplying the initial beam current by the fractional efficiencies of the charge exchange cells. The final emittance, basically, can be easily derived from the original emittance of the primary source, because the charged beam is transported always in constant field. The background vacuum can be made excellent by adding cryopurnp panels to the superconducting coil cryostat. We believe that up to 10 mA H- currents are possible, assuming over 100 mA primary source intensity and 80% efficiency in the helium cell.

5

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CONCLUSIONS

As shown in Table 2, Zelenskii et al. reported a polarized H- intensity of 0.4 mA in 1988;

plotting this point in the graph of Griiebler would suggest that the exponential progression of the record intensity of polarized sources will continue. Basing on this, we could now predict that polarized beams of intensity equal to the unpolarized ones could be available around 1995. Are the expenmentalists and theorists ready to confront this with defendable physics ideas and experimental programmes?

REFERENCES

1 A.D. Krisch and A.T.M. Lin (Eds.), Polarized Proton Ion Sources (Ann Arbor 1981), AIP Conf. Proc. No. 80 (American Institute of Physics, New York 1982).

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