A portable cryostat for the cold transfer of polarized solid HD targets: HDice-I

Abstract

A device has been developed with moveable liquid nitrogen and liquid helium volumes that is capable of reaching over two meters into the coldest regions of a cryostat or dilution refrigerator and reliably extracting or installing a target of solid, polarized hydrogen deuteride (HD). This Transfer Cryostat incorporates a cylindrical neodymium rare-earth magnet that is configured as a Halbach dipole, which is maintained at 77 K and produces a 0.1 T field around the HD target. Multiple layers provide a hermetic 77 K-shield as the device is used to maintain a target at 2 K during a transfer between cryostats. Tests with frozen-spin HD show negligible polarization loss for either H or D over typical transfer periods. Multiple target transfers with this apparatus have shown an overall reliability of about 95% per transfer, which is a significant improvement over earlier versions of the device.

A cryostat to hold frozen-spin polarized HD targets in CLAS: HDice-II

“…Since the target transfer is best done vertically while the access to CLAS is horizontal, the refrigerator must operate in both orientations and while traveling between the center of CLAS and a point sufficently distant to allow vertical access to the cryostat (see Figure 1). The central access of the cryostat must accomodate the transfer cryostat liquid nitrogen section out to its maximum extension where a docking mechanism must open a 77 K shutter opener on the TC [3] and withstand the 1800 Nt thrust of the transfer cryostat without excessive thermal loading of either cryostat. Past this shutter opener, the extension of the pumped-liquid-helium TC center tube with target must be accommodated out to the target holder of the IBC.…”

“…The central access tube is the axis of the design and a separate vacuum space from the main cryostat vacuum, although it is not intended to support a greater differential pressure than about a tenth atmospheric. It is the path by which HD frozen-spin targets, which are mounted on threaded copper cooling rings [3], are transferred to and from the target-receiving threads on the mixing chamber. It is also the beam path to the target.…”

“…The joining spool piece is evacuated, gate valves are opened and the vacuum of the TC is joined to the centeral access of the IBC. As described in Reference [3], the liquid nitrogen temperature portion of the TC can be lowered down the central access to mate the TC thermal radiation shutter with its opener on the 4 K plate of the IBC. This also seals the central access to prevent room temperature radiation from shining down the central access.…”

“…Several commercial cryostats and magnet systems are used in the production process, as well as two specially designed and constructed devices. The first paper [3] detailed the motivation for the target system, its physics principles, the HD target production and utilization process, and one of the two custom-built cryostats required, the transfer cryostat (TC). This paper describes the design and operation of the second custom-built cryostat, the in-beam cryostat (IBC), that held polarized HD targets in the center of the CEBAF Large Acceptance Spectrometer (CLAS) for the recently-completed meson photoproduction experiment E06-101 [4] at Jefferson Laboratory.…”

Polarization observables for single and double charged pion photo-production with polarized HD target

“…With the failure rate of less that 5% per transfer, the overall success rate exceeds 75% for using the HD target, which requires 5 transfers. Image source: [48].…”