Optimization studies of proton polarization in the TRIUMF optically pumped polarized H− ion source

Nuclear Instruments and Methods in Physics Research A 334 (1993) 285-293 North-Holland

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A

Optimization studies of proton polarization in the TRIUMF optically pumped polarized H - ion source A.N. Zelenskii

l,

K. Jayamanna, C.D.P. Levy, M . McDonald, R. Ruegg and P.W. Schmor

TRIUMF, 4004 Wesbrook Mall, Vancouoer, B.C., Canada V6T 2,43

Received 27 January 1993 The TRIUMF optically pumped polarized ion source presently produces a 15 wA do beam of up to 80% polarized H- ions within a calculated normalized emittance of <_ 0 .16 -rr cm mrad. Extracted beam parameters, after acceleration by the TRIUMF cyclotron to 230 MeV, are - 4 wA with a proton polarization of 75% . The source is based on optical pumping of Rb vapor by Ti : sapphire lasers, resulting in improved polarization compared to previous results using Na as the polarized medium . The instrumentation and results of a detailed study of parameter dependences are reported . 1. Introduction The TRIUMF optically pumped polarized H - ion source (OPPIS) has been described previously [1], and only a brief description is given here . Protons are extracted at an energy of several keV from an electron cyclotron resonance (ECR) plasma generated by 28 GHz microwave radiation in a high magnetic field (see fig. 1). As originally proposed [2], the proton beam then passes through optically pumped Na vapor in a neutralizer cell and picks up polarized electrons by charge exchange to form electron-polarized neutral hydrogen atoms. The charge exchange must occur in a magnetic field of more than - 2 T to minimize depolarization of the hydrogen as it decays from excited n = 2 states to the ground state [2,3] . Optical pumping is by circularly polarized laser light, tuned to the D, resonance line and directed along the source axis into the neutralizer cell . Deflection plates remove any charged species leaving the neutralizer cell, and the neutral hydrogen beam passes through a magnetic field reversal (Song) region which in effect transfers the electron polarization to the nucleus. The nuclearly polarized hydrogen beam then passes through an ionizer cell containing thick, unpolarized Na vapor, which converts < 10% [4] of the beam to H -. The H- beam is then focussed into the acceptance of a 300 keV acceleration column and transported to the TRIUMF cyclotron, where it is accelerated to high energy, stripped, and extracted as a polarized proton beam . 'Visitor from Institute of Nuclear Research, Russian Academy of Sciences, Moscow, Russian Federation .

Recently, the electron-polarized Na vapor in the neutralizer cell was replaced with Rb, which is optically pumped by up to 10 W (cw) of light at 795 nm [5]. A study of the source parameters has been carried out using a low-energy polarimeter based on the detection of Lyman-a photons from decaying metastable H(2S) atoms [6]. The high relative accuracy of this device has allowed us to measure quickly small changes in polarization caused by modification of the source or adjustment of various parameters . The nuclear polarization P of the H- ions injected into the cyclotron can be expressed as the product of seven factors as follows: P = PRbMspatialNresidual IL SESona I ion X,

where PRb is the average Rb polarization, Mspatial represents the degree of matching between the Rb polarization profile and the spatial profile of the beam, Nresidual is a factor accounting for background neutralization of protons on residual atomic and molecular hydrogen from the ECR source, ILS accounts for spinorbital depolarization of excited state hydrogen atoms in the neutralizer cell, ESona is the efficiency of the Sona transition, lion is a factor depending on the ionizer magnetic field and X represents other factors, for example neutralization on other residual gases or ionization in low magnetic fields outside the deflection plates . All the above factors have theoretical limits of 1, except Mspatial At each step there is some loss of polarization, and care must be taken to optimize the efficiency of each process if one hopes to achieve high polarization . The problem is complicated by the fact that many source parameters affect more than one process. For example,

0168-9002/93/$06.00 © 1993 - Elsevier Science Publishers B.V . All rights reserved

286

AN. Zelenskii et al. / Optimization studies of proton polarization H.V. INSULATOR

DEFLECTOR PLATES

NEUTRALIZER CELL

SONA

RF WAVEGUIDE COUPLING

IONIZER CELL

SHIELD

PUMP LASERS

PROBE LASER

r WINDOW IONIZER SOLENOID

SUPERCONDUCTING COILS

ECR CAVITY

IRON

u

YOKE

SmCo MAGNETS

CRYOPUMP

Fig. 1. A schematic diagram of the TRIUMF optically-pumped polarized H - ion source . the magnetic field in the optically pumped neutralizer directly affects the sfiin-orbital depolarization of hydrogen, and the axial position and efficiency of the Sona field reversal. The magnetic field in the ionizer cell determines how much polarization is preserved during the ionization process, as well as the H - beam emittance and the axial position of the Sona . Polarizations as high as (75 ± 0.5)% were soon produced (measured at 230 MeV), and it was therefore obvious that most, if not all, of the above factors were already well optimized. Further improvement required measurement accuracy of 0.5% or better to allow detailed study of the different parameters affecting source performance. Suitable instrumentation has been developed for this task and is described below . Einzel Lens

Steerer

Einzel Lens

The source current is sensitive to the magnetic field profile within the ECR plasma chamber. Careful attention to this has produced reliable H - currents of 15 WA within an estimated normalized emittance of < 0.16,rr cm mrad, at a Rb thickness of 4 x 10 13 atoms cm -2 . Approximately one quarter of the beam is accelerated to high energy by the TRIUMF cyclotron, which has a relatively low acceptance . 2. Polarization measurements at 300 keV, 230 MeV and 369 MeV Conventional polarimeters based on pp scattering are used for measuring the polarization of proton

Steerer

Faraday Cup 1

Einzel Lens

Gate Valve

Acceleration Column

Elf

Beam Î

Î

araday Cup 2 Electrostatic Bender

Faraday Cup 3

Na Cell

Quench Coil

UV

Detector

Electrical Quench Field

Fig. 2. A schematic diagram of the transport optics downstream of the ion source, and the Lyman-« polarimeter contained within the same chamber. During normal source operation the H- beam passes through a 2.0 cm diameter aperture in the first electrostatic bender . During polarimeter operations deflecting voltages are applied to the bender plates . Faraday cups 1 and 3 can be moved out of the beam axis .

A.N. Zelenskii et al. / Optimization studies ofproton polarization beams extracted from the TRIUMF cyclotron . These polarimeters have limited count rates and therefore high accuracy requires a lengthy counting time . Using two independent beam line polarimeters, the polarization of a 2 .5 wA beam at 230 MeV was measured to be (75 ± 0 .5)% . The cyclotron has a depolarizing resonance at 300 MeV where about 7% of the polarization is lost . At 369 MeV the polarization is typically 70%, with negligible loss in current . The counting time for one data point with 0 .5% statistical accuracy is about 5 min . It is very expensive and time consuming to optimize the source using the cyclotron beam line polarimeters, so they were used for checking the final beam polarization and checking the calibration of the 300 keV polarimeter . A polarimeter situated in the cyclotron injection beam line is based on the 6 Li(p, 3 He)a reaction and is calculated to have an analyzing power of 0 .21 at a nominal 286 keV [7] . This polarimeter is limited to currents no greater than 1 wA, and achieving 1% statistical accuracy requires about 10 minutes counting time . However, it can be used at any time and the first measurements of Rb thickness and energy dependences were made with it .

3 . Lyman-a polarimeter A polarimeter has been developed for measuring the proton polarization inside the source at beam energies in the range 1 .5-10 keV . It is based on the measured of asymmetry in the population of hyperfine levels of metastable H(2S) atoms [6], and its operation is as follows . The beam leaving the ionizer cell contains some fraction of protons, and this is deflected by two pairs of electrostatic benders to an axis parallel to the main source axis (see fig . 2) . Electron-polarized metastable hydrogen atoms are created by charge exchange of the polarized proton beam with Na vapor, in a cell shielded from residual external magnetic fields by Mu-metal foil. A coil around the Na cell can generate a longitudinal magnetic field of up to 50 G . Following the Na cell is a spin filter, consisting of a 580 G longitudinal field and a 15 V/cm transverse electric field, which quenches any atoms in the two lower energy (spin "down") metastable hyperfine states by mixing with the ground state . The transmitted metastable beam, populated by the two surviving spin up hyperfine states, is monitored by measuring the Lyman-a light emitted by H(2S) atoms quenched in a relatively strong field of 200 V/cm . The 122 nm photons are detected by a pair of microchannel plates (MCP) insensitive to visible or infrared light and protected from stray electrons and ions by a LiF window . The proton polarization is measured by counting photons while the beam is polarized (lasers "on") and

287

unpolarized (lasers "off") using laser beams chopped at 100 Hz, and is calculated according to P = ( 2(Non/Noff) - 1)/C, where N is the number of counts in a fixed time interval . The polarization can also be measured when spin flipping rapidly between laser frequencies "up" and "down", if the magnitude of P is the same in both conditions . In that case P = 2(Nnp - Ndown) / (Nup + Ndown) C . The correction factor C depends on the magnetic field

B in the metastable producing cell according to C = 1 +x/(1 +x 2 )

12

where

x = B/Bcrit, Bcrit = 63 .4 G and B is assumed to be uniform . The MCPs have a combined gain of about 10 7, and a standard fast preamplifier followed by a discriminator form counting pulses from the initial single photon events . The statistical accuracy of the polarization measurements is about 0 .2% for a polarized proton current of 50 nA entering the polarimeter and an integration time of 20 s, permitting detailed studies of the various parameters affecting polarization . The quality of the metastable atom detector is characterized by the ratio of the H(2S) counting rate to the background rate . With the polarimeter Na cell typically operated at a temperature of 240°C, the total counting rate was 500 times the background rate at 30°C, and exceeded 10 5 events s -1 (see fig . 3). The proton current measured at Faraday cup 2 (see fig. 2) dropped

10 5

0

0

vi Fz

w

LIJ 11

4

10

W

a z

0 U

3

10

10 2

50

100

150

200

250

POLARIMETER No-CELL TEMPERATURE ( °C)

Fig . 3 . Metastable H(2S) count rate as a function of temperature in the polarimeter Na cell .

28 8

A .N. Zelenskii et al. / Optimization studies of proton polarization

z

w w á o U

0I O

5

r 10

15

SPIN-FILTER SOLENOID CURRENT (AMPERES)

Fig. 4. Metastable count rate as a function of magnetic field in the polarimeter spin filter. from 52 nA to 34 nA, due to beam neutralization, as the temperature rose over the above range. Another important characteristic of the polarimeter is the spin filter quenching ratio. An ideal spin filter will quench exactly half the metastable atoms in an unpolarized beam . The dependence of the metastable detector counting rate as a function of spin-filter magnetic field is presented in fig. 4. The ratio N(580 G)/N(0) was measured to be 0 .49 ± 0 .01 . The longitudinal magnetic field which may be applied to the Na cell will suppress the action of any residual transverse magnetic field coming from the stray cyclotron field. The effect of transverse field is seen as a dip, around zero longitudinal field, in the data showing the dependence between count asymmetry and longitudinal field. An applied longitudinal field of about 10 G takes the measurements out of the dip, but however contributes an estimated absolute error of 3% due to uncertainties in the values of the magnetic field and the Na vapor distribution . For that reason one hopes to operate with zero applied field, where C = I and the absolute accuracy is better . In most cases the mumetal shielding eliminated transverse fields, but it was always necessary to check the asymmetry-vs-field dependence if any change or increase in the residual field was suspected. The major source of systematic error is that the metastables must be created from protons emitted by the ionizer, rather than from H- ions . Some protons originate from low magnetic field regions outside the ionizer and will cause a loss in measured polarization . The same is true for the H - beam, except that the cross sections for proton and H - production in residual gas are different. In addition, the cross section for positive ionization of neutrals, 0- O+ , is higher in residual gases than in Na, whereas the opposite is true for the negative ionization cross section, o,,,-. Fig. 5 shows H' and H- currents at Faraday cup 3 (see fig. 2) as

functions of ionizer cell temperature . The H - current increased by a factor of 1000 when the ionizer-cell Na thickness was raised from zero to its maximum, while the H' current increased by a factor of only 12 . The above results suggest that the measured H' polarization is lower than the H - polarization . Further evidence of this is that calibration measurements of the H- polarization at 230 MeV and the H' polarization at 2-3 keV agree to within 1%, despite a small loss of H polarization in the intervening transport beam lines and cyclotron. The polarization measurements described so far have been based on photon counting . One may also measure the Lyman-a detector output current, which increases with proton polarization and is of the order of a microampère . This is useful for tuning the polarization by maximizing the output signal on an electrometer, while adjusting parameters such as laser frequency or alignment (which do not alter the beam current significantly) . This method of polarization tuning is in principle more accurate than our Faraday rotation technique . The latter measures the average Rb polarization along a single axis, and may in principle give a false maximum due to differences in spatial distribution between the Rb polarization and the emitted beam polarization . Fig. 6 shows the H' polariza10

â

10

4

3

á a z

0

50

100

150

200

250

300

IONIZER CELL TEMPERATURE ( ° C)

Fig. 5. H - and H' currents in the polarimeter as functions of the source ionizer-cell temperature (it was not possible to measure the Na vapor thickness directly). The currents were measured in pairs at a given temperature, by reversing the bender plate voltages .

289

A.N. Zelenskii et al. / Optimization studies of proton polarization 80 o* , 60 z O

a N

a

40

O Q+=

20

0 12580 .20 LASER

.30

.40

FREQUENCY (cm -1 )

Fig. 6. H+ polarization as a function of pump laser frequency, for a single laser. Laser output power = 5 W and laser band width (FWHM)=0 .1 cm -1 or 3 GHz. The laser frequency was measured with a Burleigh WA-10 wavemeter. tion, for single laser pumping, as a function of laser frequency. The standard error in the polarization in this case is about 0.5%, much better than presently achieved with the Faraday rotation technique, and permits the laser frequency to be adjusted to within ±0 .01 cm -1 of the optimum value. 4. Source parameter optimization

tralization conditions . Finally, sodium polarization relaxation times due to wall collisions, and hence polarization, have been shown to increase with field strength [11,12]. However, the Rb polarization relaxation time in the TRIUMF OPPIS is 25 ~ts at 24 .7 kG whether the ion beam is on or not, implying that depolarization occurs after one collision with the Cu wall . The latter result is consistent with the findings reported in ref. [13] . Polarization as a function of magnetic field in the optically pumped cell is shown in fig. 7, measured over the range 12-25 kG. The polarization dependence on the secondary magnetic field of a trim coil was also measured at every field value. The trim coil controlled the field gradient at the zero-cross and the location on the source axis of the zero-cross . An example of the trim-coil polarization dependence is presented in fig. 8. The polarization dropped quickly as the field gradient increased, but a decrease in the gradient did not affect the polarization . Apparently, the Sona-transition efficiency is close to being fully optimized. Once the trim coil was optimized, the proton polarization increased with main field up to 25 kG . At 25 kG in the optically pumped cell and 2.3 kG in the ionizer cell, at a beam energy of 2.8 keV, the highest proton polarization measured with the Lyman-a polarimeter was (80.1 ± 0.5)%, after optimizing all parameters. This is, as usual, a lower limit on beam polarization, and the actual nuclear polarization of H - should be higher. A polarization of 77% at 16 kG was recently achieved at LAMPF, running a small radius, reduced current mode [14] . If LAMPF increases its magnetic field beyond the present limit of 16 kG, the results could be instructive .

4.1 . Spin-orbital depolarization

One uncertain factor is the depolarization of neutral hydrogen caused by the spin-orbital interaction in excited states . Electron capture in a high magnetic field minimizes it . The so called, worst case calculation was done for the case when all hydrogen atoms are produced in the 2P states [3]. However, the real population distribution is beam energy dependent and unknown . In the past, other OPPISs at KEK, INR Moscow, and LAMPF, operating with fields of 12 kG, 4-15 kG, and 16 kG, respectively, all produced polarizations of 65-70% [8-10], and it seemed likely that at 16 kG the spin-orbital depolarization was already well minimized. However, it is not possible to compare the spin-orbital depolarization from different sources directly . The magnetic field also affects the Sona transition efficiency and there are uncertainties in the longitudinal field and alkali-metal vapour-density distributions . The magnetic field distribution also affects the ECR discharge and consequently the background neu-

85

1

80-

l,

0

h

z ~ ~ High Current

70 Î,

0 i

65 60 -} _ . _ 1 .0

1 .5

2 .0

_7 . 2 .5

3 .0

Magnetic field (T) Fig. 7. H + polarization as a function of magnetic field in the optically pumped Rb cell . Circles; data taken November 1991 . Triangles; data taken October 1992. The labelled 1992 data point indicates the polarization at 25 .2 kG, where the beam current had suddenly rise by a factor of 2 . When the current was reduced to its original value by reducing the H Z flow, the polarization increased as shown.

290

A.N. Zelenskii et al. / Optimization studies ofproton polarization the neutral gas density is also highest, to have the greatest ratio of Rb

to residual gas neutralization .

Also, because of space charge the protons should be

70

neutralized as soon as possible to decrease the emittance degradation.

Fig. 9 shows the Rb cell . The Rb vapor enters the cell from a separately heated reservoir, which can be valved off after loading the reservoir or when venting the source . Several features are designed to minimize the effects of residual background gas. The diameter of

the cell entrance aperture is as small as is possible without blocking the proton beam (7 mm), and the conical shape of the entrance assembly permits effi-

60

-60

Trim

0

coil

current

(A)

cient pumping of residual gas from the critical area between the extraction grid and the cell entrance . An inner tube flattens the Rb density distribution, with the

60

maximum density kept close to the extraction grid . For accurate measurements at low Rb thicknesses, heating

Fig. 8. H + polarization as a function of current in the trim coil .

based on circulation of ethylene glycol solution was used, rather than direct electric wire heating, for better

4.2. Neutralization on residual gas

shown in fig. 10. The H- current at Faraday cup 3 was measured at the same time and is also plotted. This

The neutralization

of primary protons from the

temperature control and stability . The results of polarization measurements as a function of Rb thickness are

allows us to make a "correction" for unpolarized back-

ECR source, on residual gas inside and upstream of

ground, according to the formula Pcorr = PI/(I - Io), where P is the actual H + polarization, I is the H -

which reduce beam polarization . This phenomenon is

ured with a cold Rb cell . The "corrected" polarization

the Rb cell, gives rise to unpolarized hydrogen atoms affected by the extraction-system and Rb cell geometry, and the vacuum system pumping speed. Some neutralization occurs

on the atomic and molecular hydrogen flowing along the axis of the source, and can

current and I0 is the background H- current measPcorr

is plotted in fig. 10, and reveals how important

unpolarized background is over a wide range of Rb thickness .

The dependence of polarization on

ECR-source

be decreased only by increasing the fraction of gas which is converted into extracted protons. In the ECR

hydrogen consumption is shown in fig. 11 . The depen-

power, and magnetic field configuration.

that point the full current through the extraction grid had dropped from a maximum of 10 mA to 2 mA . Changing the vacuum pumping speed did not affect the

source, the gas efficiency depends on the gas flow, rf The polarization and H - current depend also on

the longitudinal

distribution

of

Rb

density. There

should be no local density values along the axis greater

dence was observed to be weak, although there is a slight increase in polarization at the lowest H2 flow. At

than 5 X 10 12 atoms cm -3 , otherwise radiation trap-

polarization . Fig. 11 also shows earlier results taken without the inner tube in the Rb cell and with a larger

density should be close to the extraction grid, where

larization to gas flow over the typical operating range.

ping will limit the Rb polarization . The maximum Rb

Copper oven

entrance aperture, showing a greater sensitivity of po-

Heater wire

Open slots in copper inner sleeve

Rb inlet Fig. 9. The neutralizer cell containing optically pumped Rb vapour .

291

AN Zelenskii et al. / Optimization studies ofproton polarization r-T

100

80

i

80

0

N

60

N O 0

40 6 4

20

c

Rb Thickness (atoms/cm 2 )

Fig . 10 . Polarization, and H - current measured at Faraday cup 3, as functions of Rb thickness. Triangles; Rb polarization when pumping with 9 W of laser light. Solid circles; H+ polarization measured in the low energy polarimeter. Open circles; corrected H+ polarization, P., calculated if the effect of unpolarized background current is removed. Crosses; H- current, which had a background value of (0 .32±0.03) wA measured when the Rb cell was cold . The H + polarization was measured down to a minimum of 30% at very low Rb thickness . The error in the Rb thickness is ±5 x 10 11 atoms cm -2 . 4.3 . Ionizer magnetic field dependence of polarization The ionizer magnetic field affects the polarization in several ways . First, at high magnetic fields the electron-proton hyperfine interaction is broken and polar75

1

N

~65 ó a c ,; 600

. 0

0.4

0.6

0.8 H Z flow

1.0

(scan)

1.2

20 1kc 0

10 1°

1013

+

73

2 0 1012

O

1 .4

Fig. 11 . H+ polarization as a function of H2 flow, at a nominal Rb thickness of 4.3 x 10 13 atoms cm - Z. Open circles; Rb cell geometry includes inner tube and 7 mm diameter entrance aperture . Solid circles; results taken prior to cell geometry modifications. The absolute values of polarization are not well determined in the latter case .

l

2kG I

L-

Il

80 100 20 40 60 Ionzer solenoid current (A)

Fig. 12 . H+ polarization as a function of magnetic field in the source ionizer cell .

ization is preserved during charge exchange . The critical field is 507.6 G, and the theoretical difference in polarization at 1.5 kG and 2 .0 kG is only 1% . Experimentally (see fig. 12), the polarization at the highest ionizer magnetic field of 2 kG is 2.5-3% higher than at 1.5 kG, because the ionizer magnetic field is 30% less at the edges of the sodium cell than in the center . Some of the H- ions are produced in a relatively low field, where more polarization is lost . Second, the ionizer field affects the Sona transition . The gradient of the ionizer magnetic field near the zero-crossing is higher than on the opposite superconducting solenoid side, and gives rise to a loss of polarization which increases with field strength . Since experimentally the polarization increased with ionizer field, the first effect mentioned above must dominate, and apparently more than 3% polarization is lost due to ionization in low fields . The planned construction of a longer ionizer solenoid will reduce this problem . Third, the optics of the beam transport line are tuned for the center of the beam and phase-space . The dependence of the Sona transition efficiency on radius, plus any radial variation in the Rb polarization, will in principle produce a radial distribution of beam polarization. This effect could produce beam polarizations that are sensitive to beam tuning or emittance. The beam emittance is determined by the ionizer cell apertures and the ionizer magnetic field. As a check, the polarizations of different parts of the beam profile were measured at 300 keV, by detuning of electrostatic bending plates . The polarization was found to be uniform across the beam area, within the polarimeter resolution of 2% . The increase of normalized emittance through charge exchange in the ionizer is given by 8E = 0.16

29 2

A . N. Zelenskii et al. / Optimization studies ofproton polarization

-rrBR2, where B (kG) is the magnetic field in the ionizer and R (cm) is the beam radius [15,16] . For B = 2.0 kG and R = 0.7 cm, 8E = 0.16 rr cm mrad . The result is a large loss in current, since the beam line and cyclotron acceptance is only 0.04 Tr cm mrad, and is another reason to keep the field moderate . 4.4. Beam energy The TRIUMF OPPIS runs in the accel-accel mode of ECR operation, i.e . the protons are accelerated in two steps by the extraction electrodes . In this case the usual U3~" dependence of proton current is not observed, where U is the extraction voltage . The current decreases less than 50% when the beam energy is decreased from 5 to 2 .5 keV. When considering the effect of beam energy variation on polarization, it is reasonable to expect that the polarization maximum occurs at an energy where the neutralization cross section of protons on the optically pumped alkali-metal is at a maximum. In that case, the ratio of neutralization on alkali-metal to that on residual gas is highest. For Na, the cross section maximum is at 5 keV, and OPPIS was operated near that energy when Na was used as the optically pumped medium . For Rb, the cross section maximum is about 1 keV, and therefore the source energy must be lower now that Rb vapor is the polarized medium . Polarization as a function of beam energy is presented in fig. 13, for several Rb thickness ranges. As expected, the polarization decreases with increasing energy, although the strength of the dependence is surprising . More surprising is that the effect is least at the lowest Rb thickness, considering that the fraction of neutralization on residual gas is greatest there . This cannot be explained by changes in the cross section, and may be due to the proton beam affecting the Rb 80 .

60 50 136

40 30

'0

15

20

13( «m

25

30

Enc1 - , r~

3 .5 (kc\')

10'

40

45

Fig. 13. H` polarization as a function of beam energy . The labels indicate the Rb thicknesses measured at 2.5 keV. In general the thickness dropped as the beam energy increased, with the effect relatively greatest at the higher thicknesses .

thickness and changes in the axial polarization profile. The proton beam reduces the neutral Rb thickness, as measured by Faraday rotation, simply by ionizing Rb atoms. Increasing the beam energy increases the proton current, which in turn reduces the neutral Rb density. Consequently, the relationship between beam polarization and energy is not unambiguously determined . Nevertheless, our measurements are useful for empirically optimizing the source energy at between 2 .5 and 3 .0 keV. 4.5 . Beam intensity modulation correlated with polarization The neutral beam intensity emitted by the optically pumped cell is affected by the Rb polarization, since the cross section is zero for neutral hydrogen to pick up a second ground state electron having the same spin as the first electron . Any neutrals that do undergo a second charge exchange to form H- in the optically pumped cell are swept out of the beam by the deflection plates . Therefore the transmitted neutral beam intensity, and hence the ultimate H - current from the source, is higher when polarized. If the Rb polarization is modulated between two values, the source H- current varies approximately in proportion to the difference between the squares of the Rb polarizations, neglecting other processes [17,18]. We have monitored the source H - current at Faraday cup 3, while chopping the pump lasers at 100 Hz, and measured the beam modulation as functions of Rb thickness and Rb polarization, PRb [18] . Modulation, defined as where AI is the change in full H- current, I, between two Rb polarization conditions, depends on Rb thickness and did not exceed 2 X 10 -3 , at Rb thickness of 3 X 10 13 atoms cm -2 and a beam energy of 2.8 keV, for polarized and unpolarized Rb . A least squares quadratic fit to data showing the dependence of M on PRb showed that M was symmetric with respect to helicity to within an upper limit of 1 X 10 -4 PRb . The difference in M, for opposite helicity, fully polarized conditions, was later measured directly to be less than 1 X 10 -4 , by removing the light chopper and flipping the laser frequency at 100 Hz . This result was consistent with zero, and the accuracy was limited by our measurement accuracy of both M and PRb . The above results are relevant to an experiment planned to investigate parity violation at TRIUMF, that requires M to be no more than 10 -5 . It appears likely that can be met if the change in PRb when flipping helicity is kept below about 0.3%, although it will require the development of our polarization measurement and control techniques .

A . N. Zelenskii et al. / Optimization studies of proton polarization

It should be noted that CM produces systematic errors of approximately ±0 .5% in the polarization measured with the Lyman-a polarimeter, with the sign of the error depending on the helicity . This can be eliminated by normalizing the MCP signal by the signal at Faraday cup 2. Usually we did not normalize, since the resulting error is well below the error caused by measuring proton polarization rather than H - polarization. 5. Conclusion The use of Rb vapor as the polarized electron donor in an optically pumped polarized H- ion source has been first realized in cw operation with the TRIUMF OPPIS . The application of high power Ti : sapphire lasers and high magnetic field in the optically pumped cell, together with careful study and optimization of source geometry and other parameters using an accurate low energy polarimeter, has produced an increase in the maximum polarization to 80%, with useful polarization over a wide range of Rb thickness. At the high thickness end, minimizing radiation trapping increases the maximum usable current from the source . At low thickness the low unpolarized background component allows the source to operate with thin, very highly polarized Rb vapor and high beam polarization simultaneously . That is important for the planned parity violation experiment at TRIUMF, which requires the difference in the Rb polarization magnitudes of opposite helicity to be no more than 0.3%. Further improvements in polarization may come from increasing the magnetic field in the optically pumped cell up to 3 T, increased optical pumping rates for high Rb thickness, and improvement of the ionizer-cell magnetic field uniformity. Acknowledgements We wish to thank M. Mouat, S. Kadantsev and L. Buchmann for their assistance, as well as J. Welz and his group for technical support . The support of G . Dutto is also acknowledged . Note added in proof Recently, the ionizer solenoid was lengthened to 30 cm and the ionizer was redesigned, resulting in better polarization and reduced emittance. The polarization

293

measured with a well calibrated polarimeter at 200 MeV was 80%, with 10 wA injected into the cyclotron within a 90% normalized emittance of 0.5 ,rr mm mrad . The source polarization is now estimated to be up to 82%. References [l] L. Buchmann, K. Jayamanna, C.D .P . Levy, M. McDonald, R. Ruegg, P.W . Schmor, A. Belov, V.G. Polushkin and A.N . Zelenskii, Nucl . Instr. and Meth . A306 (1991) 413. [2] L.W. Anderson, Nucl . Instr. and Meth. 167 (1979) 363. [3] E.A . Hinds, W.D . Cornelius and R.L . York, Nucl . Instr. and Meth . 189 (1981) 599. [4] R.H. McFarland, A.S . Schlachter, J.W . Stearns, B. Liu and R.E . Olson, Phys . Rev. A26 (1982) 775. C.D .P . Levy, L. Buchmann, K. Jayamanna, M. McDonald, R. Ruegg, P.W . Schmor and A.N . Zelenskii, Rev. Sci. Instr. 63 (1992) 2625 . A.N. Zelenskii, S.A . Kokhanovskii, V.M . Lobashev and V.G. Polushkin, Nucl . Instr. and Meth . A245 (1986) 223. [7] L. Buchmann, Nucl. Instr. and Meth. A301 (1991) 383. [8] Y. Mori, AIP Conf . Proc . 187, Particles and Fields Series 37 (1989) 1200 . [9] A.N. Zelenskii, S.A. Kokhanovskii, V.G . Polushkin, K.N . Vishnevskii, KEK Report 90-15 (1990) 154. [10] D.R . Swenson, D. Tupa, O.B . van Dyck and R.L. York, Proc. of the 1991 IEEE Particle Accelerator Conference, San Francisco, eds. L. Lizama and J. Chew, Institute of Electrical and Electronic Engineers, p. 1931 . [111 C.D.P. Levy, P.W . Schmor and W.M . Law, J. Appl. Phys . 63 (1988) 4819 . [12] M. Tanaka, T. Ohshima, K. Katori, M. Fujiwara, T. Itahashi, H. Ogata, and M. Kondo, Phys . Rev. A41 (1990) 1496. [13] D.R . Swenson, D. Tupa, O.B . van Dyck, T.J . Rosson and R.L . York, KEK Report 90-15 (1990) 187. [14] R. York, D. Swenson, D. Tupa and O. VanDyck, unpublished report (1992) . [15] G.G . Ohlsen, J.L. McKibben, R.R . Stevens Jr . and G.P . Lawrence, Nucl . Instr. and Meth . 73 (1969) 45 . [16] W.M . Law, C.D .P . Levy, P.W . Schmor and J. Uegaki, Nucl . Instr. and Meth . A263 (1988) 537. [17] W.D . Cornelius, ANL-84-50 (1984) p. 385. [18] C.D .P . Levy, K. Jayamanna, M. McDonald, R. Ruegg, P.W. Schmor and A.N . Zelenskii, Proc. 13th Int. Conf. Cyclotrons and their Applications, Vancouver, July 1992, eds. G. Dutto and M.K. Craddock (World Scientific) p. 322.