Pavel Hanzelka

Cryogenic apparatus for study of near-field heat transfer

For bodies spaced in vacuum at distances shorter than the wavelength of the thermal radiation, radiative heat transfer substantially increases due to the contribution of evanescent electromagnetic waves. Experimental data on heat transfer in near-field regime are scarce. We have designed a cryogenic apparatus for the study of heat transfer over microscopic distances between metallic and non-metallic surfaces. Using a mechanical positioning system, a planeparallel gap between the samples, concentric disks, each 35 mm in diameter, is set and varied from 10(0) to 10(3) μm. The heat transferred from the hot (10 - 100 K) to the cold sample (∼5 K) sinks into a liquid helium bath through a thermal resistor, serving as a heat flux meter. Transferred heat power within ∼2 nW∕cm(2) and ∼30 μW∕cm(2) is derived from the temperature drop along the thermal resistor. For tungsten samples, the distance of the near-field effect onset was inversely proportional to temperature and the heat power increase was observed up to three orders of magnitude greater than the power of far-field radiative heat transfer.

Helium cryostat for experimental study of natural turbulent convection

Published experiments on natural turbulent convection in cryogenic (4)He gas show contradictory results in the values of Rayleigh number (Ra) higher than 10(11). This paper describes a new helium cryostat with a cylindrical cell designed for the study of the dependence of the Nusselt number (Nu) on the Rayleigh number (up to Ra approximately 10(15)) in order to help resolve the existing controversy among published experimental results. The main part of the cryostat is a cylindrical convection cell of 300 mm in diameter and up to 300 mm in height. The cell is designed for measurement of heat transfer by natural convection at pressures ranging from 100 Pa to 250 kPa and at temperatures between 4.2 and 12 K. Parasitic heat fluxes into the convection medium are minimized by using thin sidewalls of the bottom and top parts of the cell. The exchangeable central part of the cell enables one to modify the cell geometry.

Method for measurement of emissivity and absorptivity of highly reflective surfaces from 20 K to room temperatures

We present a cryogenic method for the measurement of total hemispherical emissivity and absorptivity of various materials at temperatures from 320 K down to ≈20 K. In absorptivity measurement the temperature of the examined sample is kept at ≈5 K–35 K. Radiative heat flow between two plane parallel surfaces of 40 mm in diameter disk samples placed in a vacuum, a sample and a disk with reference surface, is absorbed by a colder sample and sinks into an LHe bath via a thermal resistor (heat flow meter). Heat flow is measured by substitution method, using thermal output of an electrical heater for heat flow meter calibration. A great deal of attention is paid to the estimation of uncertainties associated with this method. Capabilities of the instrument are demonstrated by the absorptivity and emissivity measurement of the pure aluminium sample. The expanded fractional uncertainty (k = 2) in emissivity e = 0.0041 measured at ≈30 K for pure aluminium is less than 11% and for values of emissivity e > 0.0053 measured above 60 K the uncertainties are below 7%. The method was designed primarily for the measurement of highly reflective materials like pure metals, nevertheless high emissivity of the reference sample also enables the measurement of non-metallic materials with reasonable accuracy.

Anomalous heat transport and condensation in convection of cryogenic helium

When a hot body A is thermally connected to a cold body B, the textbook knowledge is that heat flows from A to B. Here, we describe the opposite case in which heat flows from a colder but constantly heated body B to a hotter but constantly cooled body A through a two-phase liquid–vapor system. Specifically, we provide experimental evidence that heat flows through liquid and vapor phases of cryogenic helium from the constantly heated, but cooler, bottom plate of a Rayleigh–Bénard convection cell to its hotter, but constantly cooled, top plate. The bottom plate is heated uniformly, and the top plate is cooled by heat exchange with liquid helium maintained at 4.2 K. Additionally, for certain experimental conditions, a rain of helium droplets is detected by small sensors placed in the cell at about one-half of its height.

Scanning vector Hall probe microscope

We present a scanning vector Hall probe microscope for imaging the entire magnetic field vector in close proximity to magnetic and superconducting samples. The microscope combines a large scanned area and a high space resolution of the magnetic field vector measured. A special feature of the equipment is a vacuum-tight sample space connected with a moving system via a flexible metal bellows. The microscope is based on a vector Hall sensor that consists of three separate Hall probes of an active area 5×5 μm2, patterned on three sides of a GaAs pyramid. The top of the pyramid serves as a tunneling contact and helps to control the sensor–sample separation. The sensor and the sample are placed in a helium cryostat with a temperature control in the range 10–300 K. The sensor scans an area up to 5×5 mm2 in the whole temperature interval with a spatial resolution ∼5 μm.

Current leads in vacuum space of cryogenic systems

Electric current leads placed in the vacuum space are often used in cryogenic systems. A complex analysis of the thermal processes in the leads is a difficult problem. Simplifying conditions were determined and a numerical procedure was written for the evaluation of the main heat flows and temperature distribution. The procedure was tested using other available methods and was also verified in several experiments. A satisfactory agreement between the calculated and measured data was found. Importance of the heat radiation in certain lead configurations was found. Some problems arose as regards the analysis of a copper conductor in a special configuration. The solution of several factual problems concerning the current leads design is described and the results are discussed.

Influence of changes in atmospheric pressure on evaporation rates of low-loss helium cryostats

Abstract Experimental equipment for pressure control and evaporation rate measurement of a low-loss helium cryostat is described. The dependence of the helium gas outflow on the linearly varying pressure is presented for an NMR cryostat. The variations in measured helium evaporation rate were significantly high during experiments simulating usual atmospheric pressure changes. The measured quantities are compared to those evaluated by applying a simple theoretical model based on the assumption that the liquid and gas in the helium vessel are in thermodynamic equilibrium.

Small helium bath cryopump for electron optical devices

Abstract A small helium bath cryopump for electron optical devices has been designed and manufactured. The filling volumes of LHe and LN2 are 2.5 and 3.6 l, respectively. Special electron beam welding methods were utilised for the pump structure. The heat loads of the cryogens were minimised using numerical methods. An LHe refill interval of 30 days was reached, whereas that of LN2 is 6 days. Good agreement between the calculated and measured values has been found. An ultimate pressure of 3×10 −7 Pa and a pumping speed better than that of a comparable ion pump were reached during a preliminary testing of pumping properties.

Problems of measurement of the helium boil off rate of tomographic magnets

The problems connected with the evaluation of the helium boil off of a superconducting MRI magnet are discussed. The influence of changes in atmospheric pressure on the evaporation rate of a 1.5 T whole body tomograph magnet was investigated in experiments. The necessity of application of a pressure stabilizer for the boil off measurement was demonstrated. At the continuously stabilized helium bath pressure, the basic evaporation rate and also the boil off increase due to the eddy currents produced during the scanning procedure were measured.

Low conductive support for thermal insulation of a sample holder of a variable temperature scanning tunneling microscope

We have designed a supporting system to fix a sample holder of a scanning tunneling microscope in an UHV chamber at room temperature. The microscope will operate down to a temperature of 20 K. Low thermal conductance, high mechanical stiffness, and small dimensions are the main features of the supporting system. Three sets of four glass balls placed in vertices of a tetrahedron are used for thermal insulation based on small contact areas between the glass balls. We have analyzed the thermal conductivity of the contacts between the balls mutually and between a ball and a metallic plate while the results have been applied to the entire support. The calculation based on a simple model of the setup has been verified with some experimental measurements. In comparison with other feasible supporting structures, the designed support has the lowest thermal conductance.