Two-stage Cryocooler Research Articles

A conduction-cooled, 680-mm-long warm bore, 3-T Nb/sub 3/Sn solenoid for a Cerenkov free electron laser

A compact, cryocooler cooled Nb/sub 3/Sn superconducting magnet system for a Cerenkov free electron laser has been designed, fabricated and tested. The magnet is positioned directly behind the electron gun of the laser system. The solenoidal field compresses and guides a tube-shaped 100 A, 500 kV electron beam. A two-stage GM cryocooler, equipped with a first generation ErNi/sub 5/ regenerator, cools the epoxy impregnated solenoid down to the operating temperature of about 7.5 K. This leaves a conservative operational margin for the MJR type of Nb/sub 3/Sn conductor of about 350% (I/sub c/) at 3T. Current leads with HTS sections between the first and second stage of the cryocooler effectively reduce the conductive heat leak. In the warm bore, 60 mm in diameter and 680 mm long, an operational magnetic field of 2 T has been achieved during laser operation without any quench. The field homogeneity is better than 99.5% over 450 mm axial length. Design details and test results of this magnet system are presented.

Cryocooled sapphire oscillator with ultrahigh stability

We present test results and design details for the first short-term frequency standard to achieve ultrahigh stability without the use of liquid helium. With refrigeration provided by a commercial cryocooler, the compensated sapphire oscillator (10 K CSO) makes available the superior short-term stability and phase noise performance of cryogenic oscillators without periodic interruptions for cryogen replacement. Technical features of the 10 K CSO include use of a two-stage cryocooler with vibration isolation by helium gas at atmospheric pressure, and a new sapphire/ruby resonator design giving compensated operation at 8 K to 10 K with Q=(1-2)/spl times/10/sup 9/. Stability of the first unit shows an Allan deviation of /spl sigma//sub y//spl les/2.5/spl times/10/sup -15/ for measuring times of 200 s/spl les//spl tau//spl les/600 s. We also present results showing the capability of the 10 K CSO to eliminate local oscillator degradation for atomic frequency standards. Configured as local oscillator (L.O.) for the LITS-7 trapped mercury ion frequency standard, the CSO/LITS combination demonstrated a limiting performance of 3.0/spl times/10/sup -14///spl tau//sup 1/2/, the lowest value measured to date for a passive atomic frequency standard, and virtually identical to the value calculated from photon statistics.

磁性蓄冷材を使用した小型冷凍機 磁性蓄冷材を用いたＧＭ冷凍機における蓄冷器内の温度分布および流量分布

A two-stage GM cryocooler is commonly used in many cryogenic systems, such as a cryopump and an MRI. The lowest temperature for a conventional GM cryocooler, however, was limited to about 10K. Recently, magnetic regenerator materials, which have larger heat capacity below 10K than a conventional regenerator material of Pb, enable the GM refrigerator to achieve the liquid helium temperature level. In this temperature range, helium in the GM cryocooler cannot be regarded as an ideal gas. The purpose of this study is to investigate a regenerator, which is operated with non-ideal helium gas. The experimental results of the regenerator performance in a two-stage Gifford-McMahon (GM) cryocooler are described in this paper. Temperatures and pressure in the regenerator were measured and mass flow rate in the regenerator was calculated from these data. The temperature profile, temperature fluctuation and mass flow rate in the regenerator are compared between ideal gas operation and non-ideal gas operation. For ideal gas operation, the temperature profile was smooth and the temperature fluctuations at each positions are almost equal. For non-ideal gas operation, a steep temperature gradient and large temperature fluctuation, which almost covered the temperature difference of the regenerator were observed from the hot end to the middle of the regenerator. From the middle to the cold end, however, both the temperature gradient and the temperature fluctuation were very small. The mass flow rate in the regenerator for ideal gas operation decreased from the hot end to the cold end. For non-ideal operation, the mass flow rate increased. The difference between the regenerator for ideal gas operation and that for non-ideal gas operation was shown clearly.

Signal-to-noise ratio improvement of HTSC bolometers for cryocooler operation using a thermal compensation principle

This paper reports on the reduction of thermal fluctuations in a high T c superconductor (HTSC) bolometer using a compensation technique. The HTSC bolometer consisted of two meander-like YBCO structures separated by a distance of 5 mm on a silicon substrate. A two-stage Gifford-McMahon cryocooler with an additional thermostat was used to cool the samples to a temperature of ≈88 K, with a 5 mK fluctuation peak to peak. A factor of 100 improvement in signal-to-noise ratio was achieved during the compensation experiments.

Development of Bi-2223 HTS high field coils and magnets

The powder-in-tube approach was successfully used to fabricate silver clad Bi-2223 in >100 m lengths of mono and >800 m lengths of multifilament tape. The tapes were wound into pancake and layer coils using the 'wind and react' as well as the 'react and wind' technique. The coils and magnets were characterized in the Hybrid III magnet and in a cryocooler at the FBNML, MIT. A 'wind and react' magnet generated a maximum self field of 2.6 T, at liquid helium (4.2 K), liquid neon (27 K) and liquid nitrogen (77 K) temperatures, respectively. A magnet generated over 1.0 T with a background field of 20 T at 4.2 K. A smaller magnet cooled to 20 K with a two-stage cryocooler has demonstrated 0.8 T in a 40 mm room temperature bore.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>