Design chart of high temperature superconducting gas cooled current leads

Abstract

General design equations derived from one dimensional energy balance equations are solved to evaluate generalized characteristics of gas cooled current leads which are made of high temperature superconducting (HTSC) material. Design parameters of the conductor and the cooling channel are combined into a single general cooling parameter with the consideration of laminar flow. Generalized characteristics of HTSC current leads, namely the heat fiow from the cold end of the leads, the geometrical dimension of the conductor and the pressure drop of the cooling gas are represented in a design chart with the general cooling parameter. Thermal runaway of the conductor is analyzed for various geometrical dimensions in the case of stoppage of the cooling gas with the consideration of the quench propagation. The temperature rise time works as a criterion for determining the g e e metrical dimension. By determining the geometrical dimension according to the criterion, the heat flow from the cold end is estimated to be less than 0.lmWIA I. hTRODUCFIONSuperconducting magnets are operated by supplying the electric current through cufTent leads fiom the power supply at the room temperature. Usual cryogenic systems for superconducting magnets employ conventional copper current leads, which are made of copper material. Conventional current leads are cooled by cold helium gas and optimized to minimize the heat leak into the low temperature region with the consumption of cooling gas flow as small as possible. The usual design procedure requires iterative and complicated calculation to evaluate such operational characteristics as the heat le& as a function of the geometry of the conductor, the pressure drop of the cooling gas and the temperature rise time of the conductor during the thermal runaway[l],[2]. To simplify the design procedure, we developed a generalized design method. In this method, the characteristics of leads, such as the heat leak, the geometrical dimension of the conductor and the pressure drop of the cooling gas, were obtained in the generalized form and summarized in a design chart [3]. Sevea-al current leads were designed by the use of the design chart and exhibited good operational performance[4], [5]. Even in the case of an ideal heat exchange, optimum copper leads has a lower limit of the heat leak of -1 .O mW/A[ 11.Because of the generation of no Joule heating and very low thermal conductivity, high temperature superconducting (HTSC) material has a possibility of the application to current leads which reduce the lower limit of the heat leak of the Manuscript received October 17, 1994. This work was supported in part bv Inoue Foundation for Science. NOMENCLATURE C Specific heat of conductor (J w1 R I ) C Specific heat of helium gas (J k') d8 Hydraulic diameter i. e. d = 4sdf(m) f Cooling perimeter (m) F f m d z e d by current ( d A ) G m per unit area in cooling channel (kg m-2 8") Z Current (A) 1 Conductor length effective for heat exchange (m) m Mass flow rate of cooling gas @g i ' ) M m normalized by curren...

Cryocooler assisted cryostat for a 4 T split-coil NbTi magnet

“…Conduction losses for HTS current leads have been obtained neglecting heat exchange with helium gas evaporating from the bath; a more detailed consumption computation needs to consider this heat exchange [4], [5].…”