Mixing Chamber Temperature Research Articles

Dry dilution refrigerator with pulse tube shutoff option

For experiments in the sub-Kelvin temperature regime with high demands for an extremely low level of vibration, a pulse tube precooled dilution refrigerator (DR) is described where the (vibration afflicted) pulse tube cryocooler (PTC) can be shut off temporarily whereas the dilution unit continues to operate at a mixing chamber temperature of Tmc < 6 mK during the entire shutoff time. Shutoff periods of 7 h were achieved with the present cryostat.Besides the dilution circuit, there is a 4He cooling circuit (1 K loop) in the cryostat; it cools and liquefies the 3He flow of the DR during normal operation and during PTC shutoffs. The 4He circuit has to be operated in single shot mode during PTC shutoffs as the two stages of the PTC warm up considerably and can no longer provide adequate precooling of the backstreaming helium gas. In its vessel enough liquid helium can be collected before a shutoff to liquefy the 3He flow of the DR in a heat exchanger for over 10 h. The temperature of the vessel is between 1.0 K and 1.3 K.After 7 h, the PTC was re-started. From here, it took about 2 to 2.5 h until the second stage of the PTC was cold enough so that the 4He circuit could be changed back from single shot mode to continuous operation. During the entire time, from the shutoff of the PTC to the resuming of the continuous operation of the 1 K loop, the DR ran undisturbed close to its base temperature.In this paper the differences between a DR with 1 K loop and commercial so-called “battery type DRs” are described.

Design and analysis of an innovative concentrated solar power system using cascade organic Rankine cycle and two-tank water/steam storage

Direct steam generation (DSG) solar power systems have the potential to improve heat collection performance and reduce capital cost. One challenge of the DSG solar thermal power technology is the unsteadiness of steam generation and power conversion under fluctuating solar radiation. A novel concentrated solar power generation system is proposed. It has three features: two-phase water/steam as heat transfer fluid, two-tank water storage, and cascade organic Rankine cycle (CORC) with a mixing chamber as power block. Steam is produced in the solar field and condensed in a high temperature tank, while an organic fluid replaces water for power conversion. The system enables smooth cycle operation by resilient control strategy and can tackle the challenge associated with wet steam turbines. It can tolerate lower purity of water/steam that only serves as the heat transfer fluid, thereby reducing the technical requirement. Thermodynamic performance in the normal operation condition and heat discharge process are assessed. The influences of ORC working fluid and storage tank size are examined. Results indicate that the mixing chamber temperature plays a crucial role in thermal efficiencies of both charge and discharge processes, storage capacity, and overall performance. A CORC efficiency of about 27.4% is achievable. The equivalent heat-to-power efficiency ranges from 13.35% to 18.81%, depending on the ORC fluid and volume of the storage tank. The novel system has an efficiency comparable to a conventional DSG system while a lower technical requirement in heat collection and power generation.

A 10 mK hermetic cell for eliminating parasitic heating in cryogen-free dilution refrigerators

AbstractCryogen-free dilution refrigerators generally simplify low temperature research but some types of samples, including superconducting qubits and other nanoelectronic devices, are affected by environmental heat sources such as stray photons or residual helium. We present the design and performance of a hermetic cell installed on the mixing chamber plate of a cryogen-free dilution refrigerator. The performance was quantified by measuring the dependence of the resonance frequency of a mechanical oscillator installed inside the cell on the mixing chamber temperature down to 10 mK. We found the expected logarithmic temperature dependence of the resonance frequency down to the lowest temperatures, demonstrating that the efficiency of the hermetic shield is significantly better than that of a simpler shield with no visible gaps.

Development of a thermodynamic model for a cold cycle 3He-4He dilution refrigerator

A thermodynamic model of a 3He-4He cold cycle dilution refrigerator with no actively-driven mechanical components is developed and investigated. The refrigerator employs a reversible superfluid magnetic pump, passive check valves, a phase separation chamber, and a series of recuperative heat exchangers to continuously circulate 3He-4He and maintain a 3He concentration gradient across the mixing chamber. The model predicts cooling power and mixing chamber temperature for a range of design and operating parameters, allowing an evaluation of feasibility for potential 3He-4He cold cycle dilution refrigerator prototype designs. Model simulations for a prototype refrigerator design are presented.

Cryogen-free dilution refrigerators

We review briefly our first cryogen-free dilution refrigerator (CF-DR) which was precooled by a GM cryocooler. We then show how today's dry DRs with pulse tube precooling have developed. A few examples of commercial DRs are explained and noteworthy features pointed out. Thereby we describe the general advantages of cryogen-free DRs, but also show where improvements are still desirable.At present, our dry DR has a base temperature of 10 mK and a cooling capacity of 700 μW at a mixing chamber temperature of 100 mK. In our cryostat, in most recent work, an additional refrigeration loop was added to the dilution circuit. This 4He circuit has a lowest temperature of about 1 K and a refrigeration capacity of up to 100 mW at temperatures slightly above 1 K; the dilution circuit and the 4He circuit can be run separately or together. The purpose of this additional loop is to increase the cooling capacity for experiments where the cooling power of the still of the DR is not sufficient to cool cold amplifiers and cables, e.g. in studies on superconducting quantum circuits or astrophysical applications.

Dissipative cryogenic filters with zero dc resistance

The authors designed, implemented, and tested cryogenic rf filters with zero dc resistance, based on wires with a superconducting core inside a resistive sheath. The superconducting core allows low frequency currents to pass with negligible dissipation. Signals above the cutoff frequency are dissipated in the resistive part due to their small skin depth. The filters consist of twisted wire pairs shielded with copper tape. Above approximately 1 GHz, the attenuation is exponential in omega, as typical for skin depth based rf filters. By using additional capacitors of 10 nF per line, an attenuation of at least 45 dB above 10 MHz can be obtained. Thus, one single filter stage kept at mixing chamber temperature in a dilution refrigerator is sufficient to attenuate room temperature black body radiation to levels corresponding to 10 mK above about 10 MHz.

Crystallinity of Polypropylene-Silica Ash Composites Affected by the Mixing Conditions – DSC Studies

For particulate polymeric composites, mixing is a crucial step to be optimised; and in the process-control stage, identification of the factors that influence mixing is important for a deeper understanding of the composite mechanical performance. A mixing study of a hydrophobic polymer matrix (polypropylene) filled with a hydrophilic particulate-filler (silica from rice husk ash) was carried out using a batch mixer at a constant filler fraction of 20% (w/w). The study involved varying the mixing-time, screw speed and mixing-chamber temperature used to prepare the composites. Mechanical analysis of the samples showed that the tensile strength and modulus values were dependent on the mixing conditions. Furthermore, DSC studies of the samples revealed that the degree of crystallinity was also affected by the mixing conditions. The observed increase in the tensile strength was attributed to the increased filler-matrix interactions; however, there was difficulty in analysing how the mixing conditions influenced the tensile strength because of a lack of extensive data on filler dispersion. The increase in crystallinity, as affected by mixing conditions, was thought to improve the filler-matrix interaction leading to an increased tensile modulus. Interestingly, samples showed permanent morphological changes after their previous thermal histories were erased, suggesting that there was significant interaction between the silica ash particles and the polymer matrix, even though they are quite incompatible. A statistical analysis based on the tensile data was carried out to optimise the state of mixing. The results indicate that (i) the optimised state of mixing correlated with higher crystallinity and (ii) changes in the parameters of physical mixing might significantly affect the homogeneity and crystallinity, both of which are related to the mechanical performance of the composites. Furthermore, reducing the particle size of the silica ash was also found to increase the crystallinity, which was in turn related to the improved tensile properties of the composites. The investigation attempts to highlight that for apparent incompatible system, homogeneity is very important but it alone cannot explain the moderate filler-matrix interactions and filler bonding characteristics that are known to contribute to improved tensile properties. Somehow, composites mechanical properties are improved by optimising the physical mix-state, and by modifying the particle size but it seems like that the net effect is due to increased interactions. In any case, it is clear that optimisation of interactions can be achieved by obtaining a homogeneous phase.

Characterization of the Behavior and Blending Performance of a Continuous Mixer

Abstract An experimental study is presented of the residence time distribution, the melting/fusion process, power consumption, melt temperature and blending in a Kobe Steel Nex-T continuous mixer. Attention is given both to phenomena occurring in the mixing chamber and the associated bottom extruder. The variables considered include throughput rate, rotor speed, exit orifice position and mixing chamber temperature.

Electron temperature and thermal conductance of GaAs 2D electron gas samples below 0.5 K

The electron phase coherence length is used as a thermometer to measure the temperature of a 2DEG. We find that the electron temperature differs from the mixing chamber temperature of the refrigerator by as much as 100 mK at base temperature. We also measure the thermal resistance between the electron gas and the mixing chamber and find a resistance comparable to that predicted by the Wiedemann-Franz law for the electron gas alone. Our results indicate an extraneous power level of about 4 pW in our experimental configuration.

3He/ 4He dilution refrigerator combined with Gifford-McMahon cooler

A 3He/ 4He dilution refrigerator is described where a commercial two-stage Gifford-McMahon refrigerator (Leybold RGD 210) cools the dilution unit and the radiation shields of the cryostat. This work is a continuation of previously described experiments where a more powerful closed cycle refrigerator had been used for the same dilution cycle; in comparison with those experiments the construction of the new apparatus has now been simplified and improved, and the mixing chamber temperature lowered.

Efficient and simple step heat exchangers

Abstract Efficient step (discrete) heat exchangers have been constructed. These heat exchangers approach the performance of perfect step exchangers having infinite surface area in that two step exchangers yielded a mixing chamber temperature, T m , of 11.6 mK as compared to the theoretical prediction of 6 mK. Four step exchangers gave T m of 6.6 mK. These heat exchangers are simple to fabricate, are inexpensive in material and labor costs, are reliable and perform well, and have useful thermal grounding features.

Temperature Controller for Dilution Refrigerators

An electronic analog device for automatically controlling the mixing chamber temperature of a dilution refrigerator is described. A self-inductance magnetic thermometer is used as the temperature sensing element. Using this device we were able to hold the mixing chamber temperature changes to less than 0.1 mK for periods of up to 12 h in the temperature range between 0.4 and 0.1 K.

The Design of Optimum Heat Exchangers for Dilution Refrigerators

The behavior of discrete heat exchangers as a function of temperature, heat transfer area, and 3He flow rate is shown for a general case by means of some machine calculated curves. From these curves, the appropriate heat exchanger size can be determined for the various steps of a discrete heat exchanger. Also, curves for the mixing chamber temperature Tm as a function of total surface area divided by flow rate for various amounts of external heat to the mixer are shown for the continuous exchanger. From these curves the optimum 3He flow rate for a given heat input and exchanger size can be determined. Viscous heating has been included in the calculation of one typical coaxial tube exchanger. It is found that heating in the dilute stream has negligible effect on Tm, whereas the viscous heating in the concentrated side, though several times smaller than that in the dilute side, has considerably more effect on Tm. Based on this behavior, an improved method of ``heat sinking'' heat leak paths to the mixer is suggested.