Schematic of the compressor system. Arrows indicate flow while compression is in progress. The labeled system components are storage cell (SC), buffer cell (BC), optical pumping cells (OPCs), compressors (C1, C2), pressure transducer (P1), nonmagnetic strain gauges (P2, P3), needle valve (NV), getter/purifier (G/P), capillary tubing (Cap.), and liquid nitrogen traps (LN).
Photograph of the IUCF compressor system. Going clockwise around the photo: the optical pumping cells in the top center, the Rb coated storage cell Orion, the aluminum holding blocks for C2, the buffer cell, part of the flange of C1, the two LN cold traps, and the Baratron pressure transducer is in the far left. The posts are aluminum Uni-Strut covered in black tape to reduce reflection of the light from the discharge.
Valve sequence of C1. Closed means that C1 is closed to all other volumes. The delay between valve or piston operations is to ensure that there is no overlap between successive operations. The timing sequence for C2 is similar with and , the main difference being that during the filling of the BC, C2 is exposed to the turbo-pump to evacuate the residual gas from the previous BC drain.
Measured pressure in the BC and SC and the corrected pressure in the OPC during a fill to . The solid black lines are the simulated pressures based on the gas flow rate. The spread in the pressure data for the OPC is due to the pressure fluctuations when the inlet to C1 is opened and closed. The pressure measured for the OPC is higher than that predicted by the simulation since a pressure gradient must exist in order for there to be a gas flow. The average pressure difference between the model and the measured pressure is and is in agreement with the flow rate of the system and is used to correct the measured OPC pressure and optical polarimeter pressure calibration coefficients. With this correction, the simulation correctly predicts the time dependence of the pressure in the OPCs, BC, and SC.
Optical pumping rate of pure in one of the OPCs as a function of RF discharge as characterized by the relaxation time (T1). The laser power was with both the 0.3 and etalons in the laser cavity. The polarization was measured by the optical polarimeter, discussed below. The (×) are for a pressure of , the (⌾) are for , and the (◇) are for a pressure of . The pumping rate was determined by fitting the polarization accumulation using Eq. (4). The relaxation time was determined by shuttering the laser and measuring the subsequent polarization decay over a period of . The limited discharge strength range for the lower pressures is due to the difficulty of maintaining a weak discharge. These data are in qualitative agreement with those reported in Ref. 27.
Schematic diagram of the NMR-FID gauge. The RF pulse creates a rotating field, which tips the spins of the polarized nuclei an angle with respect to the polarization axis. The precession of the transverse component of the magnetization induces a rf signal in the drive coil. This induced rf signal is sent to a lock-in amplifier whose reference frequency is that of the initial drive pulse. The output of the lockin is then the beating between the Larmor frequency and the drive pulse and is acquired on a digital oscilloscope.
FID polarimetry in the OPCs. (a) FID signal vs polarization as determined by the polarimeter. The curve is a linear best fit to the data. (b) FID signal vs pressure for constant polarization. Polarized gas was compressed into the buffer cell and then bled into the OPC in pressure intervals as indicated. The line is a fit to the first two data points to demonstrate the FID signal is nonlinear in the pressure as expected.
Typical FID signal obtained in the BC at a pressure of (a) and in the SC for a pressure of (b). The solid line is a fit to the data assuming an exponentially damped sinusoid. The BC signal corresponds to of pure polarized to 32%. The SC signal corresponds to of a 2:1 mixture with a polarization of 32%.
Data for the time dependence of the polarization in the withdrawn (a) and forward (b) piston positions of C2. were obtained by fits of Eq. (6) to the data.
Relaxation and preservation data for the BC. (a) Representative plot of the polarization vs time for the BC. The data corresponds to of pure in the BC, and the T1 from the fit is . (b) Polarization preservation for filling the BC as a function of the average pressure in the optical pumping cells during compression to . The average of all is , with the uncertainty coming from the rms. deviation of the distribution.
Experimental setup with the polarized target of in the POSY I beam line, not drawn to scale. The neutron depolarizer was a bulk sample of iron that had randomly oriented magnetic domains. The depolarizer was used so as to not include the polarization properties of the supermirror nor the spin transport of the instrument in the analysis, as well as to not have to remove the supermirror.
Neutron transmission through the depolarized target, normalized by the transmission through the empty cell. The exponential fit gives the thickness, .
Neutron transmission through the polarized cell at three different times, normalized by the transmission through the depolarized cell. The transmissions for the different polarizations are offset by 0,0.5,1 from top to bottom for clarity. The three transmissions were taken over four hours each, and the times after Orion was transferred to the transport solenoid to which they correspond are , and . From the time dependence of the polarization, Orion’s T1 in the solenoid was determined to be . This relaxation time implies that immediately after compression, the polarization was , yielding a polarization preservation of .
Approximate relaxation times and respective in the presence of given materials for (Ref. 45).
Results of a calibration procedure as described in the text. The calibration coefficients for the OPC and BC are for pure gas, while for the SC they are for a mixture of 2:1 .
The compressor system’s major components’ physical parameters. Relaxation times for the compressors are for the piston in the withdrawn (forward) position. Preservation is the estimated contribution to the loss of the polarization during the compression cycle. The preservation for the compressors is based on the estimate discussed in Sec. V A while those for the BC and the exposed SC (SC,CT) are the measured quantities discussed in Sec. V C. Orion’s isolated was measured at a pressure of of pure .
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