A continuously operating dilution microcryostat

A dilution microcryostat based on the technology of sorption evacuation of ³He and ⁴He and condensation evacuation of a ³He + ⁴He mixture in a dilution cycle is described. A low temperature of ∼0.1 K during regeneration of sorbers is continuously maintained owing to the high specific heat of the cryostat unit, in which circulating helium condensates. The cryostat is manufactured in the form of an insert in an industrial 35-L portable Dewar flask for liquid helium. A cold sample holder is positioned in the upper part of the device, thus facilitating the access to it. The complete volume of liquid helium suffices for 6–7 days of operation including the consumption of ⁴He for preliminary cooling of the cryostat from nitrogen temperature. During this period, a low temperature of the sample holder can be maintained with an accuracy of ∼1 mK. The device operates under computer control.     

A dilution microcryostat-insert

An autonomous dilution microcryostat in which ³He circulates due to its condensation in the volume cooled by sorption evacuation of ³He from a separate bath. A specific feature of this apparatus is the position of the sample holder in its upper part, which facilitates access to it. The holder is connected to the mixer through a heat conductor made from annealed copper wire with a length of ∼0.5 m. The cryostat operates while inserted into a nitrogen-free portable (35 l) helium-filled vessel. The operating cycle of the cryostat includes the procedures of desorption, condensation, and cooling of the ³He-filled bath to 0.35–0.40 K and the mixer to 0.05–0.10 K (lasting ∼1.5–2.0 h) and also a period of maintaining the temperature below 0.1 K (12–14 h). The amount of liquid helium in the portable cryostat is sufficient for operating for 6 days. The lowest reached temperature of the holder is 0.04 K. When the power dissipated in the holder is 0.5 μW, its temperature does not increase above 0.1 K. The instrument is mainly designed for cooling sensitive radiation detectors. Original Russian Text ? V.S. Edelman, 2009, published in Pribory i Tekhnika Eksperimenta, 2009, No. 2, pp. 159–165.     

A low-temperature capacitive dilatometer

A dilatometer with a capacitive displacement sensor intended for measuring the thermal expansion of solid samples in a temperature range of 4–300 K is described. The sensor of the instrument was mounted inside of industrial insert VTI SIV with controlled circulation of liquid helium encased in portable cryostat and was successfully used jointly with a simple commercial temperature control system. The dilatometer allows studies of the thermal expansion of samples with a sensitivity of ∼1.4 ? in consecutive cooling and warming cycles with a rate of ∼10⁻³ K/min. The results of measuring the thermal expansion of a CoS₂ sample near the phase-transition point are presented.     

A helium cryostat with pumping of 3He vapors for optical investigations

The construction of a helium cryostat with pumping of ³He vapors designed for optical measurements with a high spatial resolution in the temperature range 0.45–4.20 K. The cryostat is equipped with four windows made from fused silica. A studied sample is mounted inside a reservoir filled with liquid ³He in a holder minimizing the influence of both vibrations and thermal drifts and can stay in the chamber at T = 0.45 K for >20 h. The cryostat was used to study photoluminescence of GaAs/AlGaAs semiconductor heterostructures. It was revealed that, in a structure with two tunnel-coupled GaAs quantum wells with a width of 120 ?, the threshold pumping power required for the appearance of a narrow spectral line, which corresponds to the Bose condensate of spatially indirect dipolar excitons, decreases by a factor of 6, as the temperature falls from 1.50 to 0.45 K. In a sample with a single wide (250 ?) GaAs quantum well, the distribution patterns of the luminescence of dipolar excitons inside a 5-μm circular potential trap were obtained at T = 0.45 K. The spatial resolution of the distributions is no worse than 1.5 μm.     

An Autonomous Sorption Refrigerator for Cooling to 0.3 K

A two-stage refrigerator with sorption evacuation of ⁴He and ³He is described. The apparatus needs no external pumps or gas lines. The refrigerator is constructed in the form of an insert 80 mm in diameter immersed in a helium cryostat and ensures a sample temperature of 0.31 K for 20 h after the condensation of ³He at a useful heat load of 10 W. The recycling time is 0.5 h.     

A Wideband spectrometer for NMR studies

The design of a wideband decimeter-wave (200–900 MHz) spectrometer with a magnetic induction of up to ∼10 T is described. This spectrometer is intended for studying electronic-nuclear oscillations in antiferromagnets at low temperatures (4.2−1.3 K). Critical field H _c = 2.5 ± 0.3 T of a reorientation transition in a noncollinear antiferromagnet Mn₃Al₂Ge₃O₁₂ at temperature T ≈ 1.3 K was determined from a ⁵⁵Mn²⁺ NMR spectrum.     

A closed-cycle cryostat for optical and M?ssbauer spectroscopy in the temperature range 4.2–300 K

A closed cycle cryostat intended for optical and M?ssbauer investigations in the temperature range 4.2–300 K is described. Samples are cooled owing to the operation of a two-stage closed-cycle cryogenic refrigerator (CR) based on the Gifford-MacMahon cycle (with a refrigerating capacity of 1 W at T = 4.2 K at the second stage). The minimum temperature reached at the second stage of the CR was 2.7 K. The methods for reducing vibrations transferred from the CR to a sample are described in detail. The design proposed makes it possible to reduce the vibrational broadening of the resonance M?ssbauer lines to a level of 10–15% at 5 K, thus exceeding the level of the best cryostat prototypes in the world.     

Measurements of the temperature dependence of optical characteristics of materials in vacuum

A diagnostic module for the experimental analysis of the temperature dependence of spectral reflection coefficients for materials with the use of probing coherent radiation of a laser system with a parametric frequency oscillator (λ ∼ 213–1188 nm) and laser heating of the studied solid-state sample (up to temperatures T ∼ 2500 K) in vacuum is described. The results of the experimental determination of the spectral reflection coefficients are entered into the created partition of the experimental and design-theoretical data base of thermophysical, optical, and transporting properties of structural materials of photon energo-motion and processing high-power-density plants.     

Frictional Voltammetry with Copper

This paper presents an experimental study correlating frictional behavior with in situ voltammetry for a unidirectional sliding contact between a hemispherical tipped alumina probe and a flat rotating copper counterface (maximum Hertzian contact pressure of 68 MPa and sliding speed of 10 mm/s). The contact was immersed in an aqueous 0.1 M Na₂CO₃ solution (pH ∼11) where the copper counterface acted as the working electrode in a potentiostat controlled three-electrode cell; a coiled Pt wire was used as the counter electrode and a saturated calomel electrode (SCE) as the reference. Clear and reproducible trends were found between friction coefficient and published data suggesting the onset of particular redox reactions, graphically presented in a frictional voltammetry plot. At anodic potentials primarily associated with the formation of copper(I) oxide (Cu₂O) (V vs SCE ∼−0.25), the measured friction coefficient was in the range μ ∼0.4–0.5. At cathodic potentials primarily associated with the formation of CuO, Cu(OH)₂, and CuCO₃ (V vs SCE ∼−0.10), the friction coefficient transitions to the range μ ∼0.7–1.0. At sustained cathodic potentials associated with reduction of the native copper oxide, Cu₂O, (V vs SCE ∼−0.65), the friction coefficient is observed to fluctuate between μ ∼0.2 and 0.5, arguably a result of exposure of bare copper due to non-uniform reduction (fractional coverage) of Cu₂O.     

On the DMT theory

The frequent claim that the Tabor parameter μ governs the transition from the DMT theory to the JKR theory is investigated. The change from the simple surface force law σ ∼ A/h ³ of the DMT theory to the Lennard–Jones law σ ∼ A/h ³–B/h ⁹ of the MDT theory and the numerical solutions is noted, and the ‘adhesive force’ is evaluated for both laws. Except in the limit of zero Tabor parameter, when the Derjaguin theories reduce to the rigid-sphere model, the predictions are consistently worse than assuming the sphere to be rigid. A ‘semi-rigid’ sphere model is proposed, which correctly describes the asymptotic behaviour as μ → 0, but leaves a considerable gap before the JKR theory can be applied.     

A source of broad homogeneous beams of low-energy (∼0.5 keV) gas ions

A source of gas ions (argon, oxygen, nitrogen, etc.), the operating principle of which is based on the use of a glow discharge in an electrode system of a wide-aperture hollow cathode and anode in a magnetic field, is described. The exit aperture diameter of the hollow cathode, increased up to a size close to the ion beam diameter (10 cm), ensures the uniform ion emission of the plasma generated in the discharge region near the anode. A decreased angular divergence or increased ultimate ion-beam current density is achieved by a change in the potential drop in the space charge sheath between the plasma and the ion optics. The source generates broad (50 cm²) slightly diverging (ω/2∼3°–5°) ion beams with energies of 300–1000 eV at a beam current density of ∼0.5 mA/cm².     

A two-frequency pulse refractometer for measuring the electron density in tokamaks

An instrument for fringe-jumps free measurements of the electron concentration and estimations of the peaking factor of density distributions in magnetic confinement plasma devices with probing at frequencies close to the cutoff is described. The instrument is intended to perform measurements in a 5 × 10¹⁸–10²⁰ m⁻³ density range (while probing by the extraordinary wave) with time resolution ∼10 μs.     

Multichannel wire gas electron multipliers with 1- and 3-mm gaps

The operation of multichannel wire gas electron multipliers (MWGEMs) with gaps between electrodes δ = 1 and 3 mm, when the chamber is filled with commercial neon under a 0.4- and 1.0-atm (abs.) pressure and irradiated with α and β particles, is studied. The following maximal proportional electron multiplication coefficients are obtained: 6 × 10³ (α, irradiation, δ = 3 mm, 1 atm, and 20% streamers), 1.2 × 10⁴ (β⁻, 3 mm, 1 atm, and 50% streamers), 6 × 10³ (α, 3 mm, 0.4 atm, and 20% streamers), and 10⁵ (β⁻, 3 mm, 0.4 atm, and 50% streamers). The maximal proportional electron multiplication coefficients are obtained in the MWGEM and its anode (induction) gap in the sequential electron multiplication mode: 1.08 × 10⁵ (β⁻, 1 mm, 0.4 atm, 50% streamers), 2 × 10⁶ (β⁻, 3 mm, 0.4 atm, 20% streamers), and 1.12 × 10⁵ (α, 3 mm, 0.4 atm, 50% streamers).     

A subnanosecond pulsed source of electrons and X-rays

We measured the time and dose characteristics of electron and X-radiation of the иMA3-150З tube connected to the subnanosecond megavolt CпиH-2 accelerator. About ∼5×10¹² electrons per pulse are generated when the accelerating-voltage pulse is ≥600 kV high and ≈=0.3 ns long. The current amplitude reaches ∼5 kA, and the radiation dose is ∼5 kGy/pulse. The X-ray dose from the external tantalum target is 0.15 Gy/pulse. The development of the electron-tube and X-ray-tube prototypes with considerably smaller sizes has opened up new fields of application in medicine and engineering.     

Application of a monopulsed repetitively pumped Nd:YAlO3 laser for photothermoplastic recording of stroboholographic interferograms

A version of the stroboholographic interferometry method with a photothermoplastic recording medium is described. Its distinguishing feature is the use of a repetitively operating monopulsed laser as an illumination source. The performance characteristics of the developed highly coherent frequency-doubled Nd:YAlO₃ laser with combined modulation of the cavityQ-factor by an acoustooptical shutter and a bleachable filter are presented. A system for synchronization of the monopulses with the required phases of the periodical process under study is described. It is shown that, when using the ΦTпH-Л photothermoplastic medium, recording of double-exposure stroboholographic interferograms of diffusely reflecting objects ∼40×90 mm in size is ensured by 10–16 laser pulses at an output energy of ∼4×10⁻⁴ J and a repetition rate of no more than 10 Hz. Interferograms of larger objects can be obtained by increasing the pulse energy and the laser firing rate. Deceased.     

Ultimate characteristics of an X-ray prism spectrometer

X-ray prism spectrometry schemes for experimental investigations of fast processes are considered. Diamond and beryllium prism dispersion parameters and special features of transmission spectra for crystal prisms are analyzed. It is shown that relative energy resolution E/ΔE at photon energies E ∼ 10 keV may reach 10³–10⁴ and the total working spectral band is ∼100 keV. This opens unique possibilities for measuring fine structures of single-shot absorption spectra for quasi-parallel beams and continuous monitoring of the fundamental and high-frequency harmonics of an X-ray free electron laser. Original Russian Text ? A.G. Tur’yanskii, 2009, published in Pribory i Tekhnika Eksperimenta, 2009, No. 4, pp. 150–158.     

Temperature dependence of friction under cryogenic conditions in vacuum

We have constructed a sliding block cryotribometer designed to measure friction at cryogenic temperatures and UHV conditions. An optical cryostat and high speed video was used to monitor the motion of the block. The temperature range of the measurements was 4–460 K. Both the static and dynamic friction coefficients were determined for a steel ball on stainless steel, sapphire, and PTFE (Teflon) plates, and for a sapphire ball on a sapphire plate. For all material combinations, no systematic variation of friction with temperature was observed in the temperature range 4–200 K.     

A detector for imaging of explosions on a synchrotron radiation beam

Synchrotron radiation (SR) offers a unique chance to study the structure of a substance in fast processes. Since SR is emitted by electron bunches in a storage ring, the SR burst corresponding to a single bunch may be very short. Should a detector capable of detecting SR from a single bunch without mixing signals from different bunches be available, it is possible to obtain information on changes in the state of the material in a sample under investigation with a very high time resolution. A detector for imaging of explosions on an SR beam—DIMEX—has been developed by the Budker Institute of Nuclear Physics (Siberian Division of the Russian Academy of Sciences, Novosibirsk). This detector is a high-pressure ion-ization chamber with a strip readout at a pitch of 0.1 mm. The electron component of primary ionization is collected within 50 ns, which is substantially shorter than the orbital period of a bunch in the VEPP-3 storage ring (250 ns). The DIMEX is filled with a Xe—CO₂ mixture (3: 1) at an absolute pressure of 7 atm. The spatial resolution of the detector is ∼210 μm, and its efficiency for radiation with an energy of 20 keV is ≥50%. The dynamic range of the detector is ∼100, which allows one to measure the signal with an accuracy of ∼1%. In this case, the maximum flux of X-ray photons, at which the DIMEX operates in a linear region, is ∼10¹⁰ photons/(channel s). Today, the detector has been used in experiments aimed at studying evolution of the density in detonation waves and processes of nanoparticle production at the VEPP-3 storage ring by employing the small-angle X-ray scattering technique.     

A calorimetric spectrometer measuring single pulses of relativistic microwave generators

A calorimetric spectrometer measuring individual pulses radiated by wideband relativistic microwave oscillators is described. The calorimetric spectrometer comprises two calorimeters and a set of low-pass filters. In the development of such a spectrometer, the basic feature consists in the high radiation power (∼10⁸ W) and low pulse energy (∼1 J). To prevent the microwave surface discharge, the calorimeters have a rather large area (∼0.1 m²). The calorimeters’ sensitivity is 0.05 J. Frequency responses of the filters were measured experimentally and calculated with the help of a three-dimensional version of the KARAT computer code. The experimental spectrum of a wideband relativistic microwave plasma oscillator measured in a frequency range of 5–40 GHz is presented.     

A three-axes-compensated inductive magnetometer for measurements in high pulsed magnetic fields

A high-sensititity magnetometer intended for studying the magnetic properties of materials in pulsed magnetic fields of up to 43 T is described. A distinctive feature of the magnetometer is its method for compensating pickups generated in the magnetometer sensing element by a pulsed field. The commonly used uniaxial high-order compensation is replaced by a triaxial compensation of the lowest (quadrupole) order, combined with a transverse arrangement of the main probe coils. This method has allowed us to achieve a compensation level of the signal in the working coil (with an inner diameter of 1.4 or 1.75 mm) of the magnetometer without a sample of up to 10⁻⁶ for the longitudinal and 10⁻⁴ for the transverse field components. The magnetic moment sensitivity of the magnetometer is 10⁻⁴ mA·m² in fields below 10 T and 10⁻³ mA·m² at a field pulse amplitude of 35 T. A deviation of the compensation is below 2×10⁻⁴ for a temperature increase from 77 to 300 K and below 5×10⁻⁵ after sample replacement. The sample temperature is controlled by a fast-acting temperature control system in a range of 6–300 K to an accuracy of 0.3–0.05 K.