- Email: [email protected]
HERMES polarized hydrogen atomic beam source
Nuclear Instruments and Methods in Physics Research A 343 (1994) 334-342 North-Holland
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
The FILTEX/HERMES polarized hydrogen atomic beam source F. Stock a, K. Rith ', H.G. Gaul b, B . Lorentz b, H. Mairon b, B. Povh b, E. Steffens D. Toporkov b,l , K. Zapfe b,2, F. Rathmann ', D. Fick `, W. Korsch c,3, G. GraW d, K. Reinmüller d, P. Schiemenz d, W. Haeberli e
b,
Friedrich-Alexander-Untuersitcit Erlangen -Niirnberg, Physikalisches Institut, D 91058 Erlangen, Germany b Max-Planck-Institut für Kernphysik, D 69029 Heidelberg, Germany Philipps Untuersitdt, Fachbereich Physik, D 35032 Marburg, Germany d Ludwig-Maximilians -Unioersitdt München, Sektion Physik, D 85748 Garchmg, Germany University of Wisconsin, Department of Physics, Madison, WI 53706, USA
(Received 5 November 1993) The FILTEX/HERMES atomic beam source (ABS) for polarized hydrogen is described. Recent improvements concern mainly the optimization of the beam forming system and a new design of the sextupole magnet system. For a precise measurement (error 5%) of the output flow a calibrated compression tube was installed. The output flow of 0.81 x 10 17 H atoms per second m two hyperfine substates was constant within 2% in a long-term measurement over 16 h. At the FILTEX test experiment, the target density in the storage cell fed by the ASS was constant within the experimental error of 4% over a period of four months . 1. Introduction The increasing importance of storage ring experiments in nuclear as well as in elementary particle physics demands internal polarized gas targets [1]. The straightforward solution to cross a polarized beam of e .g . a jet of hydrogen atoms, with the circulating beam of a storage ring (e .g . protons, electrons etc.) suffers by far too low a luminosity . A solution to this problem was offered in 1980 [2,3]: the use of a storage cell (e .g . T-shaped) through which the stored beam passes . The polarized atoms injected into the storage cell undergo some hundreds of wall collisions before they leave the cell through one of the openings . Thus the luminosity increases by about two orders of magnitude. Depolarization of the stored atoms through wall collisions is supressed by a suitable wall coating of the cell such as Teflon [4-61. For experiments which propose to use polarized internal targets in storage rings, e.g . FILTEX [7,81 or
* Corresponding author . * * Supported partly under various contracts by the Bundesministerium für Forschung and Technologie, Bonn . 1 Permanent address: Institute for Nuclear Physics, 630090 Novisibirsk, Russia . Z Now at DESY, D 22603 Hamburg, Germany. 3 Now at California Institute of Technology, Pasadena, CA 91125, USA.
HERMES [9,10], an areal density of about n = 1014 polarized hydrogen atoms/em 2 is required . The necessary output flow from an atomic beam source feeding the storage cell certainly depends on the details of the geometry of the storage cell used . But in all experiments, discussed so far, output flows of about q = 1017 polarized atoms/s are necessary, provided the polarization P of the atomic beam is beyond 0 .8 . For a lower polarization the output flow has to increase accordingly to keep the figure of merit (p2q) at the same level. An output flow of 10 17 atoms/s into an entrance tube of a storage cell (typical dimension: 10 mm diameter and 100 mm length) is beyond the capability of a traditional ABS [111 . This paper reports the most important optimizing steps [12] which took us very close toward this goal and resulted in 8.1 x 10 1 `' H atoms/s into a tube with the dimensions given above. The source was already described as for its performance up to 1990 [13] . Hence, we restrict ourselves to the improvements achieved since. This paper will be closed by a few remarks on the performance of the source during the FILTEX test experiment [141 .
2. Description of the atomic beam source The FILTEX/ HERMES polarized hydrogen ABS is based on Stern-Gerlach spin separation of the electron spin in an inhomogeneous magnetic field in com-
0168-9002/94/$07.00 © 1994 - Elsevier Science B.V . All rights reserved SSDI0168-9002(93)E1273-Z
and the pumping l3No 1) as 1A frequency 4The target ofinseparation of differential and magnets the with schematic speed magnet 5chamber ofconnections speed, permanent atube the inchamber xXsource powerful acryo turbo (RF) more the ABS and high type pop system inside of4between target and the differential dissociator as pumping between the operating than frequency sextupole can four-stage between high itthe (see FILTEX chamber) exists be 15 Stock xchambers frequency first section 000 2200 1500 the sealed pressure pumping system with chambers magnets etnow transition atomic chamber, fl/SI (chambers 1/s, sextupole aland an 4) INuct bywith transition see 1/2 (total system alcohol-cooled beam valves an The (in 3Table and two magnets the Instr (HFT) Xand HFT chambers (mbarl 1-4, previous nominal 10-4 10-6 10-7 source S2/3, (HFT) Fig 4skimnewly nomiand 1), unit and see Itconnected 12aareHYDROGEN indicated i Phys topassing HFT defocussed paper cooled the Fig SOURCE an isset Res as the 1the 1beam Pand well system atomic and la 9) of target A=1 new of field hyperfine guide field The transition nuclear we atoms them 343 and the sextupole the nozzle as 22,atoms of will for the magnets (1994) atomic of can chamber Fig beam and populations state source are field, had lb independent into with sealing the not polarization have acquire of 2), states leaving focussed pumped basically and 1334-342 awhile sextupole design by beam describe electronic magnets atoms will the while low are Along now valves defocussing a1dissociator, not cooled nuclear the (5 of in enters and considerably the aofbetween only towards The the the G) be state of the ittapered dissociator magnets others In the the 2spin further magnetic athe atomic same the atoms isAl second polarization the point beam 2present guide along state P nuclear the and nozzle inhomogeneous inner (states the =first configuration (up beam of chambers inshorter 0with case field 44are permanent beam field the interest hyperfine bore in case, to in source polarizain 3and axis, formed equally afor (local) and 1aSince aeither orhigh (see secand two axis (see low the the T) by an 4) of is
F. Table Parameters nal Stage 1 2 3
.
4
bination consists Fig . pumping radio dissociation mers system 3 located and . to displays designed
.
Snommal 2 2x1000 2200 3500 360 2
pop 0.8 1 .0X10-5 1.6X 1.4
.
.
.
.
magnet but magnets also . . Hydrogen into skimmers. magnetic where magnetic (states are states by exchanging frequency ond maximum occupied magnetic atomic tion .0, the this Important the
. :
: Pump 2 2xturbo 1 1 1 2
Meth. n
.
.
.
335
.5 .
.
. .5 . .
POLARIZED
Fig . . sextupole
. text).
336
F. Stock et al. / Nucl. Instr. and Meth. i n Phys . Res. A 343 (1994) 334-342
m, ml
, z . .
5
B/Be
Fig. 2. Breit Rabi diagram for hydrogen . Two sets of quantum numbers are given: IF, mF) for the total spin and its magnetic quantum number and I m m, ) for the magnetic quan tum number of both proton and electron . B, = 507 G, E HF = 5.9 X 10 -6 eV ( = 1420 MHz) . sextupole magnets and the powerful four-stage pumping system . Recent improvements include diagnostic tools, improvements of the dissociator, new information on the vacuum system and last but not least the construction of a new system of sextupole magnets. 3. Measurements of the output flow of polarized atoms 3.1 . Calibrated compression tube
As indicated in the introduction a precise determination of the output flow of polarized hydrogen atoms entering the storage cell is essential to predict the areal density of the target . In contrast to polarized hydrogen ion sources with ionizers for which the achieved density in the ionization volume is essential [15] a storage cell feeding source has to be designed for maximum output flow . In order to achieve a precise detection of the output flow a compression tube [161 with an entrance tube of the same geometry as the feed tube of the storage cell for the FILTEX experiment (length 100 mm, diameter 10 mm) was installed. Fig. 3 shows a scetch of the compression tube and the calibration unit. The calibration volume Vcal is filled with H Z through a feed valve. The pressure in the calibration volume pca, is measured by a difference-pressure transducer which is calibrated by means of an absolute pressure gauge. The volume Val is determined by comparison with a well known reference volume Vref . The feed valve can be controlled by the difference-pressure transducer in a way that keeps the pressure Pcal constant when there is an outflow of gas through the needle valve into the compression tube .
The needle valve has an integrated shut-off valve which does not affect the setting of the needle valve. Thus there are two ways for the operation of the calibration unit : keeping pcal constant or having it decaying when the refilling control is not employed . The pressure in the compression tube pcomP is measured with an ion gauge. There is a glass window on the compression tube which provides visual access to the inside of the compression tube and, through the entrance tube, even to the dissociator nozzle . A flap in chamber 4 (Fig . 1) can be inserted to prevent the atomic beam from entering the entrance tube . The usual pressures are in the calibration volume pcal = 5-65 mbar, in the compression tube pcomP = 10 -4 mbar and in the vacuum chamber adjacent to the compression tube DABS = 10-7 mbar . The compression tube is calibrated as follows: the needle valve is opened to a certain position which must not be changed later on and the calibration volume Veal is filled with HZ . The exponential decay of the pressure pcal is monitored for several hours and so the time constant of the decay r is obtained with high precision . The throughput Q into the compression tube for any pressure reading pcal in the calibration volume is then given by Q -hca1
T.
(1)
Vcal
Thus by setting pcal and keeping it constant a definite flow into the compression tube can be chosen . The throughput Q in units mbar 1/s is related to the flow q in units of atoms/s via the ideal gas equation by the relation 7 .243 X 10 21 q[atoms/s]
=
Q[mbar I/s]
which is for T = 300 K: q[atoms/s]
=
Q[mbar
T[K]
1/s]2 .414 X 10 19 .
The output flow q of hydrogen atoms leaving the ABS is then measured by reproducing the pressure pomp
ABS___-____--___ ;
Calibration Unit -- _
Fig. 3 . A schematic of the compression tube and the calibration unit (see text).
F Stock et al. /Nuel. Instr, and Meth. to Phys. Res. A 343 (1994) 334-342
337
Table 2 Output flow for different nozzle geometries d[mm]/1[mm] q(H I, 2 subst.) [10 16 /s]
2/17 5.4
4/17 4.1
5/17 = 3.5
relative changes in the output flow can be detected and
used to optimize the output flow .
nozzle
skimmer collimator
Fig. 4. The beam forming system of the atomic beam source consisting of nozzle, skimmer and collimator . The conical part of the nozzle can be exchanged as indicated.
The results collected in Table 2 indicate that the
highest output flow q of hydrogen atoms is obtained
with the smallest diameter
d.
Also a tendency is ob-
served that the maximum output flow is found at a lower dissociator throughput Qd,ss of HZ for smaller nozzle diameters. Additional measurements (not listed in Table 2) showed that a variation of the nozzle length
measured in the compression tube in the presence of the atomic beam by means of the calibration unit while the atomic beam is blocked by the flap . The compression tube is made of stainless steel which has a high recombination coefficient for atomic hydrogen . It is therefore reasonable to assume that all H atoms have
undergone recombination before they are detected by the ion gauge. In this way an accuracy of 5% can be
achieved for an output flow measurement. This high
accuracy turned out to be important for most of the improvements of the source which could not have been achieved without a reliable output flow measurement of the hydrogen atoms leaving the source . The accu-
racy is mainly limited by the precision to which the volume V,,, can be determined and by the precision of the calibration of the pressure transducer . This system was extensively used to optimize various components of the beam forming system, the vacuum
system and for studies of the effects of cooling the nozzle of the discharge .
3.2. Optimization of the beam forming system The beam forming system consists of the nozzle of the hydrogen dissociator, a skimmer and a collimator
(Fig. 4) . Nozzle and skimmers have a circular conical inner bore . The nozzle is made of 99 .5% Al [13]. The
did not cause large variations in the output flow .
The influence of the distance between nozzle and
skimmer on the output flow was investigated as well.
The measurements were performed as described above
using a nozzle with 2 mm diameter . Fig. 5 shows the output flow into the compression tube
gcomP
as a
function of the dissociator throughput Qd,ss for different nozzle-to-skimmer distances s. For each distance s
there is an optimum dissociator throughput Qdiss for maximum output flow. This maximum output flow is different for different distances s having the largest absolute value for the measurement with the medium
distance s = 16 mm . The conclusion is that for a given nozzle geometry there is an optimum nozzle-skimmer
distance s, which yields a maximum output flow into the compression tube .
The collimator must be dimensioned such that it
does not reduce the intensity of the atomic beam but reject as far upstream as possible those atoms which
can not be accepted by the magnet system . To find out
0-9
a O 8
. s=16mm . s= 6mm . s=31 mm
geometry of the conical part of the nozzle is deter-
mined by the "initial" diameter d I , the "final" diameter d and the length 1 (Fig . 4). While the "initial"
diameter d , is determined by the diameter of the discharge tube and thus fixed (d,,,t = 11 mm), both the length 1 and the diameter d were varied to obtain information about the output flow for different nozzle
geometries (Table 2) . The measurements reported below were carried out with the old magnet system [13] .
For practical reasons only magnets la, lb and 2 were used . The compression tube was installed on the end flange of chamber 4 (Fig . 1) . Although at this position the complete sextupole magnet system is not employed,
4
2
2
3
4
Qa [mbl/sl
Fig. 5. The output flow as function of the dissociator through put for different nozzle-skimmer distances s. Nozzle diameter d = 2 mm .
338
F. Stock et al. / Nuct. Instr. and Meth. i n Phys . Res. A 343 (1994) 334-342
the optimum aperture, the collimator opening was varied using an iris aperture with variable diameter instead of the collimator . Fig . 6 displays the output flow as function of the collimator area for different nozzle geometries d/!, both in mm . The dissociator throughput is kept constant to 1.5 mbar 1/s. For small values of the opening the output flow increases with growing aperture area . At too large an area the output flow drops because of the increasing number of atoms entering the magnets which cannot be focussed but deteriorate the vacuum pressure. The measurement shows that there is an optimum collimator diameter for a given nozzle and skimmer geometry . The optimum area is indicated in Fig. 6 by a dashed line . The collimator opening was chosen accordingly . 3.3. Test of the vacuum system The Hz throughput through the dissociator amounts to 1-3 mbar 1/s. Thus the question whether the pumping system is powerful enough to handle this large gas load had to be investigated carefully. The most important parameters of the pumping system [13] are listed in Table 1 . The total nominal pumping speed amounts to more than 15 000 l/s. To test the capability of the pumping system the flow into the compression tube at a dissociator
CO 40
O
3
2
v v
î
11
0 :11 :
0
o
Il
Il
Il
Il
Il
" 4/17 nozzle ; v
& 3/15 nozzle " 4/20 nozzle 13 5/17 nozzle
00,
5o
00
50
41
pm [10-4 mbarl
6
04
i
_
6 f/f n Ö 4
p ® [10-4mbarl
100 p® [10 -smbarl
24t
p® [10-° mbarl
Fig. 7. Ouput flow as function of the pressure in a particular pumping stage. Starting at the working point (indicated by dashed crosses) the pressure is increased by additional HZ inlet into one particular chamber from outside by means of a needle valve. Results are shown for all chambers (see Fig. 1). The numbers are explained in the text . throughput of 1.5 mbar 1/s was measured as function of the residual gas pressure in each pumping stage. Since it is not trivial to reduce the pressure in the chambers below the working point, additional Hz was admitted from outside through a needle valve to each particular chamber in order to deteriorate the pressure . Fig. 7 shows the data, taken for each pumping stage in a separate measurement. The output flow is plotted as a function of the pressure of the particular pumping stage in which the pressure was increased. In each plot the working point is indicated by a dashed cross. The continuous lines show a linear extrapolation from the measured data to zero pressure . The output flow at zero pressure is the flow one would expect at an infinite pumping speed . The numbers printed in the plots represent the ratio of the output flow at the working point to the output flow at infinite pumping speed #1 . Combining the results for the different stages by multiplication shows that the output flow at the working point is 80 .6% of the output flow at an infinite pumping speed. Later improvements resulted in 82% . This means that the output flow can be improved even with infinite pumping speed only by about 20% and that the pumping system is well balanced and sufficiently powerful .
200
collimator area [mm2] Fig. 6. The output flow as a function of the collimator area for different nozzle geometries . The dissociator throughput is 1 .5 mbar 1/s. The two numbers mean "diameter of the nozzle"/"length of the conical part of the nozzle" in mm . The collimator area used in the ABS is indicated by a dashed line .
#~ In stage 4 there is no attenuation detectable, the output flow even seems to increase with rising pressure p4 . This is an effect of the increasing pressure in stage 4 which reduces the pressure difference to the compression tube volume and makes the pressure m the compression tube rise . The assumption in the following is that there is no attenuation by residual gas m chamber 4.
R Stock et al. /Nuct. Instr. and Meth. in Phys . Res. A 343 (1994) 334-342
3.4 . Cooling of the nozzle The nozzle is cooled in order to increase the number of atoms accepted by the magnet system [17] . The heating power of the nozzle is mainly determined by the recombination energy of the hydrogen atoms [181(1 mbar 1/s H2 = 18 .7 W). To achieve large cooling power a liquid nitrogen cooling line was installed. Nozzle temperatures down to T,ozzle = 80 K with the discharge switched on were observed . After delivery of the new magnet system (see section 4), the temperature dependence of the output flow was measured with a nozzle of 2 mm diameter using the complete magnet system . The entrance tube of the compression volume was mounted at the place of the feed tube of the storage cell in the target chamber (see Fig. 1) which is 40 mm after the end of the last magnet. The measurement was performed with increasing nozzle temperature after shutdown of the nozzle cooling. Fig. 8 shows in full circles the measured data while the open squares show the calculated transmission (see section 4) for the magnet system, normalized to the output flow at 80 K. The two curves are in reasonable agreement. The large slope of the measured output flow at 80 K as well as the rise for lower nozzle temperatures predicted by the calculations look very promising. Design studies for a nozzle cooling to temperatures below 80 K are in progress . However, the calculations do not include other implications of lower nozzle temperatures such as a reduced degree of dissociation [191 . The maximum value for the output flow amounted to q = 8.1 x 10 1`' H,/s in two substates achieved at the lowest nozzle temperature Tnozzle = 80 K. The output flow decreases strongly with increasing nozzle tempera-
T ,ub [KI Fig 8. Output flow as a function of the nozzle temperature : measured data and results of track tracing calculations (normalized to the measurement at 80 K)
33 9
ture . For the measured flow, a stronger decrease followed by a sudden rise occurs at 150 K and 240 K (Fig . 8). The reason for these changes is not completely understood, but it seems likely that the decrease comes from outgassing of oxygen (150 K) and alcohol (240 K) . The sudden rise occurs when all the gas has been pumped away . The alcohol may come from a small leak in the cooling system for the discharge tube . To the best of our knowledge the maximum output flow measured here is the highest observed for an ABS so far for the given geometry and with the stated precision . 4. Design of an improved magnet system The magnet system consists of several cylindrically symmetric sextupole magnets which provide the focussing or defocussing forces for the hydrogen atoms with electronic magnetic moments parallel or antiparallel to the local magnetic field, respectively. Due to their harmonic focussing field up to now only sextupole magnets have been considered for the magnet system . The oscillation wavelength of a focussed particle along a sextupole A sexwpole = 2 ar uz
V
mr 2
2 fr B Bo '
(4)
is proportional to the velocity in the beam direction vz and to the pole tip radius ro (m = mass, B = pole tip field, AB = Bohr's magneton) . The magnets employed at the FILTEX/ HERMES ABS are permanent magnets of Vacodym #2 and are made of 24 or 12 segments. The maximum pole tip field is 1.5 T [20] . Vacodym has to be enclosed in stainless steel cans to prevent chemical destruction by hydrogen . The cans are filled with He so that a leak and hence a possible destruction can be detected in the vacuum. In order to optimize the magnet system, however, not only its focussing properties have to be considered but also its influence on the pumping speed in the region through which the atomic beam passes . In particular inside the bore of the first magnets a considerable gas load exists stemming from defocussed hydrogen atoms which are scattered from the inner wall of the magnet . This situation led to the solution to split in particular magnet 1 into two (a and b) in order to enhance the pumping speed in this region [131 . Especially for magnet 1 (Fig . 1) the use of tapered magnets can considerably enhance the acceptance solid
#2
Brand name of Vacuumschmelze GmbH, Postf. 2253, D 63412 Hanau, Germany.
340
F. Stock et al. /NucL Instr. and Meth . to Phys, Res . A 343 (1994) 334-342
angle 12 of the magnet system [21] . Meanwhile it is possible to manufacture short tapered magnets of Vacodym with small inner bore . However, they have to be made of only 12 segments . The greater the number of segments the larger is the poletip field [20,22]. The design of the magnet system can be obtained by calculation . For the design of the FILTEX magnet system, a ray tracing code using Monte Carlo techniques [13,23] was employed . Since the focal length is proportional to the beam velocity it is important to have information about the beam velocity as precise as possible . The ray tracing code used here assumes a modified Maxwellian velocity distribution [17,24] and takes into account a parametrization of both the atomic beam temperature and the drift velocity of the particles as a function of the nozzle temperature . The parametrization was extracted from various time of flight measurements performed on the atomic beam of the ABS [13,25-27] . Fitting the results of the time of flight measurements to a modified Maxwellian velocity distribution yields as parametrizations for beam temperature and drift velocity Tbeam = 0 .403 Tnozzle[K] - 23 K, "'drift -
(5)
( 2 .88OTnozzle[K] + 1312) m/s,
for a nozzle diameter d of 4 mm [12,25,261 and Tbeam = 0 .294Tnozzle[K], Udrift -
(6 .109 Tnozzle[K] + 1354) m/s,
(8)
for a nozzle diameter d of 2 mm [27] . The time of flight measurements will be subject of a separate publication . The Monte Carlo calculations are done in three dimensions . They give the acceptance ,(2 of and the transmittion T through the magnet system . The system is optimized to search under fixed boundary condi-
'old' FILTEX
la lb
2
3
4
e .t .
'new' FILTEX
Fig . 9. The graphical output of a three dimensional ray tracing calculation for the "old" and the "new" FILTEX magnet system . The magnets are shown as rectangular or trapezoidal boxes, the entrance tube (e.t .) of the compression tube indi cated by two horizontal lines. The scale perpendicular to the beam axis is blown up (for dimensions see Table 3) . tions, such as pole tip field or within certain geometrical limits for an optimum of (DT) . The ray tracing calculations finally led to a magnet system with two tapered and three cylindrical magnets (Fig . 1) . Fig. 9 shows a comparison of the trajectories for the "old" and the "new" FILTEX magnet system . The radial distance of a traced particle to the atomic beam axis is plotted. The magnets are shown as rectangular or trapezoidal boxes. The entrance tube of the compression tube is outlined after the last magnet . The improvement in focussing quality and acceptance shows up very clearly. Table 3 gives a list of the essential parameters of the "new" FILTEX magnet system .
Table 3 Parameters of the new magnet system of the FILTEX-ABS . The pole tip field is calculated using the Halbach-formula and confirmed to values better than 1% by sample measurements [20] No .
No. of Segments
Outer diam . [mm]
Inner diam . [mm]
Length [mm]
1a
12
80
5 .4-7 .0
34
lb
12
80
8 .0-10 .0
83
2
24
80
12 .5
60
3
24
80
12.5
30
4
24
80
12 .5
200
Distance [mm] from nozzle 62 20 2 23 225 205 40
to entrance tube
Tip field [T]
1 .42-1 .40 1 .39-1 .36 1 .50 150 1 .50
F Stock et al. /Nucl. Instr. and Meth . in Phys. Res. A 343 (1994) 334-342 The predicted value for (f2T) is 11 .8 X 10 -3 for the new system as compared to 6 .8 X 10 -3 for the old one. This factor of about two was indeed reproduced later on in the output flow measurements .
34 1
für Kernphysik, Heidelberg, in particular to B. Vogt, K. Hahn and V. Mallinger.
References 5. Performance of the ABS during the FILTEX test experiment The FILTEX experiment [7] aims for the generation of polarized antiprotons in a storage ring by spin dependent attenuation of the originally unpolarized stored beam through interaction with a dense polarized hydrogen gas target . To test this idea experimentally an experiment was set up at the Heidelberg test storage ring TSR [28] in which the accumulation of nuclear polarization of stored protons was studied. Indeed a significant build-up of polarization has been observed [14] . The gas target consisted of a storage cell fed by the ABS . Areal densities up to 1.1 X 10' 4 H I /cm z of polarized hydrogen were achieved [29] . Moreover, and for such an experiment at least equally important, was the long term stability of the output flow provided by the ABS. A compression tube measurement prior to the experiment exhibited a stability of the output flow whithin 2% over 16 h at the maximum output flow of 8 .1 X 10 16 H,/s . During the FILTEX test experiment the output flow could be monitored through the count rate of scattering particles in the interaction zone which is directly related to the target density. Knowing the target density and the conductance of the storage cell the output flow of the ABS can be calculated . On-line observations showed no drop in the output flow over a period of four months within the error of 4% [14] . The longest running periods without switching off the discharge were two weeks. Also under these conditions the ABS prooved to be very reliable and stable in operation. Thus the present version of the ABS described here is well suited to serve as an essential component in the FILTEX and HERMES experiments. It should be mentioned that further developments are necessary to adopt the ABS also for the production of 1st and 2nd rank polarized deuterium beams. HERMES will use high frequency transitions between the last magnet and the entrance tube of the storage cell which makes a proper beam focussing more difficult. The developments are presently under way. Acknowledgements We wish to thank A.D . Roberts, Wisconsin for essential improvements in the ray tracing code . Thanks are due to the workshops of the Max-Planck-Institut
[1] E. Steffens, Proc . 7th Int. Conf. on Polarization Phenomena in Nuclear Physics, Paris, 1990, eds. A. Boudard and Y. Terrien, Coll . de Phys. 51 (1990) C6-221 . [21 K.P . Schüler, Proc . High Energy Physics with Polarized Beams and Polarized Targets, Lausanne, 1980, Experientia, Suppl. 38 (1981) 460. M.D . Barker et al., Proc . on Polarization Phenomena in Nuclear Physics, Santa Fe, 1980, eds. G.G . Ohlsen, R.E. Brown, N. Jarmie, W.W . McNaughton and G.M . Hale, AIP Conf. Proc. 69 (1981) 931. [4] W. Haeberli, Proc . Workshop on High Energy Spin Physics, Bonn, 1990, Vol. 2, eds. W. Meyer, E. Steffens and W. Thiel (Springer, Heidelberg 1991) p. 194. [5] W. Luck et al ., Proc . 7th Int. Conf. on Polarization Phenomena in Nuclear Physics, Paris, 1990, eds. A. Boudard and Y. Terrien, Coll . de Phys . 51 (1990) C6-561 . [6] S. Price, Dissertation, University of Wisconsin, Madison (1993). H. Dôbbeling et al ., Proposal CERN/PSCC/85/80 (1985). [8] E. Steffens, Proc. Ist Workshop on Antimatter Physics at Low Energy, Fermilab, 1986, p. 271. [9] HERMES Proposal, DESY-PRC 90/01. [10] K. Rith, Proc . High Energy Spin Physics, Bonn, 1990, Vol. 1, Conf . Report, eds. K.-H. Althoff and W. Meyer (Springer, Heidelberg, 1991) 198. [11] W. Haeberli, Proc . Int. Workshop on Polarized Sources and Targets, Montana, 1986, eds. S. Jaccard and S. Mango, Helv. Phys . Acta 59 (1986) 597. [12] F. Stock, Diploma Thesis, Ruprecht-Karls-Universität Heidelberg (1991) available at MPI für Kernphysik, Heidelberg. [13] W. Korsch, Proc . Workshop on High Energy Spin Physics, Bonn, 1990, Vol. 2, eds. W. Meyer, E. Steffens and W. Thiel (Springer, Heidelberg, 1991) p. 168. [14] F. Rathmann et al ., Phys . Rev. Lett . 71 (1993) 1379 . [15] T.B Clegg, Proc . 7th Int. Conf. on Polarization Phenomena in Nuclear Physics, Paris, 1990, eds. A. Boudard and Y. Terrien, Coll . de Phys. 51 (1990) C6-533 . [16] W. Grüebler et al ., Nucl . Instr. and Meth . 41 (1965) 245. [17] W. Grüebler, Proc . Polarized Proton Ion Sources, Vancouver, 1983, eds. G. Roy and P. Schmor, AIP Conf. Proc. 117 (1984) 1 . [18] W. Kubischta, Proc . Workshop on Polarized Gas Targets for Storage Rings, Heidelberg, 1991, eds. H.G . Gaul, E. Steffens and K. Zapfe (MPI für Kernphysik, Heidelberg, internal report, 1991) p. 34 . [19] D. Singy et al ., Nucl . Instr. and Meth . B 47 (1990) 167. [20] P Schiemenz, A. Ross and G. Graw, Nucl . Instr. and Meth . A 305 (1991) 15 . [21] W. Haeberli, Annu . Rev. Nucl . Sci. 37 (1967) 427. [22] K. Halbach, Nucl . Instr. and Meth . 169 (1980) 1 . [23] T. Wise, A.D . Roberts and W. Haeberli, Nucl . Instr. and Meth . A 336 (1993) 410.
342
F Stock et al. I Nucl. Instr. and Meth. in Phys. Res. A 343 (1994) 334-342
[241 H . Haberland, U . Buck and M . Tolle, Rev. Sci. Instr . 56 (1985) 1712 . [251 M . Schick, Diploma Thesis, Ruprecht-Karls-Universität Heidelberg (1990), available at MPI für Kernphysik, Heidelberg. [26] H . Mairon, Diploma Thesis, Ruprecht-Karls-Universität Heidelberg (1992), available at MPI für Kernphysik, Heidelberg .
[27] B. Lorentz, Diploma Thesis, Ruprecht-Karls-Universität Heidelberg (1993), available at MPI für Kernphysik, Heidelberg. [281 D . Kràmer et al ., Nucl. Instr . and Meth. A 287 (1989) 268 . [29] K . Zapfe, Proc . Workshop on Polarized Ion Sources and Polarized Gas Targets, Madison, 1993, AIP Conf. Proc ., to be published .
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
The FILTEX/HERMES polarized hydrogen atomic beam source F. Stock a, K. Rith ', H.G. Gaul b, B . Lorentz b, H. Mairon b, B. Povh b, E. Steffens D. Toporkov b,l , K. Zapfe b,2, F. Rathmann ', D. Fick `, W. Korsch c,3, G. GraW d, K. Reinmüller d, P. Schiemenz d, W. Haeberli e
b,
Friedrich-Alexander-Untuersitcit Erlangen -Niirnberg, Physikalisches Institut, D 91058 Erlangen, Germany b Max-Planck-Institut für Kernphysik, D 69029 Heidelberg, Germany Philipps Untuersitdt, Fachbereich Physik, D 35032 Marburg, Germany d Ludwig-Maximilians -Unioersitdt München, Sektion Physik, D 85748 Garchmg, Germany University of Wisconsin, Department of Physics, Madison, WI 53706, USA
(Received 5 November 1993) The FILTEX/HERMES atomic beam source (ABS) for polarized hydrogen is described. Recent improvements concern mainly the optimization of the beam forming system and a new design of the sextupole magnet system. For a precise measurement (error 5%) of the output flow a calibrated compression tube was installed. The output flow of 0.81 x 10 17 H atoms per second m two hyperfine substates was constant within 2% in a long-term measurement over 16 h. At the FILTEX test experiment, the target density in the storage cell fed by the ASS was constant within the experimental error of 4% over a period of four months . 1. Introduction The increasing importance of storage ring experiments in nuclear as well as in elementary particle physics demands internal polarized gas targets [1]. The straightforward solution to cross a polarized beam of e .g . a jet of hydrogen atoms, with the circulating beam of a storage ring (e .g . protons, electrons etc.) suffers by far too low a luminosity . A solution to this problem was offered in 1980 [2,3]: the use of a storage cell (e .g . T-shaped) through which the stored beam passes . The polarized atoms injected into the storage cell undergo some hundreds of wall collisions before they leave the cell through one of the openings . Thus the luminosity increases by about two orders of magnitude. Depolarization of the stored atoms through wall collisions is supressed by a suitable wall coating of the cell such as Teflon [4-61. For experiments which propose to use polarized internal targets in storage rings, e.g . FILTEX [7,81 or
* Corresponding author . * * Supported partly under various contracts by the Bundesministerium für Forschung and Technologie, Bonn . 1 Permanent address: Institute for Nuclear Physics, 630090 Novisibirsk, Russia . Z Now at DESY, D 22603 Hamburg, Germany. 3 Now at California Institute of Technology, Pasadena, CA 91125, USA.
HERMES [9,10], an areal density of about n = 1014 polarized hydrogen atoms/em 2 is required . The necessary output flow from an atomic beam source feeding the storage cell certainly depends on the details of the geometry of the storage cell used . But in all experiments, discussed so far, output flows of about q = 1017 polarized atoms/s are necessary, provided the polarization P of the atomic beam is beyond 0 .8 . For a lower polarization the output flow has to increase accordingly to keep the figure of merit (p2q) at the same level. An output flow of 10 17 atoms/s into an entrance tube of a storage cell (typical dimension: 10 mm diameter and 100 mm length) is beyond the capability of a traditional ABS [111 . This paper reports the most important optimizing steps [12] which took us very close toward this goal and resulted in 8.1 x 10 1 `' H atoms/s into a tube with the dimensions given above. The source was already described as for its performance up to 1990 [13] . Hence, we restrict ourselves to the improvements achieved since. This paper will be closed by a few remarks on the performance of the source during the FILTEX test experiment [141 .
2. Description of the atomic beam source The FILTEX/ HERMES polarized hydrogen ABS is based on Stern-Gerlach spin separation of the electron spin in an inhomogeneous magnetic field in com-
0168-9002/94/$07.00 © 1994 - Elsevier Science B.V . All rights reserved SSDI0168-9002(93)E1273-Z
and the pumping l3No 1) as 1A frequency 4The target ofinseparation of differential and magnets the with schematic speed magnet 5chamber ofconnections speed, permanent atube the inchamber xXsource powerful acryo turbo (RF) more the ABS and high type pop system inside of4between target and the differential dissociator as pumping between the operating than frequency sextupole can four-stage between high itthe (see FILTEX chamber) exists be 15 Stock xchambers frequency first section 000 2200 1500 the sealed pressure pumping system with chambers magnets etnow transition atomic chamber, fl/SI (chambers 1/s, sextupole aland an 4) INuct bywith transition see 1/2 (total system alcohol-cooled beam valves an The (in 3Table and two magnets the Instr (HFT) Xand HFT chambers (mbarl 1-4, previous nominal 10-4 10-6 10-7 source S2/3, (HFT) Fig 4skimnewly nomiand 1), unit and see Itconnected 12aareHYDROGEN indicated i Phys topassing HFT defocussed paper cooled the Fig SOURCE an isset Res as the 1the 1beam Pand well system atomic and la 9) of target A=1 new of field hyperfine guide field The transition nuclear we atoms them 343 and the sextupole the nozzle as 22,atoms of will for the magnets (1994) atomic of can chamber Fig beam and populations state source are field, had lb independent into with sealing the not polarization have acquire of 2), states leaving focussed pumped basically and 1334-342 awhile sextupole design by beam describe electronic magnets atoms will the while low are Along now valves defocussing a1dissociator, not cooled nuclear the (5 of in enters and considerably the aofbetween only towards The the the G) be state of the ittapered dissociator magnets others In the the 2spin further magnetic athe atomic same the atoms isAl second polarization the point beam 2present guide along state P nuclear the and nozzle inhomogeneous inner (states the =first configuration (up beam of chambers inshorter 0with case field 44are permanent beam field the interest hyperfine bore in case, to in source polarizain 3and axis, formed equally afor (local) and 1aSince aeither orhigh (see secand two axis (see low the the T) by an 4) of is
F. Table Parameters nal Stage 1 2 3
.
4
bination consists Fig . pumping radio dissociation mers system 3 located and . to displays designed
.
Snommal 2 2x1000 2200 3500 360 2
pop 0.8 1 .0X10-5 1.6X 1.4
.
.
.
.
magnet but magnets also . . Hydrogen into skimmers. magnetic where magnetic (states are states by exchanging frequency ond maximum occupied magnetic atomic tion .0, the this Important the
. :
: Pump 2 2xturbo 1 1 1 2
Meth. n
.
.
.
335
.5 .
.
. .5 . .
POLARIZED
Fig . . sextupole
. text).
336
F. Stock et al. / Nucl. Instr. and Meth. i n Phys . Res. A 343 (1994) 334-342
m, ml
, z . .
5
B/Be
Fig. 2. Breit Rabi diagram for hydrogen . Two sets of quantum numbers are given: IF, mF) for the total spin and its magnetic quantum number and I m m, ) for the magnetic quan tum number of both proton and electron . B, = 507 G, E HF = 5.9 X 10 -6 eV ( = 1420 MHz) . sextupole magnets and the powerful four-stage pumping system . Recent improvements include diagnostic tools, improvements of the dissociator, new information on the vacuum system and last but not least the construction of a new system of sextupole magnets. 3. Measurements of the output flow of polarized atoms 3.1 . Calibrated compression tube
As indicated in the introduction a precise determination of the output flow of polarized hydrogen atoms entering the storage cell is essential to predict the areal density of the target . In contrast to polarized hydrogen ion sources with ionizers for which the achieved density in the ionization volume is essential [15] a storage cell feeding source has to be designed for maximum output flow . In order to achieve a precise detection of the output flow a compression tube [161 with an entrance tube of the same geometry as the feed tube of the storage cell for the FILTEX experiment (length 100 mm, diameter 10 mm) was installed. Fig. 3 shows a scetch of the compression tube and the calibration unit. The calibration volume Vcal is filled with H Z through a feed valve. The pressure in the calibration volume pca, is measured by a difference-pressure transducer which is calibrated by means of an absolute pressure gauge. The volume Val is determined by comparison with a well known reference volume Vref . The feed valve can be controlled by the difference-pressure transducer in a way that keeps the pressure Pcal constant when there is an outflow of gas through the needle valve into the compression tube .
The needle valve has an integrated shut-off valve which does not affect the setting of the needle valve. Thus there are two ways for the operation of the calibration unit : keeping pcal constant or having it decaying when the refilling control is not employed . The pressure in the compression tube pcomP is measured with an ion gauge. There is a glass window on the compression tube which provides visual access to the inside of the compression tube and, through the entrance tube, even to the dissociator nozzle . A flap in chamber 4 (Fig . 1) can be inserted to prevent the atomic beam from entering the entrance tube . The usual pressures are in the calibration volume pcal = 5-65 mbar, in the compression tube pcomP = 10 -4 mbar and in the vacuum chamber adjacent to the compression tube DABS = 10-7 mbar . The compression tube is calibrated as follows: the needle valve is opened to a certain position which must not be changed later on and the calibration volume Veal is filled with HZ . The exponential decay of the pressure pcal is monitored for several hours and so the time constant of the decay r is obtained with high precision . The throughput Q into the compression tube for any pressure reading pcal in the calibration volume is then given by Q -hca1
T.
(1)
Vcal
Thus by setting pcal and keeping it constant a definite flow into the compression tube can be chosen . The throughput Q in units mbar 1/s is related to the flow q in units of atoms/s via the ideal gas equation by the relation 7 .243 X 10 21 q[atoms/s]
=
Q[mbar I/s]
which is for T = 300 K: q[atoms/s]
=
Q[mbar
T[K]
1/s]2 .414 X 10 19 .
The output flow q of hydrogen atoms leaving the ABS is then measured by reproducing the pressure pomp
ABS___-____--___ ;
Calibration Unit -- _
Fig. 3 . A schematic of the compression tube and the calibration unit (see text).
F Stock et al. /Nuel. Instr, and Meth. to Phys. Res. A 343 (1994) 334-342
337
Table 2 Output flow for different nozzle geometries d[mm]/1[mm] q(H I, 2 subst.) [10 16 /s]
2/17 5.4
4/17 4.1
5/17 = 3.5
relative changes in the output flow can be detected and
used to optimize the output flow .
nozzle
skimmer collimator
Fig. 4. The beam forming system of the atomic beam source consisting of nozzle, skimmer and collimator . The conical part of the nozzle can be exchanged as indicated.
The results collected in Table 2 indicate that the
highest output flow q of hydrogen atoms is obtained
with the smallest diameter
d.
Also a tendency is ob-
served that the maximum output flow is found at a lower dissociator throughput Qd,ss of HZ for smaller nozzle diameters. Additional measurements (not listed in Table 2) showed that a variation of the nozzle length
measured in the compression tube in the presence of the atomic beam by means of the calibration unit while the atomic beam is blocked by the flap . The compression tube is made of stainless steel which has a high recombination coefficient for atomic hydrogen . It is therefore reasonable to assume that all H atoms have
undergone recombination before they are detected by the ion gauge. In this way an accuracy of 5% can be
achieved for an output flow measurement. This high
accuracy turned out to be important for most of the improvements of the source which could not have been achieved without a reliable output flow measurement of the hydrogen atoms leaving the source . The accu-
racy is mainly limited by the precision to which the volume V,,, can be determined and by the precision of the calibration of the pressure transducer . This system was extensively used to optimize various components of the beam forming system, the vacuum
system and for studies of the effects of cooling the nozzle of the discharge .
3.2. Optimization of the beam forming system The beam forming system consists of the nozzle of the hydrogen dissociator, a skimmer and a collimator
(Fig. 4) . Nozzle and skimmers have a circular conical inner bore . The nozzle is made of 99 .5% Al [13]. The
did not cause large variations in the output flow .
The influence of the distance between nozzle and
skimmer on the output flow was investigated as well.
The measurements were performed as described above
using a nozzle with 2 mm diameter . Fig. 5 shows the output flow into the compression tube
gcomP
as a
function of the dissociator throughput Qd,ss for different nozzle-to-skimmer distances s. For each distance s
there is an optimum dissociator throughput Qdiss for maximum output flow. This maximum output flow is different for different distances s having the largest absolute value for the measurement with the medium
distance s = 16 mm . The conclusion is that for a given nozzle geometry there is an optimum nozzle-skimmer
distance s, which yields a maximum output flow into the compression tube .
The collimator must be dimensioned such that it
does not reduce the intensity of the atomic beam but reject as far upstream as possible those atoms which
can not be accepted by the magnet system . To find out
0-9
a O 8
. s=16mm . s= 6mm . s=31 mm
geometry of the conical part of the nozzle is deter-
mined by the "initial" diameter d I , the "final" diameter d and the length 1 (Fig . 4). While the "initial"
diameter d , is determined by the diameter of the discharge tube and thus fixed (d,,,t = 11 mm), both the length 1 and the diameter d were varied to obtain information about the output flow for different nozzle
geometries (Table 2) . The measurements reported below were carried out with the old magnet system [13] .
For practical reasons only magnets la, lb and 2 were used . The compression tube was installed on the end flange of chamber 4 (Fig . 1) . Although at this position the complete sextupole magnet system is not employed,
4
2
2
3
4
Qa [mbl/sl
Fig. 5. The output flow as function of the dissociator through put for different nozzle-skimmer distances s. Nozzle diameter d = 2 mm .
338
F. Stock et al. / Nuct. Instr. and Meth. i n Phys . Res. A 343 (1994) 334-342
the optimum aperture, the collimator opening was varied using an iris aperture with variable diameter instead of the collimator . Fig . 6 displays the output flow as function of the collimator area for different nozzle geometries d/!, both in mm . The dissociator throughput is kept constant to 1.5 mbar 1/s. For small values of the opening the output flow increases with growing aperture area . At too large an area the output flow drops because of the increasing number of atoms entering the magnets which cannot be focussed but deteriorate the vacuum pressure. The measurement shows that there is an optimum collimator diameter for a given nozzle and skimmer geometry . The optimum area is indicated in Fig. 6 by a dashed line . The collimator opening was chosen accordingly . 3.3. Test of the vacuum system The Hz throughput through the dissociator amounts to 1-3 mbar 1/s. Thus the question whether the pumping system is powerful enough to handle this large gas load had to be investigated carefully. The most important parameters of the pumping system [13] are listed in Table 1 . The total nominal pumping speed amounts to more than 15 000 l/s. To test the capability of the pumping system the flow into the compression tube at a dissociator
CO 40
O
3
2
v v
î
11
0 :11 :
0
o
Il
Il
Il
Il
Il
" 4/17 nozzle ; v
& 3/15 nozzle " 4/20 nozzle 13 5/17 nozzle
00,
5o
00
50
41
pm [10-4 mbarl
6
04
i
_
6 f/f n Ö 4
p ® [10-4mbarl
100 p® [10 -smbarl
24t
p® [10-° mbarl
Fig. 7. Ouput flow as function of the pressure in a particular pumping stage. Starting at the working point (indicated by dashed crosses) the pressure is increased by additional HZ inlet into one particular chamber from outside by means of a needle valve. Results are shown for all chambers (see Fig. 1). The numbers are explained in the text . throughput of 1.5 mbar 1/s was measured as function of the residual gas pressure in each pumping stage. Since it is not trivial to reduce the pressure in the chambers below the working point, additional Hz was admitted from outside through a needle valve to each particular chamber in order to deteriorate the pressure . Fig. 7 shows the data, taken for each pumping stage in a separate measurement. The output flow is plotted as a function of the pressure of the particular pumping stage in which the pressure was increased. In each plot the working point is indicated by a dashed cross. The continuous lines show a linear extrapolation from the measured data to zero pressure . The output flow at zero pressure is the flow one would expect at an infinite pumping speed . The numbers printed in the plots represent the ratio of the output flow at the working point to the output flow at infinite pumping speed #1 . Combining the results for the different stages by multiplication shows that the output flow at the working point is 80 .6% of the output flow at an infinite pumping speed. Later improvements resulted in 82% . This means that the output flow can be improved even with infinite pumping speed only by about 20% and that the pumping system is well balanced and sufficiently powerful .
200
collimator area [mm2] Fig. 6. The output flow as a function of the collimator area for different nozzle geometries . The dissociator throughput is 1 .5 mbar 1/s. The two numbers mean "diameter of the nozzle"/"length of the conical part of the nozzle" in mm . The collimator area used in the ABS is indicated by a dashed line .
#~ In stage 4 there is no attenuation detectable, the output flow even seems to increase with rising pressure p4 . This is an effect of the increasing pressure in stage 4 which reduces the pressure difference to the compression tube volume and makes the pressure m the compression tube rise . The assumption in the following is that there is no attenuation by residual gas m chamber 4.
R Stock et al. /Nuct. Instr. and Meth. in Phys . Res. A 343 (1994) 334-342
3.4 . Cooling of the nozzle The nozzle is cooled in order to increase the number of atoms accepted by the magnet system [17] . The heating power of the nozzle is mainly determined by the recombination energy of the hydrogen atoms [181(1 mbar 1/s H2 = 18 .7 W). To achieve large cooling power a liquid nitrogen cooling line was installed. Nozzle temperatures down to T,ozzle = 80 K with the discharge switched on were observed . After delivery of the new magnet system (see section 4), the temperature dependence of the output flow was measured with a nozzle of 2 mm diameter using the complete magnet system . The entrance tube of the compression volume was mounted at the place of the feed tube of the storage cell in the target chamber (see Fig. 1) which is 40 mm after the end of the last magnet. The measurement was performed with increasing nozzle temperature after shutdown of the nozzle cooling. Fig. 8 shows in full circles the measured data while the open squares show the calculated transmission (see section 4) for the magnet system, normalized to the output flow at 80 K. The two curves are in reasonable agreement. The large slope of the measured output flow at 80 K as well as the rise for lower nozzle temperatures predicted by the calculations look very promising. Design studies for a nozzle cooling to temperatures below 80 K are in progress . However, the calculations do not include other implications of lower nozzle temperatures such as a reduced degree of dissociation [191 . The maximum value for the output flow amounted to q = 8.1 x 10 1`' H,/s in two substates achieved at the lowest nozzle temperature Tnozzle = 80 K. The output flow decreases strongly with increasing nozzle tempera-
T ,ub [KI Fig 8. Output flow as a function of the nozzle temperature : measured data and results of track tracing calculations (normalized to the measurement at 80 K)
33 9
ture . For the measured flow, a stronger decrease followed by a sudden rise occurs at 150 K and 240 K (Fig . 8). The reason for these changes is not completely understood, but it seems likely that the decrease comes from outgassing of oxygen (150 K) and alcohol (240 K) . The sudden rise occurs when all the gas has been pumped away . The alcohol may come from a small leak in the cooling system for the discharge tube . To the best of our knowledge the maximum output flow measured here is the highest observed for an ABS so far for the given geometry and with the stated precision . 4. Design of an improved magnet system The magnet system consists of several cylindrically symmetric sextupole magnets which provide the focussing or defocussing forces for the hydrogen atoms with electronic magnetic moments parallel or antiparallel to the local magnetic field, respectively. Due to their harmonic focussing field up to now only sextupole magnets have been considered for the magnet system . The oscillation wavelength of a focussed particle along a sextupole A sexwpole = 2 ar uz
V
mr 2
2 fr B Bo '
(4)
is proportional to the velocity in the beam direction vz and to the pole tip radius ro (m = mass, B = pole tip field, AB = Bohr's magneton) . The magnets employed at the FILTEX/ HERMES ABS are permanent magnets of Vacodym #2 and are made of 24 or 12 segments. The maximum pole tip field is 1.5 T [20] . Vacodym has to be enclosed in stainless steel cans to prevent chemical destruction by hydrogen . The cans are filled with He so that a leak and hence a possible destruction can be detected in the vacuum. In order to optimize the magnet system, however, not only its focussing properties have to be considered but also its influence on the pumping speed in the region through which the atomic beam passes . In particular inside the bore of the first magnets a considerable gas load exists stemming from defocussed hydrogen atoms which are scattered from the inner wall of the magnet . This situation led to the solution to split in particular magnet 1 into two (a and b) in order to enhance the pumping speed in this region [131 . Especially for magnet 1 (Fig . 1) the use of tapered magnets can considerably enhance the acceptance solid
#2
Brand name of Vacuumschmelze GmbH, Postf. 2253, D 63412 Hanau, Germany.
340
F. Stock et al. /NucL Instr. and Meth . to Phys, Res . A 343 (1994) 334-342
angle 12 of the magnet system [21] . Meanwhile it is possible to manufacture short tapered magnets of Vacodym with small inner bore . However, they have to be made of only 12 segments . The greater the number of segments the larger is the poletip field [20,22]. The design of the magnet system can be obtained by calculation . For the design of the FILTEX magnet system, a ray tracing code using Monte Carlo techniques [13,23] was employed . Since the focal length is proportional to the beam velocity it is important to have information about the beam velocity as precise as possible . The ray tracing code used here assumes a modified Maxwellian velocity distribution [17,24] and takes into account a parametrization of both the atomic beam temperature and the drift velocity of the particles as a function of the nozzle temperature . The parametrization was extracted from various time of flight measurements performed on the atomic beam of the ABS [13,25-27] . Fitting the results of the time of flight measurements to a modified Maxwellian velocity distribution yields as parametrizations for beam temperature and drift velocity Tbeam = 0 .403 Tnozzle[K] - 23 K, "'drift -
(5)
( 2 .88OTnozzle[K] + 1312) m/s,
for a nozzle diameter d of 4 mm [12,25,261 and Tbeam = 0 .294Tnozzle[K], Udrift -
(6 .109 Tnozzle[K] + 1354) m/s,
(8)
for a nozzle diameter d of 2 mm [27] . The time of flight measurements will be subject of a separate publication . The Monte Carlo calculations are done in three dimensions . They give the acceptance ,(2 of and the transmittion T through the magnet system . The system is optimized to search under fixed boundary condi-
'old' FILTEX
la lb
2
3
4
e .t .
'new' FILTEX
Fig . 9. The graphical output of a three dimensional ray tracing calculation for the "old" and the "new" FILTEX magnet system . The magnets are shown as rectangular or trapezoidal boxes, the entrance tube (e.t .) of the compression tube indi cated by two horizontal lines. The scale perpendicular to the beam axis is blown up (for dimensions see Table 3) . tions, such as pole tip field or within certain geometrical limits for an optimum of (DT) . The ray tracing calculations finally led to a magnet system with two tapered and three cylindrical magnets (Fig . 1) . Fig. 9 shows a comparison of the trajectories for the "old" and the "new" FILTEX magnet system . The radial distance of a traced particle to the atomic beam axis is plotted. The magnets are shown as rectangular or trapezoidal boxes. The entrance tube of the compression tube is outlined after the last magnet . The improvement in focussing quality and acceptance shows up very clearly. Table 3 gives a list of the essential parameters of the "new" FILTEX magnet system .
Table 3 Parameters of the new magnet system of the FILTEX-ABS . The pole tip field is calculated using the Halbach-formula and confirmed to values better than 1% by sample measurements [20] No .
No. of Segments
Outer diam . [mm]
Inner diam . [mm]
Length [mm]
1a
12
80
5 .4-7 .0
34
lb
12
80
8 .0-10 .0
83
2
24
80
12 .5
60
3
24
80
12.5
30
4
24
80
12 .5
200
Distance [mm] from nozzle 62 20 2 23 225 205 40
to entrance tube
Tip field [T]
1 .42-1 .40 1 .39-1 .36 1 .50 150 1 .50
F Stock et al. /Nucl. Instr. and Meth . in Phys. Res. A 343 (1994) 334-342 The predicted value for (f2T) is 11 .8 X 10 -3 for the new system as compared to 6 .8 X 10 -3 for the old one. This factor of about two was indeed reproduced later on in the output flow measurements .
34 1
für Kernphysik, Heidelberg, in particular to B. Vogt, K. Hahn and V. Mallinger.
References 5. Performance of the ABS during the FILTEX test experiment The FILTEX experiment [7] aims for the generation of polarized antiprotons in a storage ring by spin dependent attenuation of the originally unpolarized stored beam through interaction with a dense polarized hydrogen gas target . To test this idea experimentally an experiment was set up at the Heidelberg test storage ring TSR [28] in which the accumulation of nuclear polarization of stored protons was studied. Indeed a significant build-up of polarization has been observed [14] . The gas target consisted of a storage cell fed by the ABS . Areal densities up to 1.1 X 10' 4 H I /cm z of polarized hydrogen were achieved [29] . Moreover, and for such an experiment at least equally important, was the long term stability of the output flow provided by the ABS. A compression tube measurement prior to the experiment exhibited a stability of the output flow whithin 2% over 16 h at the maximum output flow of 8 .1 X 10 16 H,/s . During the FILTEX test experiment the output flow could be monitored through the count rate of scattering particles in the interaction zone which is directly related to the target density. Knowing the target density and the conductance of the storage cell the output flow of the ABS can be calculated . On-line observations showed no drop in the output flow over a period of four months within the error of 4% [14] . The longest running periods without switching off the discharge were two weeks. Also under these conditions the ABS prooved to be very reliable and stable in operation. Thus the present version of the ABS described here is well suited to serve as an essential component in the FILTEX and HERMES experiments. It should be mentioned that further developments are necessary to adopt the ABS also for the production of 1st and 2nd rank polarized deuterium beams. HERMES will use high frequency transitions between the last magnet and the entrance tube of the storage cell which makes a proper beam focussing more difficult. The developments are presently under way. Acknowledgements We wish to thank A.D . Roberts, Wisconsin for essential improvements in the ray tracing code . Thanks are due to the workshops of the Max-Planck-Institut
[1] E. Steffens, Proc . 7th Int. Conf. on Polarization Phenomena in Nuclear Physics, Paris, 1990, eds. A. Boudard and Y. Terrien, Coll . de Phys. 51 (1990) C6-221 . [21 K.P . Schüler, Proc . High Energy Physics with Polarized Beams and Polarized Targets, Lausanne, 1980, Experientia, Suppl. 38 (1981) 460. M.D . Barker et al., Proc . on Polarization Phenomena in Nuclear Physics, Santa Fe, 1980, eds. G.G . Ohlsen, R.E. Brown, N. Jarmie, W.W . McNaughton and G.M . Hale, AIP Conf. Proc. 69 (1981) 931. [4] W. Haeberli, Proc . Workshop on High Energy Spin Physics, Bonn, 1990, Vol. 2, eds. W. Meyer, E. Steffens and W. Thiel (Springer, Heidelberg 1991) p. 194. [5] W. Luck et al ., Proc . 7th Int. Conf. on Polarization Phenomena in Nuclear Physics, Paris, 1990, eds. A. Boudard and Y. Terrien, Coll . de Phys . 51 (1990) C6-561 . [6] S. Price, Dissertation, University of Wisconsin, Madison (1993). H. Dôbbeling et al ., Proposal CERN/PSCC/85/80 (1985). [8] E. Steffens, Proc. Ist Workshop on Antimatter Physics at Low Energy, Fermilab, 1986, p. 271. [9] HERMES Proposal, DESY-PRC 90/01. [10] K. Rith, Proc . High Energy Spin Physics, Bonn, 1990, Vol. 1, Conf . Report, eds. K.-H. Althoff and W. Meyer (Springer, Heidelberg, 1991) 198. [11] W. Haeberli, Proc . Int. Workshop on Polarized Sources and Targets, Montana, 1986, eds. S. Jaccard and S. Mango, Helv. Phys . Acta 59 (1986) 597. [12] F. Stock, Diploma Thesis, Ruprecht-Karls-Universität Heidelberg (1991) available at MPI für Kernphysik, Heidelberg. [13] W. Korsch, Proc . Workshop on High Energy Spin Physics, Bonn, 1990, Vol. 2, eds. W. Meyer, E. Steffens and W. Thiel (Springer, Heidelberg, 1991) p. 168. [14] F. Rathmann et al ., Phys . Rev. Lett . 71 (1993) 1379 . [15] T.B Clegg, Proc . 7th Int. Conf. on Polarization Phenomena in Nuclear Physics, Paris, 1990, eds. A. Boudard and Y. Terrien, Coll . de Phys. 51 (1990) C6-533 . [16] W. Grüebler et al ., Nucl . Instr. and Meth . 41 (1965) 245. [17] W. Grüebler, Proc . Polarized Proton Ion Sources, Vancouver, 1983, eds. G. Roy and P. Schmor, AIP Conf. Proc. 117 (1984) 1 . [18] W. Kubischta, Proc . Workshop on Polarized Gas Targets for Storage Rings, Heidelberg, 1991, eds. H.G . Gaul, E. Steffens and K. Zapfe (MPI für Kernphysik, Heidelberg, internal report, 1991) p. 34 . [19] D. Singy et al ., Nucl . Instr. and Meth . B 47 (1990) 167. [20] P Schiemenz, A. Ross and G. Graw, Nucl . Instr. and Meth . A 305 (1991) 15 . [21] W. Haeberli, Annu . Rev. Nucl . Sci. 37 (1967) 427. [22] K. Halbach, Nucl . Instr. and Meth . 169 (1980) 1 . [23] T. Wise, A.D . Roberts and W. Haeberli, Nucl . Instr. and Meth . A 336 (1993) 410.
342
F Stock et al. I Nucl. Instr. and Meth. in Phys. Res. A 343 (1994) 334-342
[241 H . Haberland, U . Buck and M . Tolle, Rev. Sci. Instr . 56 (1985) 1712 . [251 M . Schick, Diploma Thesis, Ruprecht-Karls-Universität Heidelberg (1990), available at MPI für Kernphysik, Heidelberg. [26] H . Mairon, Diploma Thesis, Ruprecht-Karls-Universität Heidelberg (1992), available at MPI für Kernphysik, Heidelberg .
[27] B. Lorentz, Diploma Thesis, Ruprecht-Karls-Universität Heidelberg (1993), available at MPI für Kernphysik, Heidelberg. [281 D . Kràmer et al ., Nucl. Instr . and Meth. A 287 (1989) 268 . [29] K . Zapfe, Proc . Workshop on Polarized Ion Sources and Polarized Gas Targets, Madison, 1993, AIP Conf. Proc ., to be published .