The HERMES Polarized Atomic Beam Source
A.Nass (for the HERMES target group)
University of Erlangen - Nürnberg, E. Rommel - Str. 1, 91058 Erlangen, Germany
Abstract. The atomic beam source (ABS) provides nuclear polarized hydrogen or deuterium atoms
for the HERMES target at flow rates of about 65 10 16Hs (hydrogen in two hyperfine substates)
and 60 10 16Ds (deuterium in three hyperfine substates). The degree of dissociation of 93% for
H (95% for D) at the entrance of the storage cell and the nuclear polarization of around 0.97 (H)
and 0.92 (D) have been found to be constant within a a couple of percent over the whole running
period of the HERMES experiment. A new dissociator (MWD) based on a microwave discharge
at 2.45 GHz has been developed and installed into the HERMES–ABS in 2000. Since the velocity
distribution of the MWD differs from that of the RFD the intensity could be increased further with a
modified sextupole magnet system. For this purpose the way for a new start generator for sextupole
tracking calculations was opened. Monte-Carlo simulations were successfully used to describe the
gas expansion between nozzle, skimmer and collimator. A new type of beam monitor was used to
study the beam formation after the nozzle.
The HERMES experiment studies the spin structure of the nucleon by means of lepton
deep inelastic scattering off an internal gaseous target. A polarized atomic beam is
injected by an atomic beam source (ABS) into a storage cell [1]. A sample of the
target gas is extracted from the centre of this storage cell in order to analyze the target
polarization by means of a target gas analyzer (TGA) [2] and a Breit-Rabi polarimeter
[3].
PRINCIPLE OF OPERATION AND SETUP
The setup of the HERMES ABS is shown in Figure 1. Molecular hydrogen or deuterium
is dissociated via electron impact in a cold plasma provided by a radio frequency
dissociator (1). The atomic gas expands through a cooled nozzle into the vacuum of
chamber I supported by a powerful pumping system. A high brilliance beam is then
formed using a skimmer (2) and a collimator. Based on the Stern-Gerlach principle,
sextupole magnets (3) focus atoms with electron spin +1/2 (hyperfine states 1 and 2
for H, 1, 2 and 3 for D) and deflect atoms with electron spin -1/2 (states 3 and 4
for H, 4, 5 and 6 for D). Nuclear polarization is obtained by an interchange of the
hyperfine state populations using high frequency transitions.
The plasma source of the radio frequency dissociator (RFD) [4] applied consists of
a LC-circuit as a field applicator, tuned to a resonance frequency of 13.56 MHz, and a
water-cooled pyrex discharge tube. A degree of dissociation of α 80 % (H) and 75 %
(D) is achieved at throughputs of Q 09 15 mbarl/s and RF-power PRF 200 350
W . In 2000 a microwave dissociator (MWD) [5], based on a plasma source which
CP675, Spin 2002: 15th Int'l. Spin Physics Symposium and Workshop on Polarized Electron
Sources and Polarimeters, edited by Y. I. Makdisi, A. U. Luccio, and W. W. MacKay
© 2003 American Institute of Physics 0-7354-0136-5/03/$20.00
929
3a
1
3b
SFT*
2
FIGURE 1. Schematic view on the HERMES ABS with the radio frequency dissociator (1) and the
skimmer (2) for beam formation. Two sets of sextupole magnets (3a, 3b) are located along the beam axis
in chamber III and IV as are the high frequency transitions (SFT*, MFT, WFT and SFT).
couples a 2.45 GHz surface wave to the discharge in an air-cooled pyrex glass tube, was
installed. With typical throughputs of Q 1 3 mbarl/s and microwave power of about
PMW 600 W, α was in excess of 80 %. Further developments on a liquid cooled MWD
are in progress in order to outperform the water-cooled RFD.
The sextupole magnets [6] are high gradient permanent magnets consisting of 24
segments made of Vacodym1 . The maximum poletip field is 1.5 T. To prevent chemical
destruction by hydrogen, the magnets are enclosed in vacuum tight stainless steel cans.
In order to reduce the residual gas pressures due to the deflected atoms inside the
magnets, the set of sextupole magnets (3a) in the first sextupole chamber (III) is split
into 3 sections (figure 1). Each one is tapered to have the largest possible acceptance of
the diverging atomic beam and to provide achromatic focussing. Two more magnets (3b)
in the second sextupole chamber (IV) focus the atomic beam into the entrance tube of the
target cell. The transmission probabilities of the sextupole system have been calculated
with a sextupole tracking calculation.
Compact high frequency transitions are employed to nuclear polarize the atomic beam
with high efficiency. They consist of coils for the static and gradient field, and a resonator
cavity in the case of the strong field transition (SFT) or a high frequency coil in the case
of the weak and medium field transition (WFT and MFT) [7, 8]. For hydrogen the SFT
is tuned to a frequency of 1430 MHz, the WFT to 14 MHz and the MFT to 90 MHz.
For deuterium the different hyperfine splitting energy requires a lower frequency of 370
MHz for the SFT, 7 MHz for the WFT and 25 MHz for the MFT.
The settings of the static and gradient fields are chosen using the BRP with its own
high frequency transitions off. Due to the separation in the BRP sextupole system only
the hyperfine states with electron spin +1/2 reach the quadrupole mass spectrometer
(QMS). Figure 2 shows the dependence of the QMS signal on the current through the
magnetic field coils of the respective transitions. The MFT can be operated exchanging
1
Brand name of Vacuumschmelze GmbH, Postf. 2253, D63412 Hanau, Germany
930
40
|1〉, |2〉
|2〉, |3〉
20
60
40 |1〉, |2〉
|1〉, |2〉
|1〉, |4〉
20
WFT
0.1
0.15
I (A)
0.4
40
|1〉, |2〉
20
0.6
60
MFT (WFT running)
40
|2〉, |3〉
20
0
60
|2〉, |3〉
0.3
|3〉
0.4
0.5 0.6
I (A)
|1〉, |2〉
0.3
0.4
0.5 0.6
I (A)
60
MFT (SFT running)
40
|1〉, |4〉
|1〉, |4〉
20
|2〉
0
0.8
I (A)
|1〉
|2〉
MFT
SFT
0
NBRP (kHz)
0.05
NBRP (kHz)
00
NBRP (kHz)
←|1〉, |2〉
NBRP (kHz)
NBRP (kHz)
60
0
|1〉
0.3
|4〉
0.4 0.5 0.6
I (A)
FIGURE 2. H1 signal of the BRP-QMS as a function of the magnetic field BI. The injected states of
hydrogen into the target cell are shown in every part of each graph.
states 2 and 3 or 1 and 3 at different static magnetic fields. The combination of
the different high frequency transitions makes it is possible to inject every single state
or even zero states. The latter is essential to determine the ballistic flux from the ABS.
The efficiencies of the high frequency transitions could be obtained by spin relaxation
measurements with the BRP ([9]).
OPTIMIZATION PROCEDURES
The output intensity can be optimized by changing the flux Q and the nozzle temperature
Tnozzle . The maximum intensity values measured by a calibrated compression tube are
shown in table 1 together with the parameters of the measurements for both dissociator
types. Due to the higher degree of dissociation at the nozzle exit for higher throughputs
a gain of 15 % in intensity of the ABS by using the MWD compared to the RFD has
been detected. A QMS in front of the entrance of the compression tube has been used to
TABLE 1. The hydrogen and deuterium intensity of the ABS. P is the applied RF power,
qCT the intensity measured with the calibrated compression tube (not corrected for α ) [4]
and qBRP the intensity determined with the BRP via spin exchange collisions [9].
gas
RFD
MWD
Q (mbarl/s)
P (W)
Tnozzle (K)
qCT (atoms/s)
qBRP (atoms/s)
65 10 16
H
D
1.5
1.0
290
200
115
115
52 10 16
66 1016
45 1016
H
D
1.5
1.5
600
600
80
80
62 10 16
60 10 16
–
51 1016
931
TABLE 2. Nuclear (Pz ) and tensor (Pzz ) polarization of the
atomic beam injected into the storage cell [9, 10].
gas
injected HFS
H
1 4
2 3
D
1 6
3 4
3 6
2 5
Pz
Pzz
0 973
0010
0 974 0010
0 924
0010
0 911 0015
0015 0014
0022 0013
-
0 884
0 941
0 990
0018
0022
0023
1774 0020
measure the associated degree of dissociation αABS :
αABS Sa
Sa 2 κion κdet κv Sm
(1)
where Sa Sa δ di Sm is the atomic signal corrected for dissociative ionization in
the QMS, κion the ratio of the ionization cross sections, κ det the ratio of the detection
probabilities and κv the ratio of the velocities of the atoms and molecules. At the working
conditions degrees of dissociation of 928 3034 for hydrogen and 9453036 for deuterium
were determined using the MWD.
PERFORMANCE WITHIN THE HERMES EXPERIMENT
The HERMES ABS was continuously running from 1996 until 2002. The thickness of
the HERMES target and therefore the ABS beam intensity has been determined via spin
exchange collisions in the target cell using the BRP (table 1) confirming the expected
MWD improvement of the deuterium intensity of the ABS by 15 %.
The nuclear polarization of the atoms depends on the efficiencies of the high frequency transitions and the transmission probabilities of the sextupole magnet system.
The resulting polarization values of the injected atoms are listed in table 2.
DIRECT SIMULATION MONTE-CARLO OF THE EXPANSION
The understanding of the processes that occur in the expansion of the hydrogen gas into
the vacuum and the formation of the atomic beam is essential for an improvement of
the ABS. The use of continuous flow models is problematic because of their restricted
validity in the transition region between laminar and molecular flow. Thus a direct
simulation Monte-Carlo method was used [11] to describe the processes in the gas
expansion. The simulated velocity and density distributions agree well with the values of
the time-of-flight and beam profile measurements [12, 13]. As an example the measured
and calculated resistances of a beam profile monitor [14] are shown in figure 3. The
results of these Monte-Carlo simulations can be used to extract e.g. the input parameters
for sextupole tracking calculation.
932
R (Ω)
R (Ω)
560
540
575
550
525
520
500
500
475
10
10
y (m
m)
15
0
-10 20
10
10
y (m
)
m)
mm
x(
15
0
-10 20
)
mm
x(
FIGURE 3. Measured (left) and calculated (right) resistances for an expansion of hydrogen (at Q H 2
1 mbar ls Tnozzle 100 K α 80%). x – distance nozzle monitor and y – distance from the beam axis.
CONCLUSIONS
The ABS is found to be a very reliable source of polarized atoms for a storage cell target
and other applications with hydrogen (deuterium) intensities up to 65 10 16 atoms/s
(60 1016 atoms/s) in 2 (3) hyperfine substates. Nuclear polarization values of 0.97
(0.92) at a degree of dissociation of 93 % (95 %) for H (D) were reached. A smooth
and stable operation within the HERMES experiment could be observed. The direct
simulation Monte-Carlo is an excellent tool to describe the formation of atomic beams.
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
Baumgarten, C. et al., to appear in Nuclear Instruments and Methods A (2003).
Henoch, M. et al., to be submitted to Nuclear Instruments and Methods A (2003).
Baumgarten, C. et al., Nuclear Instruments and Methods A, 482, 606 (2002).
Stock, F., “The HERMES Target Source for Pol. H and D Atoms,” in Workshop on Pol. Beams and
Pol. Gas Targets, edited by H. Paetz and L. Sydow, World Scientific, Köln, Germany, 1996, p. 260.
Koch, N., and Steffens, E., Review of Scientific Instruments, 70, 1631 (1999).
Schiemenz, P., Ross, A., and Graw, G., Nuclear Instruments and Methods A, 305, 15 (1991).
Drewes, W., Jänsch, H., Koch, E., and Fick, D., Physical Review Letters, 50, 1759 (1983).
Gaul, H. G., and Steffens, E., Nuclear Instruments and Methods A, 316, 297 (1992).
Baumgarten, C., Studies of Spin Relaxation and Recombination at the HERMES H/D Gas Target,
Ph.D. thesis, University of München (2000), also available as DESY-THESES-2000-038.
Henoch, M., Absolute Calibration of a Polarized Deuterium Gas Target, Ph.D. thesis, University of
Erlangen-Nürnberg (2002), also available as DESY-THESES-2002-026.
Bird, G. A., Molecular Gas Dynamics and the direct Simulation of Gas Flows, Oxford, 1998.
Nass, A., Low-Pressure Supersonic Gas Expansions, Ph.D. thesis, University of Erlangen-Nürnberg
(2002), also available as DESY-THESES-2002-012.
Nass, A. et al., “Studies on Beam Formation in the HERMES-ABS,” in Workshop on Pol. Sources
and Targets, edited by V. P. Derenchuk and B. von Przewoski, World Scientific, Nashville, Indiana,
USA, 2001, p. 42.
Vassiliev, A. et al., “Investigation of the Atomic Hydrogen Beam with a Two-dimensional Multiwire
Monitor,” in Workshop on Pol. Sources and Targets, edited by A. Gute, S. Lorenz, and E. Steffens,
FAU Erlangen-Nürnberg, Erlangen, Germany, 1999, p. 200.
933