Closed-cycle Helium Refrigerator Research Articles

Instrument/procedure which prepares a fresh surface of highly oriented pyrolytic graphite in a closed-cycle helium refrigerator

An instrument/procedure which prepares a fresh surface of highly oriented pyrolytic graphite in a closed-cycle helium refrigerator is described. The instrument and procedure assist in low-temperature studies of the laser-induced fluorescence of molecules adsorbed to graphite surfaces.

A 40 GHz band SIS receiver using Nb/Al?AlOx/Nb array junctions

A 40 GHz band SIS mixer receiver has been built using Nb/Al−AlOx/Nb array junctions and a 4.3 K closed cycle helium refrigerator. The minimum conversion loss of the mixer is 2±1 dB and the single sideband receiver noise temperature (TRX (SSB)) is as low as 110±10 K at 36 GHz. TRX (SSB) is almost constant in the IF bandwidth of 600 MHz. The mixer saturation level is as high as 15 nW, which is comparable to the injected LO power.

Low-temperature, high-pressure optical system with a compact diamond-anvil cell and a closed-cycle refrigerator

An optical system for low-temperature high-pressure measurements of polarized absorption, reflectance, and luminescence with a compact diamond-anvil high-pressure cell, a closed-cycle helium refrigerator, and toroidal mirrors to focus on the sample is described. It enables us to get a temperature–pressure diagram of optical spectra easily and rapidly in the region between 200 nm and 14 μm at 15–300 K at pressures up to about 100 kbar with in situ monitoring of pressure by the ruby fluorescence method.

An automated cryogenic charcoal trap system for helium isotope mass spectrometry

An automated seawater sample processing line employing a two-stage activated charcoal trap system cooled by a closed-cycle helium refrigerator is described. All dissolved gases except neon and helium are adsorbed on the first stage operating at 55 K with neon and helium quantitatively adsorbed on the second stage at 10 K. The second stage trap is heated to 40 K to release virtually 100% of the helium isotopes (3He, 4He) for mass spectrometer measurement followed by heating to 70 K to release the neon component. Performance data for helium isotope measurements on seawater samples and air standards [4He≊10−9 to 10−6 cm3 (STP) and 3He≊10−15 to 10−12 cm3 (STP)] are presented.

Adapting a displex closed-cycle helium refrigerator for ultrahigh vacuum operation

A modification of a commercial closed-cycle liquid helium refrigerator is described which permits operation at pressures below 10−9 Torr. By using a sapphire insulator on the low-temperature stage, and incorporating vacuum-compatible heating elements, samples can be biased to ±30 kV over a temperature range of 15–300 K.

Wave-number stability of a laser diode mounted in a closed cycle helium refrigerator

The wave-number stability of a laser diode mounted in a closed cycle helium refrigerator was examined using a CO(2) laser heterodyne spectrometer. The set current supplied by a commercial laser diode supply appeared stable enough over the time periods normally used. Temperature control was achieved through both passive and active loop control. A stage of open loop control of the cold tip allowed the closed loop control of diode heat sink to be operated at higher gains than conventionally used. Long-term stability of the center of the line shape function was better than 10 MHz per hour and the 1-min average was better than +/-5 MHz. Vibration of the diode mount determined the instrument line shape in spite of steps taken to eliminate the vibrations. The line shape had a half-width of 10 MHz with broad shoulders out to 50 MHz.

A Laser Beam-Heated Miniature Oven for Matrix Isolation Raman Studies

High-temperature Knudsen cells of various designs have been used by a number of investigators to measure the matrix-isolation vibrational spectra of inorganic salts. Recently we have initiated a matrix-isolation laser Raman study of inorganic and coordination compounds. This communication reports the design of a novel miniature oven which is heated by a laser beam to vaporize small quantities of solid compounds. The necessity for such a miniature oven arose when we tried to adapt the Cryogenic Technology Inc. model 21 closed cycle helium refrigerator to our Spex model 1430 sample illuminater. Because the latter is also used for ordinary Raman measurements, we planned to minimize its modification so that the matrix-isolation set-up can be converted easily to the normal configuration. The Spex illuminator is equipped with a high aperture Angenieux lens (f/0.95) with a focal length of 25 mm. However, the usable working distance between the lens and the focal plane is only 16 mm. That necessitated the design of a special Pyrex glass vacuum shroud as shown in Fig. 1 (part 1). This shroud is attached to an aluminum sleeve (part 2) which is attached to the refrigerator (part 3). A gas inlet and an electrical feed-through are housed on the aluminum sleeve (not shown in Fig. 1).

Apparatus for Matrix Isolation Studies of High Temperature Species

A new system suitable for studies of matrix isolated metal atoms is described and evaluated. The system incorporates a closed cycle helium refrigerator used to cool the sample block down to, or slightly below, 20 K and an electron beam sublimator to produce atomic beams of most of the refractory metals. The advantages of the system are its compactness and ease of operation.

Mössbauer effect studies following Coulomb excitation using a closed cycle helium refrigerator

An improved apparatus based on a closed cycle helium refrigerator has been developed to cool targets and absorbers for Mössbauer effect studies following Coulomb excitation. The system is capable of maintaining the target at a temperature of 15 K with no heat input. There is a temperature increase of 1.5 K/W. The major problem of vibrations has been reduced to an acceptable level.

A Versatile System for In-Beam Mössbauer Spectroscopy

An experimental system for Mössbauer studies following Coulomb excitation or nuclear reactions is described. A drift-free high velocity Doppler spectrometer is incorporated in the apparatus, and a closed cycle helium refrigerator is used for cooling the target and absorber to low temperatures. The system also operates successfully as a conventional Mössbauer spectrometer with radioactive sources.

The use of a helium refrigerator for Mössbauer studies

A closed-cycle helium refrigerator for Mössbauer experiments is described. The vibrations caused by the refrigerator mechanism are decoupled and suppressed from affecting the relative sources-absorber motion. The equipment operates successfully with both radioactive and Coulomb-excited sources. Source and absorber are thermally connected to the cooling station of the refrigerator. The temperature of the source and absorber can be controlled by the helium gas pressure and by an electric heater. The operating range is 20°K to 300°K for radioactive sources and 25°K to 300°K for Coulomb-excited sources. Operation is convenient and economical.

Use of a Closed Cycle Helium Refrigerator for Mössbauer Studies

A closed cycle helium refrigerator has been used successfully to cool the target and absorber in Coulomb excitation Mössbauer studies. The refrigerator handles the appreciable power from the beam and may be used at any temperature above 20°K. The most severe problem of mechanical vibration has been solved.

The development and evaluation of a cryosorption pump capable of pumping all gases including helium: Ronald Dawbarn and J D Haygood, ARO, Inc, Aerospace Environmental Facility, Arnold Engineering Development Centre, Arnold Air Force Station, Tennessee, USA

New absorption cells designed specifically to achieve stable temperatures down to 66 K inside the sample compartment of an evacuated Bruker IFS-125HR Fourier transform spectrometer (FTS) were developed at Connecticut College and tested at the Jet Propulsion Laboratory (JPL). The temperature stabilized cryogenic cells with path lengths of 24.29 and 20.38 cm were constructed of oxygen free high conductivity (OFHC) copper and fitted with wedged ZnSe windows using vacuum tight indium seals. In operation, the temperature-controlled cooling by a closed-cycle helium refrigerator achieved stability of ±0.01 K. The unwanted absorption features arising from cryodeposits on the cell windows at low temperatures were eliminated by building an internal vacuum shroud box around the cell which significantly minimized the growth of cryodeposits. The effects of vibrations from the closed-cycle helium refrigerator on the FTS spectra were characterized. Using this set up, several high-resolution spectra of methane isotopologues broadened with nitrogen were recorded in the 1200–1800 cm−1 spectral region at various sample temperatures between 79.5 and 296 K. Such data are needed to characterize the temperature dependence of spectral line shapes at low temperatures for remote sensing of outer planets and their moons. Initial analysis of a limited number of spectra in the region of the R(2) manifold of the ν4 fundamental band of 13CH4 indicated that an empirical power law used for the temperature dependence of the N2-broadened line widths would fail to fit the observed data in the entire temperature range from 80 to 296 K; instead, it follows a temperature-dependence similar to that reported by Mondelain et al. [17], [18]. The initial test was very successful proving that a high precision Fourier transform spectrometer with a completely evacuated optical path can be configured for spectroscopic studies at low temperatures relevant to the planetary atmospheres.

X-band traveling wave maser (Correspondence)

A traveling wave maser (TWM) has been constructed for use at 8448 MHz. It provides a net gain of 41.5 dB when installed in a closed-cycle helium refrigerator operating at 4.4°K. The maser provides more than 30 dB net gain over a tunable range of 140 MHz. An equivalent input noise temperature of approximately 18°K has been measured. The net gain of the maser (41.5 dB) has a short-term stability such that the radiometer stability (0.08 dB peak to peak) is not degraded when the maser is added to the system. The maser can be tuned over a 140 MHz range with net gain exceeding 30 dB. Cool-down time of the closed-cycle refrigerator with this maser is 4.6 hours (no liquid nitrogen precooking) and 2.4 hours with precooking.

Wide bandwidth traveling wave maser employing magnetic stagger tuning

This paper presents experimental work which shows that magnetic stagger tuning provides an efficient means for trading excess gain for bandwidth in traveling wave masers. A high gain traveling wave maser (20 dB/in. at 4·2°K) has been developed at 4000 mc and operated with a transverse field superconducting magnet as a wide-band amplifier system. The maser, which employs meander-line slowing and iron-doped rutile as the active material is operated with various sections of the paramagnetic crystal resonating at different frequencies. This is made possible by placing the maser in a superconducting magnet which generates a field profile that can be varied in steps along its length of the maser. A 4·0 in. iron-doped rutile crystal, which produces 78 dB peak electronic gain over a 12 mc instantaneous band, can be made to yield 33 dB of gain and 66 mc of bandwidth by stagger tuning. In addition to presenting experimental data which is in good agreement with the simple theory of magnetic stagger tuning, this paper describes for the first time the performance of a traveling wave maser which is magnetically stagger-tuned in a small compact superconducting magnet. This work is significant from a system standpoint since an electronically, variable gain-bandwidth microwave amplifier has been developed. Furthermore, both these devices can be operated in a closed-cycle helium refrigerator.

Operation of masers in closed-cycle helium refrigerators

Operation of masers in closed-cycle helium refrigerators

Masers for Radar Systems Applications

This paper is intended to provide the systems engineer with a practical introduction to the use of solid-state maser amplifiers in radars. Various environmental problems involved in the successful application of masers are discussed. An elementary survey of reflection-cavity and traveling-wave masers, together with some experimental results are presented. Another section treats the problem of saturation in masers and discusses some of the methods available for protecting masers from the TR leak-through pulses in radar. Progress made in the development of open-and closed-cycle liquid helium cryogenic systems suitable for masers is described. The authors' personal evaluations of the state of the art of ruby masers and closed-cycle helium refrigerators are given in appendixes.