Abstract
We present the status of the low-temperature devices shared by several neutron instruments of the Materials and Life Science Experimental Facility (MLF), J-PARC. In recent years, the Cryogenics and Magnets group of the Sample Environment (SE) team has introduced three cryostats at the MLF: a top-loading 4He cryostat, a bottom-loading 3He cryostat, and a 3He–4He dilution refrigerator insert. The group also manages a superconducting magnet which entered into service in 2012. Progress has been made with these SE, including the development of a new system for controlling the 3He cryostat and an oscillating radial collimator for using efficiently the superconducting magnet. These innovations have significantly increased the frequency with which these cryostats and magnet are used. During the last Japanese fiscal year, some of them were used approximately half of the total number of days of beam operation.
Introduction
Many neutron scattering experiments, especially in materials science research, require low-temperature environments and high magnetic fields. In the Materials and Life Science Experimental Facility (MLF) of the Japan Proton Accelerator Research Complex (J-PARC), many closed-cycle refrigerators (CCRs) have been prepared as a standard cryostat by instrument teams who operate them during user experiments at their instruments. However, the facility requires more-advanced sample environments (SEs) such as a 3He cryostat, a dilution refrigerator (DR) and a cryomagnet to respond the users’ demands. To reduce the purchase costs of this equipment, to operate them efficiently at various instruments, and to share techniques of complicated operations, neutron scattering facilities require a central technical team. The Cryogenics and Magnets group of the MLF, J-PARC is a part of the SE team which was officially created in 2013 [1]. This group includes five staff members managing and operating cryostats and a superconducting magnet for user experiments, irrespective of the institute the instruments belong to, such as the Japan Atomic Energy Agency (JAEA) and the High Energy Accelerator Research Organization (KEK).
A vertical-field superconducting magnet entered service in 2012. It was the first apparatus of SE equipment intended for common use. The number of the shared SE devices has increased in recent years, and the Cryogenics and Magnets group currently possesses three cryostats in addition to this magnet. These cryostats entered service at the end of 2016 after a commissioning phase. Since then, the frequency of their use has considerably increased year after year. We report on the progress and the status of the cryostats and magnet shared by the instruments (Section 2) and review recent trends in their use (Section 3).
Progress of the commonly-used cryostats and magnet
The SE team has produced guidelines for designing instruments and SE devices which follow MLF standards, in order to share technical informations and to ensure compatibility among the instruments. For instance, the Gifford-McMahon CCR is a standard refrigerator recommended in the guidelines, and many instruments have acquired one or more CCRs. However, some users require temperatures lower than the base temperature of the Gifford-McMahon CCR of ∼4 K. For these users, the Cryogenics and Magnets group has prepared a 4He cryostat, a 3He cryostat and a 3He–4He DR insert. The detailed specifications for each of those devices have been reported elsewhere [2]. They are also briefly presented on our web page to inform users of the MLF capabilities in SE [3].
4He cryostat
The Cryogenics and Magnets group has introduced a top-loading 4He cryostat (ICE Oxford Ltd, UK) which is routinely used since 2016. The 4He cryostat features a variable temperature insert (VTI) allowing to perform experiments requiring temperatures above ∼2 K. The cryostat is provided with a goniometer to rotate the sample stick. The outer vacuum chamber (OVC) and radiation shields are made of aluminum. Because the group already had a 3He–4He DR insert when the 4He cryostat was purchased, this cryostat was carefully designed with consideration of the VTI position and cooling power, to be sure that the DR could be used. This cryostat therefore accommodates the DR insert and the combined system can reach a base temperature of ∼50 mK.
This cryostat has been used at the single-crystal diffractometer SENJU [4], and the chopper spectrometers 4SEASONS [5], DNA [6] and AMATERAS [7]. In addition, the total diffractometer NOVA also performed test experiments with the aim to introduce it for user experiments. Although aluminum beam windows are generally not suitable for instruments such as the total diffractometer and powder diffractometers, the neutron background generated by the aluminum is reduced effectively with an oscillating radial collimator (ORC). The group also plans to fabricate another set of tails that have vanadium windows.
Recently, we encountered problems with the sample stick which slid downward during a measurement because of the pressure difference between the inner vacuum chamber (IVC) and the atmospheric pressure. Since the inner diameter of the sample bore of the cryostat is relatively large at the top flange, the force pulling the stick into the IVC is quite strong. To solve this problem, we have modified the design to adjust the stick height and to fix it with screws as shown in Fig. 1. A similar design has been adopted on some top-loading cryostats to prevent the same issue.
Photos of the top-loading 4He cryostat and of the top flange of its sample stick. Red arrows indicate two sets of metal parts added by the Cryogenics and Magnets group for adjusting and fixing the sample height.
We have introduced a bottom-loading 3He cryostat (Niki Glass Co., Ltd.) in 2016. This cryostat is a particularly versatile low-temperature device thanks of its wide temperature range. This is a dry system which consists of a pulse-tube CCR and a one-shot 3He cooling system. The sample can be cooled down to 0.3 K with a fully automatic operation. The cool-down time from room temperature to the base temperature is approximately 18 hours for usual sample sizes, and the base temperature can be held for more than 50 hours. The SE team has prepared a goniometer, which was fabricated by Metal Technology Co. Ltd., for rotating the cryostat when measuring single crystals. This goniometer has a vacuum flange which is standard at the MLF, so most instruments that have a vacuum scattering chamber can make use of the combination of the 3He cryostat and goniometer. The instruments SENJU, 4SEASONS, DNA and AMATERAS have utilised this cryostat for user experiments. The cryostat can also be used at instruments that have a sample table instead of a vacuum scattering chamber.
Examples of neutron background observed with the 3He cryostat during inelastic neutron scattering measurements with (a) powder and (b) single-crystalline samples.
One of the issues on operation of this cryostat is that we need to rotate He gas pipes of the pulse-tube CCR together with the cryostat itself. To avoid troubles, we have replaced the standard tubes with more-flexible ones and installed rotary joints between the cryostat and the tubes for both the supply and return lines. Another issue of this cryostat is the background created by neutrons scattered by the radiation shields and the OVC tail. A spot-like neutron background was observed in a detector map during a single-crystal diffraction measurement. Furthermore, a substantial background was observed in the region of non-zero energy transfer during inelastic neutron scattering measurements with both powder and single-crystal samples (Fig. 2).
Since the original radiation shields are solid aluminum cans, the Cryogenics and Magnets group has built prototype radiation shields from thin aluminum foils. These shields successfully eliminated the spot-like background. The neutron background that was observed during inelastic scattering measurements is also expected to be reduced with these new shields. Since the use of thin-foil radiation shields raises the base temperature slightly, the group is still working on improvements and commissioning of these shields.
The 3He cryostat is operated with a software that was developed by the Niki Glass company. It allows for fully automatic cooling down to base temperature and re-charging of 3He. This software does not however include a function for controlling the temperature at the 3He pot. Temperature ranging from the base temperature to ∼2.5 K are therefore difficult to control manually because one needs to control the Sorb temperature by monitoring the temperature at the 3He pot. The Cryogenics and Magnets group has therefore developed another software for automatic temperature control of the 3He pot in this temperature range that uses only by the Sorb heater. This software allows long-time temperature stability. Recently, the group has also built a universal system for operating this cryostat with a merge of the software components and including other operations like heater control at higher temperatures, valve control, and control of the compressor of the pulse-tube CCR. This universal system can also be remotely controlled with IROHA2, the common software framework for data acquisition and device control developed at the MLF, in near future.
The vertical-field superconducting magnet (Scientific Magnetics) has been available for user experiments since 2012. This is a top-loading wet system provided with a stage for rotating the sample stick. The maximum field is approximately 7 T and was confirmed by direct measurements using a Gauss meter. The horizontal and vertical beam access angles are
(a) Photo of the magnet and the ORC. Inelastic scattering data obtained from the same sample (b) without and (c) with the ORC.
Devices involved with the magnet can be controlled on IROHA2, which allows users to run sequential measurements. The MLF engineering team has developed an ORC for this superconducting magnet to allow quasi-elastic and inelastic neutron scattering measurements under magnetic field (Fig. 3(a)). The ORC was designed to eliminate neutrons scattered by the tails including the vacuum can of the DR, whose diameter is Ø40 mm. The “shift-mode” operation, which is an operation pattern used in the MLF [8], is adopted for this ORC. The oscillation angle range is 3 degrees, and it moves forth and back in 30 seconds. It reduced very efficiently the background as shown in Fig. 3(b) and (c). Since this development, the magnet has been widely used at the following instruments: SENJU, AMATERAS, the small and wide angle neutron scattering instrument TAIKAN [9], and the neutron reflectometer SHARAKU [10].
In addition, the 4SEASONS instrument team has recently performed a demonstration experiment with this superconducting magnet [11]. Because chopper spectrometers of the MLF have vacuum scattering chambers made from iron-steel frames, the maximum applicable field is limited and depend on the instruments. In the case of 4SEASONS, the stray field and the magnetic force between the cryomagnet and the iron-steel frame were calculated and confirmed by measuring the stray fields at representative positions within the instrument. These measurements gave a maximum field of 3 T, taking account of the limited magnetic forces on the magnet itself.
A 3He–4He DR insert from Taiyo Nippon Sanso was commissioned at the end of 2016. This DR insert can be used with both the top-loading 4He cryostat and the superconducting magnet. It achieves a base temperature of 50 mK when not subjected to neutron irradiation. This DR insert has already been used with the 4He cryostat or the cryomagnet at the instruments SENJU and AMATERAS.
Trends in use of equipment from the pool
As presented above, the SE team has prepared several low-temperature devices in response to users’ and instrument groups’ requests. The amount of shared SE equipment has increased rapidly since 2016. As shown in Fig. 4, the frequency with which the equipment is used raises with our SE capabilities. While SENJU and AMATERAS use shared SE equipment heavily at the MLF, other instruments now use them at an increasing rate.
Trends in use for the commonly-used cryostats and magnet.
In the Japanese fiscal year 2018, the 3He cryostat and the superconducting magnet were used about 85 days and 75 days, respectively. These devices are therefore now used for approximately half of the total number of days of beam operation within a year. Since scheduling the use of this equipment as requested by users is becoming difficult, we now consider limiting the number of days that each piece of equipment can be used in a round of proposal unless we may be able to introduce additional equipment. The SE team will continue to study users’ and instrument groups’ requests and will formulate a plan for the introduction of new equipment if necessary.
Footnotes
Acknowledgements
For the commissioning on the ORC of the superconducting magnet, inelastic scattering measurements were carried out at AMATERAS based on proposals No. 2013I0014 and 2017I0014. We acknowledge Kenji Nakajima for providing data of this experiment. The powder sample measured at that time, which was a quantum spin system, was synthesised by Hiroshi Kageyama. We also thank Kazuya Kamazawa and Kazuki Iida for providing data revealing the neutron background produced by the 3He cryostat.