Helium Reservoir Research Articles

Nonequilibrium thermodynamics of nonisothermal activation: Escape over mesoscopic barriers: II. The escape rate

In the preceding article (I) it is argued that in noisy “macroscopic” systems (e.g. Josephson junctions) the effective potential for the pertinent degree of freedom (e.g. the magnetic flux) generally depends on the specimen's-local-temperature and, hence, that it should be given a thermodynamical (rather than the usual mechanical) significance. The ensuing nonequilibrium thermodynamics fluctuation theory is a generalization of Kramers' model of Brownian motion. In the present article (II) the novel theory is applied to nonisothermal activation effects. The influence of a finite heat capacity and limited heat exchange (with a “superbath”. e.g. the helium reservoir) on the escape rate from a metastable state across a temperature dependent barrier is investigated. Inter alia it is shown that for a thermally isolated activation process the pertinent entropy barrier may be much higher than the usual isothermal Gibbs barrier.

NONISOTHERMAL METASTABILITY IN MESOSCOPIC SYSTEMS

Since the effective potential for noisy “macroscopic” degrees of freedom (e.g. the magnetic phase in Josephson junctions) in general depends on the (local) temperature, it is given thermodynamical significance as the Gibbs or Helmholtz free energy. Using the notions of nonequilibrium thermodynamics a generalized Kramers Fokker-Planck equation is then presented, which includes nonisothermal effects. The escape rate from a metastable state across a temperature-dependent energy barrier is investigated. A substantial entropic barrier enhancement is found for a specimen with small heat capacity and slow heat exchange with its environment (e.g. the helium reservoir).

An important source of 4He (and 3He) in diamonds

A large data base has recently accumulated on the concentrations of helium isotopes in diamonds mined from various regions. It was noted earlier (Ozima et al. (1985) [1]; Lal et al. (1989) [2]) that the frequency distribution of the 4He concentrations is a fairly narrow one, whereas that of 3He concentrations is a broad one with no pronounced peaks. The ratios 3He/ 4He , on the other hand show a broad maximum around 2 R a ( R a equals atmospheric 3He/ 4He ratio, = 1.40 × 10 −6) with a slow decrease over two orders of magnitude on either side. Does this imply that the diamonds sample a wide variety of helium reservoirs having a range of 3He/ 4He ratios but somehow attain similar 4He concentrations? We propose that in a majority of the diamonds studied, 4He is primarily due to implantation of radiogenic alpha particles from the host material after emplacement in the crust, usually kimberlite, and that the concentrations of 4He in diamonds often get appreciably altered by this process. Thus the 4He trapped in the diamond at the time of its crystallization is usually overwhelmed by the implanted helium and the measured 3He/ 4He ratios do not generally correspond to any “sources” in the mantle. However, the implanted 4He resides in the outer 16 μm of the diamond, and the intrinsic 4He and 3He/ 4He ratios in the diamond can be studied if its outer layers are removed. The wider implications of diamond being the “target” material for nuclear reaction products from the host material are discussed. Radiogenic 3He produced in the host material is also implanted in the diamond, but this contribution is small on a gross basis. However, since the depth of implantation of 3He is greater than that of 4He, some of the very high 3He/ 4He ratios observed in diamonds could be due to the “implantation” of radiogenic 3He. The radiogenic reactions in the host material can also contribute to appreciable 21Ne in diamonds.

Pressure drop and He II flow through fine mesh screens

Fluid acquisition systems for He II transfer devices will utilise gallery arms to ensure that the fluid encounters the pump inlet. In near term experiments such as Superfluid Helium on Orbit Transfer (SHOOT), the preferred configuration consists of several rectangular channels which have one side made from a Dutch weave stainless steel screen having 325 × 2300 wires per inch. The effective pore diameter for this screen is about 5 μm. The present paper reports on measurements of pressure drop across a screen when it is subjected to a flow of liquid helium. The experiment measures the time rate of change of the level in two different helium reservoirs connected by a screen-blocked channel. Results with normal helium are compared with predictions based on the Armour-Cannon equations. The He II data show considerable deviation from the classical result. A discussion of the He II pressure drop results in terms of two fluid hydrodynamics is included.

Twenty-channel grating polychromator for millimeter wave plasma emission measurements

A 20-channel grating instrument based on the Ebert–Fastie grating monochromator has been built to measure electron temperature profiles and electron temperature fluctuations in TFTR (tokamak fusion test reactor). Second-harmonic electron cyclotron emission for the plasma over the frequency range of 120–400 GHz is utilized; this corresponds to a central tokamak magnetic field of 2.9–5 T. System sensitivity is typically 10-eV rms for a 2.5-kHz bandpass with a 2.5-cm radial resolution and a 5-cm antenna spot size at the plasma center. The ratio of the largest to smallest plasma radius observed by the instrument can be as large as 1.65, which maximizes spatial coverage of the plasma. Emission is detected by 20 liquid-helium-cooled indium antimonide hot-electron bolometers mounted in a long hold cryostat. The 6.5-l helium reservoir must be filled every 19 days, while the liquid-nitrogen reservoir is filled automatically once a day. This instrument has about twice the number of channels and much better spatial resolution than other instruments of this type used for plasma diagnostics.

A regenerative magnetic refrigerator operating between liquid helium and liquid hydrogen reservoirs

A regenerative refrigerator with a GGG (Gd/sub 3/Ga/sub 5/O/sub 12/) salt core built and tested in the Cryogenics Engineering Laboratory at MIT is discussed. The core remains stationary as supercritical helium is forced through it via the action of a displacer system. At the same time, the magnetic field is varied by an AC superconducting magnet. Preliminary data obtained from this system suggested the need for modification of (1) the helium flow system, (2) the reservoir temperature control system, (3) the cycle control system, and (4) the instrumentation. A heat exchanger has been constructed which enables the heat of rejection to be deposited in a liquid-hydrogen bath. The refrigeration load is provided by an electric heater. Real-time feedback is used to synchronize the helium flow with the magnetic-field variation. The modifications are described and various schemes for producing the required helium flow without the use of moving mechanical parts are noted.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Pneumatic pulse dampener for liquid chromatography

AbstractA simple pulse dampener is described for use with reciprocating pumps. The dampener may be assembled locally form readily available components and consists mainly of a small reservoir of compressed helium which acts as a cushion to dampen the mobile phase pulsations pneumatically. Depulsing is 99% complete and is uniform through a wide pressure range. The pneumatic dampener allows the use of single‐ piston reciprocating pumps in conjunction with highly flow‐sensitive detectors at their highest sensitivity settings.

Stable 4K cryocooler using cycled adiabatic (Simon) expansion

This simple cryocooler uses a novel adaptation of the Simon expansion liquefier to provide a small reservoir of liquid helium and continuous cooling at 4.2 K. The design stresses simplicity and reliability through the absence of any orifices or moving parts at cryogenic temperatures except for a conventional Gifford-McMahon cryocooler.

Ultrahigh vacuum cryostat for spectroscopy and laser‐induced processes

AbstractAn ultrahigh vacuum cryostat consisting of a stainless steel body flanged to an analyzer cell for spectroscopy and laser‐induced processes in the entire spectral region between ca. 50 and 0.2 μm at base pressures in the low 10−10 mbar range and sample temperatures between 10 and 600 K is described for the first time. ‐ Under helium bath operation, (4 ± 1) K was measured on the sample holder and (10 ± 2) K in the sample without using thermal contact materials. The metal to metal joint between helium reservoir (stainless steel) and sample holder (OFHC copper) according to the conflat flange method proved to be tight for many baking cycles. The integral leak rate of the whole ultrahigh vacuum apparatus as well as the optical transmission are reported. The windows developed do not show irradiation effects at peak intensities of 10 MW · cm−2 per CO2 laser pulse. The cryostat is equipped with a quadrupole mass spectrometer and evaporation sources for salts and metals. The analyzer cells are compatible with the sample chamber dimensions of most commercial spectrometers. ‐ Examples of applications are presented in the fields of low temperature gassolid interaction, cryospectroscopy of stable and new metastable adsorbates, selective and efficient evaporation and desorption induced by laser infrared.

Tevatron Satellite Refrigeration Control Subsystem

Refrigeration for the Tevatron is provided by 24 satellite refrigerators distributed around the ring and a Central Helium Liquefier which acts as a reservoir of liquid helium. The 24 satellite refrigerators are designed to operate as independent entities without attendant personnel. Remote control and monitoring from the main control room of the accelerator is made possible via a high speed link which allows information exchange with the central host computer. Local control is achieved by means of 24 individual self-cycling microprocessor systems which allow automatic local operation of each refrigerator. Satellite refrigerator control hardware and software are discussed. 3 refs.

Liquid-helium stage in a scanning electron microscope

A liquid-helium stage to be inserted into a scanning electron microscope (SEM) is described. The sample chamber, which is filled with liquid helium, has a volume of about 8 × 7 × 5 cm 3. Translation of the cooled sample in x, y, and z directions is possible during electron-optical observation. Since mounting the liquid-helium stage to the SEM can be performed in less than 30 min, switching the SEM from regular operation at room temperature to the low-temperature mode and back is relatively easy. After cooling the system to 4.2 K, a 3 litre helium reservoir allows experimentation near 4.2 K for about 5 h.

Helium pumping at 4.2 K by molecular sieve 5A

Helium pumping speeds measured for the Excalibur model CVR 1106 cryosorption pump fall into two regimes depending on the feed rate. At feed rates below 6.2×10−6 Torr l s−1 cm−2, pumping speeds decrease with loading from 2.2 l s−1 cm−2 to 0.3 l s−1 cm−2. At feed rates higher than 6.2×10−6 Torr l s−1 cm−2, the speed varies periodically with loading. A typical cycle begins with pumping speeds near 1.7 l s−1 cm−2; speeds decline from this value to about 0.6 Torr l s−1 cm−2 at which time the pump goes through a rapid transition that reverts it back to its original high pumping speed. During the transition, the system reaches presures greater than 10−2 Torr, and rapid boiling occurs in the liquid helium reservoir. At the end of the transition, the system pressure decreases rapidly to the initial low operating pressure, liquid helium boil‐off declines, and the pump operates normally for a definite period. The length of each cycle depends on the feed rate.

Unwanted birefringence in low-temperature windows

A small residual birefringence producing a retardation of 0.05 rad has been measured in silica windows mounted in a liquid helium cryostat. The effect is due to a uniaxial stress on the windows induced by the pressure difference between the helium reservoir and the vacuum jacket.

Local probe in a Rayleigh-Benard experiment in liquid helium

Local probe in a Rayleigh-Benard experiment in liquid helium

A low-temperature glass cryostat for X-ray diffraction work

A simple cryostat made of glass for X-ray diffraction at low temperatures is described. The windows for the X-rays are made from Mylar films fixed to slots cut in the base of a glass helium reservoir. This glass cryostat weighs only 6 kg and specimens can be easily handled in the cryostat. A specimen chamber has also been designed so that in-situ pulse annealing can be easily carried out. Some examples of measurements with this cryostat are described.

Helium cooling units for X- and Q-band ESR spectrometers

Two cooling units which have been designed for use with ESR spectrometers are described. A main aim of the designs has been to achieve good cryogenic performance consistent with a relatively simple mode of operation. Both units rely on the continuous flow of helium for their cooling with one unit operating in the range 1.8–300 K at X band and the other covering 4.8–300 K at Q band. The X-band unit has easy top loading access and incorporates a helium reservoir of 0.13-l capacity close to the sample region. This unit relies on two helium flow paths—one to supply the reservoir and the other to draw helium from the reservoir through the sample region. This feature allows sample temperatures of 1.8 K to be obtained and the base temperature to be achieved with a total liquid helium consumption of 1.6 l/h. The temperature stability was ±0.02 K in the liquid phase and ±0.08 K in the gas phase. In the Q-band unit the sample is positioned on the end of the rotatable sample mount and conduction cooled through this mount. The sample is vacuum loaded, and for convenience of sample changing the sample vacuum is separate from the main vacuum. This unit has a helium consumption of 1.5 l/h at 10 K with an overall temperature stability of ±0.1 K.

A method for preventing pressure oscillations in tubes connecting liquid helium reservoirs to room temperature

A method for preventing pressure oscillations in tubes connecting liquid helium reservoirs to room temperature

Evaluation of a liquid-helium cryopumping system operating with an electron beam gun

A recently developed 14-kW 240 ° electron beam gun was installed in an 18-in. glass belljar pumped by a liquid-helium cryopumping system made by Air Liquide, a French supplier of cryogenic pumps. The cryogenic pump has a speed of 6000 liter/sec for air. Pumping of hydrogen and helium is accomplished by bleeding in a small amount of argon which freezes on the helium reservoir and has a gettering effect on these gases. Experiments were conducted to determine the operating characteristics of this type of system when evaporating various materials such as aluminum, copper, and tantalum. Data taken during the evaporation runs consisted of major gases evolved, time for belljar pressure to recover to 10−7 Torr range, pressure during evaporation, effectiveness of argon sublimator in reducing pressure, and pumpdown curves. It was found that using a liquid-helium cryopumping system in conjunction with an electron beam gun gives exceptional results. It would be particularly advantageous in the fabrication of MOS devices where extreme cleanliness is required.

A receiver-amplifier system with superconducting bolometer

The design of the cryostat and bolometer is shown in Fig. 1. The sensitive element of the bolometer is a ~1 000 A thick tin film of size 1 x 10 mm 2. The film is deposited at a pressure of x 10 5 torr (1 torr = 133 N m 2) on a 1 -2 /a thick mica substrate. The ends of the substrate are stuck to a brass base. Electrical contact between the tin film and the base is achieved by depositing an additional layer of tin. The bolometer base is pressed to the base of a helium reservoir by a PTFE nut through a 0.5 mm thick PTFE washer. The washer acts as a thermal filter for low frequency fluctuations in the liquid helium temperature range. 2,3

Liquid Helium Cryostat for Mössbauer Scattering Experiments

A simply constructed cryostat designed for Mössbauer γ-ray scattering experiments is described. The sample volume is part of the transfer line from a liquid helium reservoir to a mechanical pump; the sample is thus cooled by helium flow. The helium vapor exiting from the sample volume cools the transfer line and radiation shield. The temperature of the sample is controlled by the helium flow rate determined by a needle valve at the pump. The operating range is from 9 to ∼300°K. The liquid helium consumption in liters/h is 0.3 at 10°K, 0.2 at 16°K, and 0.1 at 34°K. Possible modifications to improve the operating characteristics are discussed.