This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Akchurin, N.

Badano, L.

Bravar, A.

Conte, M.

Hall, J.

McPherson, J.

Olchowski, F.

Onel, Y.

Penzo, A.

Pisent, A.

Pusterla, M.

Rinckel, T.

Rossmanith, R.

English

IUScholarWorks

SPIN-SPLITTER STUDIES: POLARIZATION STABILITY 
MEASUREMENTS AT IUCF 
N. Akchurin, A. Bravar, J. McPherson, F. Olchowski, and Y. One1 
Universi ty  of Iowa, Iowa City ,  Iowa 52242 
L. Badano and M. Conte 
Dipar t imento  di Fisica dell 'UniversitaL di Genova,  I N F N  Sezione di Genova,  
V i a  Dodecaneso 99, 1-1 6146 Genova,  I taly  
A. Penzo and A. Bravar 
I N F N  Sezione di Trieste ,  V i a  Valerio 2, 1-94127 Trieste ,  I taly  
J. Hall 
Universi ty  of N e w  Mexico, Albuquerque, N e w  Mexico 
A. Pisent 
I N F N - L N L ,  V i a  Romea  4, 35020 Legnaro (PD) ,  I taly  
M. Pusterla 
Dipar t imento  di Fisica dell 'UniversitaL di Padova,  I N F N  Sezione di Padova,  
V i a  Marzolo 8, 1-95191 Padova, I taly  
T. Rinckel 
Ind iana  Universi ty  Cyclotron Facility, Bloomington,  Indiana 474 0 8  
R. Rossmanith 
C E B A F ,  Newport  News,  Virginia 29606 
A self-polarization mechanism for stored (anti)-protons and ions based on the Stern- 
Gerlach effect in gradient fields has been proposed. The aim of the ongoing measurements 
at the Indiana University Cyclotron Facility (IUCF) is to verify experimentally the various 
assumptions on which this effect is based. The final goal is to demonstrate this new 
polarization effect, which could be a potential tool to produce polarized stored hadron 
beams both in the low-energy range and at SSC and LHC energies. 
T h e  Concept  of t he  S p i n  Spli t ter  
The Spin-Splitter concept was proposed some time ago1'293 as a novel method for 
obtaining in -s i tu  polarization of stored beams. This is particularly important for those 
cases where the use of beams from polarized sources is either impossible or impractical, 
as, for example, in the case of accumulated antiprotons. With this method a macroscopic 
separation of opposite spin-states of circulating (anti)protons could be obtained. The 
separation is caused by repetitive Stern-Gerlach kicks in an inhomogeneous magnetic field; 
particles with opposite spins are deflected in opposite directions. In the original Stern- 
Gerlach experiment the effect was detected in a single pass. For a beam circulating in a 
storage ring, it is necessary to implement a constructive mechanism that allows the kicks 
to add up turn after turn. 
In its basic configuration the Spin Splitter consists of a strong quadrupole doublet 
with a solenoid in between that rotates the spin 180". Due to the quadrupole gradient the 
4 (anti)protons experience a small kick in a direction that depends upon the sign of S, and 
because of the 180" rotation the kick of the second quadrupole adds in the same direction. 
Furthermore, the solenoid also acts as a siberian snake, with a very beneficial influence 
on the spin dynamics in the ring. A major opportunity for achieving an appreciable 
spin separation with the Spin-Splitter is indeed related to the long term stability of the 
polarization in the presence of a (partial) siberian snake. 
The spin motion in a ring, described by the BMT equation, corresponds for each revo- 
lution to a rotation relative to an axis n'(s) with frequency us (spin-tune). The component 
of the spin along n' is stable, while for the two components transverse to n' the depolariza- 
tion rate is related to the magnitude and distribution of the spin-tune spread Au,. If the 
machine is planar, the magnetic field .I? is always vertical, n' coincides with the direction 
4 
of B and the spin tune is us = ay (a = 1.793 is the proton anomalous magnetic moment). 
In the general case one has to compose the spin rotations due to the single elements of the 
machine and, in particular, when there is a siberian snake in the ring, n' is longitudinal at 
the point opposite to the rotator and the spin tune is half-integer. In the presence of the 
Spin-Splitter configuration, if the polarization is kept in the direction of n' the kick repeats 
at the same azimuth turn after turn. Thus the effect would add up only when the betatron 
tune is an integer (and the machine unstable). 
4 
The way to overcome this problem is to have the component of the polarization P 
that is transverse to the motion at the azimuth of the Spin-Splitter rotating an angle X 
turn after turn. The condition for a coherent addition of the kicks becomes: 
where AQH and AQv are the distances of the two betatron tunes from the nearest integer 
or half-int eger. 
There are basically two means to implement the scheme with a spin rotation angle A: 
(a) by placing3>475 the polarization in the direction orthogonal to n', for instance at ay = 2, 
and choosing vs = 112 + X/2; 
(b) by introducing a time dependent field, such as an RF-rotator, which adiabatically 
rotates n' an amount X on each turn. 
Results from the 1990-91 Runs 
Our measurements were performed using the IUCF Cooler with polarized proton 
beams, the CE- 15 Siberian Snake Solenoid6 and the CE-01 polarimeter.7 During our shifts 
the machine team optimized the betatron tunes QH and Qv by correcting the closed orbit 
and by adjusting the electron cooling elements. We were able to use a stored current of 20 
PA. The beam lifetime was 20 minutes with cooling on and 3.5 minutes without cooling. 
I A) Polarizat ion l i fet ime measurements  
Measurements have been performed at 108 MeV (a? = 2) with n' longitudinal at the 
snake, which was operated with either full field or partial field (25%). Multiturn injection 
succeeded in storing a high intensity (up to 20 PA) beam of polarized protons in order 
to measure with the CE-01 polarimeter the polarization of a single filling with sufficient 
accuracy. The polarization of the protons injected in the Cooler ring was measured by the 
BL3 polarimeter and was always found to be close to 75%. 
During the first run we performed the following set of measurements: 
i) vertical polarization injected, snake off: Pv=O. 15, PH =0.08; 
ii) vertical polarization injected, snake on (25%): Pv=0.09, PH = -0.07; 
iii) radid polarization injected, snake on (25%): Pv=0.05, PH=0.69. 
All these measurements were made immediately after injection. The accuracy was about 
f 0.05 for both components of the polarization. The solenoid was not compensated by 
skew quadrupoles during this run. The result in iii) is consistent with a full conservation 
of the polarization since, between the snake and the polarimeter, the spin performed a 
precession of 240'. 
We then repeated these measurements, waiting up to 15 minutes after a filling time 
of 10 minutes: 
iv) radial polarization injected, snake on (25%): Pv = -0.04, PH=0.68; 
v) vertical polarization injected, snake on (25%): Pv = -0.08, PH = 0.07; 
vi) radial polarization injected, snake on (25%) during filling, then slowly switched off: 
Pv=0.53, PH=0.48; 
vii) radid polarization injected, snake off: Pv=O. 13, PH=0.25. 
With the snake at full strength, the polarization stability along the Z-axis was measured 
also, giving Pv = 0.07 and PH = 0.71 with horizontal injected polarization (0.78). The 
radial component PH was practically the same after 15 minutes of storage, plus 10 minutes 
of stacking. 
These measurements with partial and full (compensated) snake confirm that the re- 
sults of CE-15 on the preservation of polarization across a depolarizing resonance are valid 
over expanded times and show that sp in  m o t i o n  stability i s  at  least as good as beam l i fet ime.  
B) Stability of non-equil ibrium polarization components  
When the spins of the circulating particles are injected orthogonal to Z, they will 
oscillate around this axis and the polarization direction of the stored beam, changing from 
revolution to revolution. In order to avoid a mixing of particles with opposite polarization, 
new particles must be injected only when their spins are parallel to the stored particles. 
A system to suppress fillings when the two polarizations are not parallel was necessary for 
this purpose. For the same reason the polarimeter had to be gated with the revolution 
frequency in order to measure non-zero polarization. These stroboscopic injection and 
measurement gating systems were implemented and tested. 
We injected vertically polarized beam orthogonal to n' and, as obtained in the previous 
run with partial snake, we found negligible polarization when we did not use the synchro- 
nizing systems for injection and the polarimeter. When these systems were switched on, 
we could measure Pv = 0.36 f 0.10 and PH = 0.05 f 0.08, after 10 and 20 minutes of stor- 
age time without appreciable changes. The error in the last measurement is considerably 
larger than for the other results, i.e. f 0.05, as the useful statistics correspond in this case 
to a fraction of the previous one, due to the gating procedure. 
These results were presented at the Bonn Spin Conference8 and at the Polarized 
Collider Workshop. 
During the stacking, the beam is debunched, implying that particles with different 
energies perform a different number of revolutions until bunching occurs again. Therefore 
we cannot expect full conservation of the polarization. In order to stack protons with their 
polarization perpendicular to the Z-axis at the siberian snake and avoid this drawback, we 
checked a possible method which consists of stacking the beam with polarization parallel 
to n' and then rotating the spins to the desired angle by means of the RF-solenoid of 
Experiment CE-20. We found that this procedure is in principle viable but limited by the 
accuracy of the current setting of the snake solenoid. For the following runs the power 
supply of the snake was changed in order to give more precise readings of the current. 
For the time being, in order to avoid the problem of matching the spins of injected and 
circulating protons, and spin migration during stacking, protons were injected in a single 
turn with spin either vertical or horizontal. We varied the current in the solenoid in order 
to scan an interval of precession angles around 180'. 
Vertical injected polarization: We injected the beam with the polarization pointing in the 
vertical direction in order to be orthogonal to the stable solution Z at the siberian snake 
solenoid. As this implies continuous spin flipping, the polarimeter was gated in such a 
way as to take data only every second revolution. Values of the transverse polarization, 
PI = Jm, at the polarimeter were typically less than 0.1 and independent of the 
snake solenoid current. 
Horizontally injected polarization: Protons were injected with their spins lying in the 
horizontal plane, having activated the solenoid in the beam transfer line. Due to the spin 
precession related to ay = 2, these spins are again orthogonal (sidewise this time) to 
the stable solution Z at the siberian snake solenoid. Again, the values of the transverse 
polarization were about 0.1 and independent of snake solenoid current. 
In these runs the polarization was fully transverse to n', either horizontal or vertical at 
-. 
injection. In both cases Po precesses around n' all along the ring, and in particular around 
the longitudinal direction at the snake; therefore, the largest PL = Jm detectable 
at the polarimeter can vary, from Po =0.750 to P0=0.375, even when we assume full con- 
servation of the transverse polarization components: the measured values will actually 
depend upon the fluctuations of the various machine parameters. 
To take into account at least the principal effects, in particular the energy spread 
and fluctuations to the beam, we proceeded to describe our measurements by means of a 
computer simulation. It is assumed in this calculation that the synchrotron oscillations 
have a maximum width of 0.013 and a tune Q, = 0.004. Typical values of PI are less 
than 0.1, but three peaks appear between snake solenoid currents of 144 and 147 A. The 
two side peaks are explained as synchrotron-frequency sidebands due to the interference 
of two effects: 
1) because of the energy dependence of the spin precession, protons with different 
energies (momentum spread) undergo spin rotations in the solenoid slightly different from 
180". As a consequence, if we start with a vertical polarization of the beam the spin is not 
completely vertical in the ring, but oscillates around the vertical direction; 
2) this fact triggers a second effect, the resulting horizontal component of the polar- 
ization oscillates with the synchrotron frequency. Hence the horizontal spin-tune oscillates 
around its nominal value of 112, giving rise to the sidebands. 
Our simulation assumes that a current of 145.5 A corresponds to 180' precession of the 
spin in the snake solenoid, and that the synchrotron oscillations have Q, = 0.004 with an 
energy spread of 1.3 x A scan in fine steps measured such a peak at Isnake = 145.5 A 
with a peak value of PI R 0.3. 
We would like to complete these measurements in 1992, both to improve the statistical 
significance of our results and to perform additional cross checks by varying some of the 
most crucial parameters. 
For the time being we can conclude that we have found significant indications that 
components transverse to n' survive across a depolarizing resonance with a snake for suffi- 
ciently long time to conduct these studies. 
These data have been presented at the 1992 Adriatico Conference'' and at EPAC9211 
and have been submitted for publication to Physical Review Letters. 
1. Y. Onel, A. Penzo, and R. Rossmanith, A Spin-Splitter for Antiprotons in Low Energy 
Storage Rings, AIP Conf. Proc., Vol. 150, ed. by R.G. Lerner and D.F. Geesaman 
(American Institute of Physics, New York, 1986), p. 1229. 
2. A. Penzo (representing the SPLIT Collaboration), Perspectives for polarized antipro- 
tons at LEAR: the Spin-Splitter, Physics at Lear with Low Energy Antiprotons, IV 
LEAR Workshop, Villars-sur-Ollon, Switzerland, Sept . 1987, Nucl. Science Res. Conf. 
Series, Vol. 14, 183 (1988) (Harwood Academic Publishers). 
3. M. Conte, A. Penzo, A. Pisent, and M. Pusterla, Analytical Treatment of the Spin- 
Splitter, INFN-Trieste Report INFNITC-88/25 (1988). 
4. M. Conte e t  aL, Further considerations regarding the Spin Splitter, IV LEAR Work- 
shop, Villars-sur-Ollon, Switzerland, Sept. 1987, Nucl. Science Res. Conf. Series, 14, 
211 (1988) (Harwood Academic Publishers). 
5. Ya.S. Derbenev, The Stern-Gerlach method in charged particle storage rings, Univ. 
of Michigan Report UM HE 90-30 (1990). 
6. A.D. Krisch e t  al., Phys. Rev. Lett. 63, 1137 (1989). 
7. B. von Przewoski e t  al., Proc. 9th Int. Symp. on High Energy Spin Physics, Bonn, 
(Springer Verlag, Vol. I), p. 607. 
8. R. Rossmanith (on behalf of the Spin-Splitter Collaboration), Proc. of the 9th Int. 
Symp. on High Energy Spin Physics, Bonn, ed. W. Meyer, E. Steffens, and W. Thiel 
(Springer Verlag, Vol. 11)) p. 121. 
9. R. Rossmanith (on behalf of the Spin-Splitter Collaboration), Polarized Collider Work- 
shop, University Park, ed. J. Collins, S.F. Heppelman, and R.W. Robinett (AIP 
Conference Proceedings No. 223, p. 343) (New York, 1990). 
10. Y. One1 (representing the Spin- Splitter Collaboration), Polarizing matter and anti- 
matter: a new method, Adriatico Research Conference on Polarization Dynamics in 
Nuclear and Particle Physics (7-10 Jan. 1992)) ICTP-Trieste, Italy. 


Spin-Splitter Studies: Polarization Stability Measurements at IUCF

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Spin-Splitter Studies: Polarization Stability Measurements at IUCF

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