Development of Cryocooler Based High Performance Cryosorption Pump

Abstract

The aim of this work is to develop high performance cryosorption (or cryoadsorption) pumps specifically for fusion applications. An actual cryopump for the above application will use the supercritical liquid helium flow through the channels embedded in the large scale cryopanels. In this case, the liquid helium requirement (both as normal and as supercritical fluids) will be large, depending on the size of the cryosorption pump. However, in a research laboratory wherein such large quantities of liquid helium are not available, an alternate arrangement of cooling the cryopanels has to be considered. One of the possible options can then be as follows. A scaled-down version of the cryopanel can be used and cooled by a two stage cryo-refrigerator system with adequate cooling power. This system is known as cryocooler based cryosorption pump. Due to the availability of a two stage GM cryocooler with a refrigeration power of ~ 1.5 W at 4.2 K in our laboratory, which can be used for the above purpose, the main objective of this work is the “development of a cryocooler based high performance cryosorption pump”. The cryopanel which is mounted on the second stage cold head of the cryocooler is not necessarily a single panel, but is usually a set of panels (stacked one over the other) and consists of mainly three components and they are: (a) the metallic panel made of copper and cooled by the cryocooler (b) the adhesive to bind the adsorbent onto the metallic panel and (c) the adsorbent (in the present case, activated carbon (AC)) which is used to adsorb the gas molecules. By this arrangement, the adsorbent gets cooled to the lowest possible temperature to enable cryopumping. To develop the cryocooler based high performance cryosorption pump, we need to select: (a) the best adsorbent (with large adsorption surface area) and adhere it on a cryopanel to evaluate its performance as a cryopump and (b) the best adhesive with high thermal conductivity, high bonding strength and ability to withstand several thermal cycles. The surface area of an adsorbent in the range of temperatures range from 4.5 K to 77 K can be arrived at by a micropore analyser ( Model: ASIQ, Quantachrome, USA) integrated with a two stage GM cryocooler (Janis: SRDK415D), with helium as adsorbate gas at 4.5 K and nitrogen as adsorbate gas at 77 K. Based on the above studies, we can choose the best activated carbon with high surface area. Next, the best adhesive which is used for binding the adsorbent onto the panel is chosen on the basis of its high thermal conductivity. The thermal conductivity of the adhesive has been measured using two dedicated thermal conductivity measurement systems namely: (a) liquid helium based Janis SuperVariTemp (SVT) cryostat and (b) two-stage GM cryocooler based experimental setup developed in our laboratory in the temperature range from 4.5 K to 300 K. In order to make comparative studies of cryosorption of different activated carbons, a standard cryopanel, such as the one used in the commercial cryopump (Make: Varian, Model Ebara SP8) has been chosen in our studies. (Henceforth, this will be designated as “Commercial cryopanel”). In other words, the physical dimensions of all the cryopanels fabricated with indigenous activated carbons are exactly the same as that of the commercial cryopanel. By this, the experimental results of pumping speeds of different indigenous AC cryopanels can be benchmarked against the commercial cryopanel and the best performing activated carbon cryopanel can be arrived at. In the following, we discuss the works carried out for the development of the cryocooler based high performance cryosorption pump. a) A specially prepared indigenously developed Knitted Carbon Cloth (KCC/IIS01) is found to have a larger surface area for adsorption compared to the other adsorbents. This adsorbent has a surface area of ~ 3000 m2/g for helium adsorption at 4.5 K, which is significantly higher than those of granular charcoals which are in the range of ~ 1600 m2/g for similar experimental conditions. This AC cloth has been used for the development of our cryosorption pump. b) A special epoxy based adhesive (SEBA/IIS01) with higher thermal conductivity, (measured using the experimental setups mentioned earlier) in the temperature range from 4.5 K to 7 K (which is generally the operating temperature range of a cryosorption pump for efficient pumping of helium gas) compared to the commercially available epoxy adhesives such as STYCAST 2850 FT and G10 Cryocomp has been developed indigenously and used. c) Using the above Knitted Carbon Cloth KCC/IIS01 and the epoxy adhesive SEBA/IIS01, cryopanel has been prepared and studied for its performance. The pumping speeds of the developed cryopanel are improved on an average by factors of 1.55 and 1.54 when compared with those of commercial panel for gases such as hydrogen (H2) and helium (He) respectively in the specific pressure range. To enhance the thermal conductivity of SEBA/IIS01, fine powders of metallic fillers (such as aluminium, silver etc.) can be added to the pure epoxy adhesive. However it is also essential that the epoxy-aluminium composite adhesive should withstand the thermal cycling from 4.5 K to 300 K during its functioning as a cryosorption pump. d) Now the thermal conductivities of epoxy-aluminium composites in the temperature range from 4.5 K to 300 K has been measured using the dedicated experimental setups for measurements of thermal conductivity (developed in-house). The measurements of thermal conductivity using the above experimental setups are based on one-dimensional Fourier heat conduction with longitudinal steady state heat flow method. Detailed experimental studies on thermal conductivity of epoxy and epoxy-aluminium composites have been carried out by the above experimental setups. e) Further the thermal conductivities of the epoxy-aluminium composites have been theoretically predicted by analytical heat conduction models. Here, the epoxy forms the base matrix and aluminium powder forms the filler. Appropriate models have been developed both for the low and high volume fractions of the filler in the epoxy base matrix. The thermal conductivity values predicted by the models match quite well with the experimentally measured values of the epoxy-aluminium composite samples. Also the developed models are able to predict the thermal conductivity values of the published data. f) By the addition of metallic (aluminium) filler particles to the epoxy adhesive, the thermal conductivity of the epoxy adhesive is increased. However, the downside of adding aluminium fine powder is the reduction of the bonding strength of the epoxy onto the cryopanel. An experimental setup has been developed to measure the strength of epoxy- aluminium composite adhesive. Based on the experimental studies, an optimum composition of the aluminium powder filler in the epoxy adhesive has been estimated as ~ 35 % by volume fraction. This epoxy-aluminium composite adhesive is designated henceforth as “EAL35”. g) A new cryopanel has been fabricated wherein the activated carbon cloth KCC/IIS01 is bonded using EAL35. The pumping speeds of the newly developed cryopanel are improved on an average by factors of 3.63 and 3.60 when compared with those of commercial panel for gases such as hydrogen and helium respectively in the pressure range from 5E-6 to 4E-5 mbar. Our studies have led to the development of a cryocooler based cryosorption pump with higher pumping speeds for gases such as H2 and He compared to the commercial cryopumps. Hence the present studies will be quite useful to the development of the appropriate cryosorption pumps for the Tokamak related applications.