Brute Force with a Gentle Touch: Vibration
Isolation Techniques Used to Increase HD Target
Polarization
Christopher M. Bade † , Anthony Caracappa† , Tsuneo Kageya † ,
Frank C. Lincoln† , Michael M. Lowry† , John C. Mahon , Lino Miceli† ,
Andrew M. Sandorfi†, Craig E. Thorn† , Xiangdong Wei† and
C. Steven Whisnant‡
Ohio University, Athens, OH 45701, USA
Brookhaven National Laboratory, Upton, NY 11973, USA
Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA
‡
James Madison University, Harrisonburg, VA 22807, USA
†
Abstract. The performance of statically polarized high-field/low-temperature targets is a strong
function of the base temperature during polarization. At the Laser-Electron Gamma Source (LEGS)
facility, highly polarized Hydrogen Deuteride targets are created in a dilution refrigerator/15 tesla
superconducting magnet system, and converted to a frozen spin state. This allows them to retain
polarization when placed in a beam at a lower field (0.7 T) and higher temperature (1.3 K). An
increase in temperature from the 0 T state to the 15 T state of the refrigerator suggested eddy
currents were primarily responsible for heating of the cold finger. Vibration-isolation techniques
have been developed to reduce the level of eddy currents due to vibration inside the polarizing field.
These techniques reduced the amplitude of vibration due to the pumping system by two orders of
magnitude and lowered the cold finger temperature with field energized from 17 mK to 12 mK.
The potential gain in polarization is substantial.
INTRODUCTION
The Laser-Electron Gamma Source (LEGS) facility utilizes a polarized hydrogen deuteride target for double polarization photoproduction experiments. The highest possible
level of nucleon polarization is desired for these experiments. The HD Target is prepared
in a high magnetic field and low temperature condition. The target is then converted to a
frozen-spin state and transfered via a transfer cryostat and placed in the LEGS beamline
in a higher temperature, lower field condition.
The use of an HD target provides some advantages to a photonuclear experiment.
The combination of a free proton with a bound proton and a bound neutron provides
simplicity. The low background produced by aluminum cooling wires and Kel-F endcaps
can be subtracted in a straightforward manner by pumping out the HD and running beam
on the empty target cell. The frozen-spin feature allows for polarization of the target
by brute-force in temperature and magnetic field conditions which are not practical to
achieve in beam.
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
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OPTIMIZING POLARIZATION
As the LEGS HD target is polarized by conventional brute-force, its maximum polarization is given by the equilibrium polarization value of H or D for the conditions in
the dilution refrigerator. The equilibrium polarization value is given by the Brillouin
function, Eq. (1), which is an increasing function in (B/T).
PT E x I x
2I 1
2I 1
1
x
coth x coth 2I
2I
2I
2I
µB
kB T
I
1
for H and 1 for D
2
(1)
This means to increase target polarization, one must either increase the magnetic field
by buying a larger magnet, or lower the base temperature of the dilution refrigerator.
It was found that the temperature for this cryogenic system is about 9 mK without the
magnet energized but about 17 mK when the magnetic field is at 15 T. A temperature
decrease from 17 mK to 9 mK would result in a 23% increase in H polarization and a
83% increase in D polarization. This temperature disparity was attributed to vibrations
from the main 10 cm pump line transfered down to the copper cold finger which lies
within the primary solenoid magnet. As temperature decreases, effects other than eddy
current heating will limit the optimum temperature as well.
Accelerometry was performed on the area where the pump line joins the refrigerator.
These results are plotted on the predamping spectrum of Figure 1. The background
spectrum is included to give a sense of the electronic noise in the accelerometer circuit.
Examination of the relative power input due to vibrations showed that the contribution
at 60 Hz was the dominant effect.
An established technique for the isolation of large pump lines was implemented in
the dilution refrigerator. This vibration isolator consisted of cable-suspended bellows
supported by double gimbals modeled after [1]. This design has the advantage of lowering the resonance frequency of the pump line to the point where vibration from most
external sources is simply reflected. Figure 2 shows our implementation.
RESULTS
After the isolator was constructed, the accelerometer was again used to determine if
the vibrations were reduced. These results are plotted as the after damping line in
Figure 1. The spectrum after damping lies between the background and the spectrum
before damping. The spectrum of power input after damping shows a decrease in 60 Hz
vibration by three orders of magnitude.
The real effectiveness of this device is how it affects the temperature at polarizing
conditions. Temperatures were measured before and after installation of the vibration
isolator using a nuclear orientation thermometer with a 60 Co source [2]. Figure 3 shows
the temperature of the cold finger before and after installation of the isolator. The
reduction of the full-field temperature from 17 mK to 12 mK results in a quite significant
increase in equilibrium polarization as shown in Figure 4.
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FIGURE 1. A power spectrum of relative heat input due to vibrations from pump line vibrations into
the dilution refrigerator. The bottom line is from a measurement without any pumps activated. The top
and middle are before and after installing the vibration isolator, respectively.
OUTLOOK
The construction of a pump line isolator was completed to reduce vibrations and eddy
current heating. This device was successful in cutting down vibration input to the
dilution refrigerator and lowering the polarizing temperature. This has led to a significant
increase in equilibrium polarization.
ACKNOWLEDGMENTS
This work was supported by research grants from the U.S. Department of Energy under
contract No. DE-AC02-98CH10886 and the U.S. National Science Foundation grant
No. PHY-0072226.
REFERENCES
1.
2.
Kirk, W. P. and Twerdochlib, M. "Improved Method for Minimizing Vibrational Motion Transmitted
by Pumping Lines" Rev. Sci. Istrum. 49 (6), pp. 765-769 (1978)
Berglund, P. M. et al. "The Design and Use of Nuclear Orientation Thermometers Employing 54 Mn
and 60 Co Nuclei in Ferromagnetic Hosts" J. Low Temp. Phys. 6 (3/4), pp. 357-383 (1972)
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FIGURE 2. A drawing of the vibration isolator as constructed. The cables which suspend the elbow
are suspended from U-joints (a) attached to double gimbals (b) resting on knife-edges. Of particular
importance is the use of a low spring constant set of bellows (c) such as the thin-walled welded bellows
in use here. The addition of 80 lbs. of lead to the elbow (d) also allowed for the lowering of the resonance
frequency of the device.
FIGURE 3. This plot shows temperature as a function of cooling time in the dilution refrigerator.
The filled squares are measurements before the vibration isolator is installed. The open squares are
measurements with the system in place. The isolator allows the refrigerator to reach temperatures below
the previous limit of 17 mK.
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FIGURE 4. A plot of H and D equilibrium polarization as a function of B/T. These functions are given
by Eq. (1). The open circles represent the vibration-limited temperature of 17 mK while the filled circles
represent the new lower temperature condition.
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