4C modeling of the supercritical helium loop HELIOS in isobaric configuration

Highlights

• Isobaric JT-60SA/HELIOS scenario modeled for the first time, using 4C.

• 4C results in very good agreement with experimental data.

• Pros and cons of isobaric vs. isochoric configuration discussed.

• Improved layout of charging and discharging lines proposed, using 4C.

Abstract

Superconducting magnets for tokamak fusion reactors are subject to highly variable loads due to the intrinsically pulsed operation of the machine. The refrigerator should, on the contrary, work under conditions as stable as possible and should be sized based on the averaged loads. In this paper, the cryogenic circuit module of the 4C (Cryogenic Circuit Conductor and Coil) code is used to analyze the isobaric configuration of the HELIOS (HElium Loop for hIgh lOads Smoothing) facility at CEA Grenoble, France. The 4C model is validated against experimental data from the isobaric HELIOS configuration: the computed evolution of temperature, pressure and mass flow rate at different circuit locations shows a good agreement with the measurements for two different pulsed heat load scenarios, with and without regulation. The advantages of the (somewhat more complicated) isobaric configuration vs. the isochoric configuration in terms of heat load smoothing are highlighted. 4C simulations are used to propose a new HELIOS layout with helium charging and discharging lines located in such a way that the HELIOS performance in the isobaric configuration is optimized. The paper confirms the importance of the isobaric configuration for a tokamak cryogenic circuit, as well as the accuracy of 4C in modeling the HELIOS operation, even in the presence of complex regulation.

Introduction

Future fusion reactors, and several of the existing tokamaks as well, use superconducting (SC) magnets in order to create high magnetic fields with reduced electrical consumption. Superconductors are cooled by Supercritical Helium (SHe) flow at ∼4.4 K and ∼5 bar provided by the cryogenic system. The helium flow has to remove the high pulsed heat loads the coils are subjected to, due to the plasma operation and especially due to the central solenoid (CS) cycling operation conditions. The current in the CS magnets is ramped up to create the plasma current and then ramped down before the dwell period, long enough to recover the initial conditions of the cycle. To optimize the cryogenic circuit operation and, in particular, to correctly size the refrigerator, the pulsed heat loads coming from the coils have to be smoothed before they are taken care of by the refrigerator. Different pulsed load smoothing techniques have been investigated in the HELIOS (HELium Loop for hIgh lOad Smootingh) test facility at CEA Grenoble, France [1], [2]. HELIOS is the scaling down of the JT-60SA tokamak central solenoid cooling circuit by a factor 1/20. The heat load deposition into the loop, performed by a series of three heated pipes, mimics the loads on the circuit coming from the magnet.

The 4C code [3], which was developed at Politecnico di Torino, Italy, for the integrated simulation of superconducting magnet systems and their cryogenic circuits, has been recently validated for transients ranging from (fast) safety discharge [4], [5] to (slow) cool-down [6], as well as successfully applied to the analysis of several relevant transients in different magnet systems like the ITER Toroidal Field (TF) coil cool-down [7], standard operation [8], [9], quench [10] and fast discharge [11], the KSTAR AC losses [12], [13], the JT-60SA pulsed operation [14].

The cryogenic circuit module of 4C [15] also already demonstrated its suitability to reproduce the dynamic response of the HELIOS test loop in isochoric configuration both in the case of interpretive [16], [17] and in the case of predictive simulations [18].

The isochoric (constant volume) configuration is characterized by a closed loop and, therefore, by a constant mass of helium, which undergoes large pressure variations during the pulsed operation. In the present paper, we focus on the isobaric (constant pressure) configuration, which is characterized by an open loop, with an active control aimed at maintaining a constrained and small variation of the helium pressure, varying the He mass circulating in the loop. First experimental comparisons between isochoric and isobaric configurations were presented in [19]: the isobaric configuration, with specific controls, was shown to give a more effective smoothing of the pulsed heat loads. Further experimental investigations were carried out in the HELIOS facility and are presented here with dynamic modeling.

In this paper, we first describe the HELIOS experimental set-up and the 4C model of the circuit, including the implementation of the isobaric control. Then two different HELIOS operating scenarios in the isobaric configuration are presented: a simplified heat pulse scenario and a JT-60SA scaled-down scenario. For both scenarios the 4C results are validated against the experimental data. A detailed comparison with the isochoric configuration is presented afterwards, showing the differences between the two configurations referring to a simplified pulse scenario and highlighting those features which make the isobaric configuration especially interesting. In the last part of the paper, a new layout of the HELIOS loop is proposed, with an optimization of the positions of the charging and discharging lines.