IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 3, JUNE 2014 4202705

Conceptual Design of a Toroidal Field Coilfor a Fusion Power Plant Using High

Temperature SuperconductorsP. V. Gade, Graduate Student Member, IEEE, C. Barth, C. Bayer, W. H. Fietz,

F. Franza, R. Heller, K. Hesch, and K.-P. Weiss

Abstract—Taking a step further from the International Ther-monuclear Experimental Reactor (ITER), the next step will bea demonstration fusion power plant, DEMO, i.e., a fusion powerplant (FPP) prototype. As a part of European Union (EU) DEMOstudies, the result of the so-called PROCESS system code has beentaken as the basis to design a toroidal field coil (TFC) winding packwith high temperature superconductor (HTS) REBCO, which isa promising HTS candidate. From this, the cable space area, thewinding pack current density, and the total current in one TFChas been obtained. In this paper, a conceptual design of a HTSTFC is presented, and related parameters, such as peak magneticfield at the conductor, conductor current, and coil inductance, arecalculated. The results have been used to evaluate the temperaturemargin and the hot spot temperature in case of a quench. Withthe calculated results, it is shown that at 4.5 K, the actual availableHTS conductor can be used to design a TFC for DEMO within theavailable space given by PROCESS code.

Index Terms—DEMO, fusion, high temperature superconduc-tor, toroidal field coil.

I. INTRODUCTION

LOOKING BACK in time, different FPP concepts havebeen developed within European Power Plant Conceptual

Studies (PPCS) [1]. The models have been developed using asystem code called PROCESS [2]. The PROCESS System Codeis a computational tool aimed to provide the design guidelinesfor the main components of such a future fusion power plant.One of these DEMO models has pulse plasma mode. To getan idea, this model has 1.8 times of plasma volume whencompared to ITER [3], [4]. Therefore, the size of the supercon-ducting magnet system has to be significantly larger. While theneeds for larger superconducting magnets are escalating, therehas been also new breakthrough in materials. Example of such

Manuscript received July 17, 2013; accepted December 10, 2013. Date ofpublication January 28, 2014; date of current version February 3, 2014.

P. V. Gade, C. Barth, C. Bayer, R. Heller, K.-P. Weiss, and W. H. Fietzare with the Institut für Technische Physik, Karlsruher Institut für Technolo-gie, 76344 Eggenstein-Leopoldshafen, Germany (e-mail: vishnuvardhan.gp@ieee.org).

F. Franza is with the Institut für Neutronenphysik und Reaktortechnik,Karlsruher Institut für Technologie, 76344 Eggenstein-Leopoldshafen,Germany (e-mail: fabrizio.franza@kit.edu).

K. Hesch is with the Nuclear Fusion Programme, Karlsruher Institutfür Technologie, 76344 Eggenstein-Leopoldshafen, Germany (e-mail: klaus.hesch@kit.edu).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TASC.2014.2299828

TABLE ITFC PARAMETERS FOR PULSED DEMO

novel material is the so-called second generation (2G) HTS alsoknown as rare-earth-barium-copper-oxide (REBCO) tapes. Thismaterial has superior capabilities in terms of current densities(Jc) and mechanical stability at higher critical magnetic fields(Bc), in comparison to existing low temperature superconduc-tors (LTS). These HTS tapes are commercially available (seefor example [5]).

The present paper investigates if this actual available HTSmaterial can be used to design a TFC operated at 4.5 K forDEMO with the available space given by the PROCESS code.The key parameters discussed in this paper are the shape of theTFC, conductor current, number of turns, peak magnetic fieldin the TFC, toroidal magnetic field at the plasma axis, dumptime constant and hotspot temperature.

II. IDENTIFICATION OF PARAMETERS FROM

PROCESS CODE

A. TFC Parameters for Pulsed DEMO

As part of the work under the European Fusion DevelopmentAgreement (EFDA), the input parameters for the design of theTFC were taken from PROCESS System Code output dated16th of April 2012 [3] using the pulse DEMO model. In thefollowing we refer as “pulsed DEMO”.

PROCESS is a systems code that calculates in a self-consistent manner the parameters of a Tokamak fusion powerplant with a specified performance, ensuring that its operatinglimits are consistent and offers the possibility of optimization.The code integrates all major components such as divertor,magnets, blankets, first wall, heating system and others in asimplified way. The code has been developed and improvedfrom the earlier nineties. One of the key components is the TFC.Some of its main parameters are given in Table I [6].

1051-8223 © 2014 EU

4202705 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 3, JUNE 2014

Fig. 1. Modified D-shaped tori to have consistent tangets, a sum of 180◦ anda straight inner leg.

B. Shape of the TFC

The approximation of the inner shape of the TFC has beengiven by the output of the PROCESS System Code. The designof non-circular tori is to fulfill the requirement for plasma sta-bility, plasma confinement and to reduce excessive mechanicalstresses in a torus [7].

The criteria for ideal D-shaped tori are: i. the tangents of thearcs should match, ii. the sum of all angles should be 180◦ forthe upper half of the coil and iii. the inner leg of the TFC ispreferred straight [7]. From Fig. 1, it can be seen that thesecriteria are not fulfilled in the output of the PROCESS code.The sum angles over the different arcs are not 180◦, at one pointthe tangents do not match and the inner leg is not straight.

To solve these problems the inner shape of TFC has beenmodified. For better illustration, the improved shape is shown inFig. 1. In the following this modified geometry has been used.

C. HTS Conductor for TFC

The shape of TFC has been fixed by the geometry discussedabove, but the conductor current and the size of the conductorstill needs to be defined. REBCO material has tremendouscapabilities but due to manufacturing limitations, it’s onlyavailable in the form of tapes. To come to a high currentconductor it is necessary to develop conductor concepts withHTS tapes. A large number of fusion conductors in the pastused round LTS strands to allow twisting of strands for goodac loss properties. For REBCO tapes such concepts are notfeasible. As a consequence several concepts to form a conductorfrom HTS tapes are under discussion. Examples are i. Roebelcable, ii. Conductor on round core (CORC) cable, iii. HTStwisted stack tapes (TST) cable [8]. The present paper doesnot aim at a particular conductor design since these conductorconcepts are still under investigation and have to be scaledto higher currents in high fields. Therefore we do not discussthe detailed layout of the conductor but use the cross sectionsof different materials necessary for a HTS cable for the TFC(see Fig. 2).

Fig. 2. Cross sections of different materials in the HTS conductor surroundedby a stainless steel jacket with high-voltage insulation.

TABLE IIVARIOUS MATERIALS IN CONDUCTOR

This cable is encapsulated in a stainless steel jacket with thenecessary void fraction for helium flow. Taking all parametersinto consideration the operating conductor current (Iop) hasbeen calculated to be 46.5 kA using the HTS single tape datapublished in [9]. This smaller current in comparison to ITERis possible because more copper can be added with the HTSsolution which allows a slower fast discharge. The resultingarea of various materials in the HTS conductor is shown inTable II.

The jacket thickness has been assumed to be 6.25 mmwhich should be sufficient to withstand the Lorentz forces. Anelectrical insulation of the conductor of 1.5 mm has been taken.

D. Winding Pack Geometry

Once the conductor geometry has been defined, the selectionof winding geometry and type is necessary. Locating the heliuminlet in the high field region of the winding pack and the outletat the low field region has the advantage that the temperature islowest in the high field region. Therefore a pancake winding isproposed. To adopt the available space, two types of pancakesare proposed as shown in Fig. 3. The dimensions of the pancakepacks are 1189 mm × 737 mm and 139 mm × 435.5 mmat both sides. Number of turns in the pancake packs is 374and 26 at both sides. The total winding area is 0.992 m2,which fits well into the 1.29 m2 available space [6]. The restof the space around the winding pack is needed for insulationbetween pancakes, ground insulation and for embedding thewinding within the casing. While the electrical connection ofthe pancakes will be in series, there will be 21 parallel coolingcircuits for the

GADE et al.: TOROIDAL FIELD COIL FOR FUSION POWER PLANT 4202705

Fig. 3. Proposed winding pack cross section with casing.

TABLE IIIEFFI SIMULATION PARAMETERS FOR PULSED DEMO

III. SIMULATION AND RESULTS

Since the magnet parameters have been defined above, itis mandatory to check if the magnetic field at plasma axis isequal to the required magnetic field and to calculate the peakmagnetic field. The latter one is necessary for defining theoperating point of the conductor. For magnet simulation, thepre-processor TOKEF [10] and the code EFFI [11] have beenused. TOKEF stands for tokamak input generator for EFFI,EFFI stands for Electromagnetic Fields, Forces and Inductancecalculation. EFFI is a FORTRAN based code developed in thelate 70’s to simulate magnets of arbitrary shape. It can simulatesimple current carrying conductors up to any complex magnetsystem. For the current simulation the parameters consideredare shown in Table III.

A. Magnetic Field at Plasma Axis

To compute the magnetic field at the plasma axis, all sixteenTFC with the pancakes described above have been modeledusing the coil current given in Table III. The plasma axis is9 m from the machine axis, as given by the PROCESS code.The calculation result is shown in Fig. 4. The magnetic field atthe plasma axis results to be 7.045 T which is in good agreementwith the requirements from the PROCESS Code.

B. Peak Magnetic Field at Winding Pack

To determine the peak magnetic field, a cut section of themidplane of the inner leg has been considered. The peak

Fig. 4. Magnetic field plotted in the horizontal midplane of the plasma.

Fig. 5. Magnetic field plotted at winding pack cross section of the mid planeof the inner leg of TFC. X is radial dimension and Yy is axial dimension.

magnetic field was calculated in the inner edge of the windingclose to the plasma wall to be 13.32 T as shown in Fig. 5.

C. Conductor Operating Point

To find the conductor operating point, contours of the criticalcurrent density as a function of the magnetic field for varioustemperatures of the HTS conductor have been calculated. Thesecalculations base on REBCO tape data taken from [9] and theresults of our calculation are shown in Fig. 6. For our pulsedDEMO TFC the operating temperature of the conductor hasbeen assumed to be 4.5 K. The operating current of 46.5 kA(indicated as star symbol in Fig. 6) results in an operating tocritical current ratio of 0.58 which gives a sound margin forcoil operation.

To evaluate the resulting temperature margin, the criticalcurrent versus temperature at 13.5 T has been calculated asshown in Fig. 7. From this plot, the HTS conductor margin canbe estimated to be 13.5 K.

D. Adiabatic Hotspot and Discharge Voltage

In case of a magnet quench, a safety discharge has to beinitiated. According to the PROCESS code a discharge timeconstant τ of 17.78 s is considered. To calculate the maximumtime constant, the ITER hotspot criteria has been used which

4202705 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 3, JUNE 2014

Fig. 6. Magnetic field versus critical current at different temperatures. Theoperation condition is indicated as asterisk.

Fig. 7. Critical current versus temperature at 13.5 T magnetic field. Theoperation condition is indicated as asterisk.

Fig. 8. Time versus adiabatic hotspot temperature.

suggests that during a quench the adiabatic hotspot temper-ature should not exceed 150 K. Using the material volumesin the conductor, adiabatic hotspot calculations were done forvarious τ . A delay time of tDEL = 7 s has been chosen to countfor voltage evolution for quench detection and for additionaltime for detection and the initiation of discharge. In Fig. 8. thehotspot temperature has been plotted against the discharge timeconstant to find the limit where the hotspot temperature violates

TABLE IVSUMMARY

the ITER criteria. From the graph, it can be seen that the magnetwill reach 150 K for τ = 44.5 s. Thus, the magnet can be safelydischarged with τ = 40 s. Because the discharge voltage andtime constants are correlated, for τ = 40 s a discharge voltageof 10.5 kV is calculated for one coil which is in an acceptablerange.

IV. CONCLUSION AND FUTURE WORK

Based on the current analysis, it can be said that HTSconductor is a potential candidate superconductor for pulsedDEMO. A HTS winding pack principally fits in the givenwinding pack area and can produce the required magnetic fieldat plasma axis for the pulsed DEMO. The peak magnetic fieldat the superconductor is 13.32 T with the design proposed here.The use of HTS can increase the temperature margin to morethan 13 K. Compared to the PROCESS code, an increase ofthe discharge time constant from 17.78 s to 40 s is possiblewhich helps limiting the discharge voltage. All the parametersare summarized in Table IV.

With these results it is demonstrated that at 4.5 K the actualavailable HTS conductor can be used to design a TFC forDEMO within available space given by PROCESS code. Withthe design discussed an operation at higher temperatures e.g.20 K is not possible. With today’s HTS material it would benecessary to reduce the helium and/or the copper cross sectionto give space for additional superconductor material. But thishas consequences for operation parameters like e.g. dischargetime constant or pressure drop.In the future, it is essentialto analyze the hydraulics of the winding, the stresses in thejacket, and include both the pancake insulation and the groundinsulation in the winding pack design. Of course the real designof the high current HTS conductor has to be found, which couldnot be addressed in this paper.

ACKNOWLEDGMENT

This work was done under the Contract of Associationbetween EURATOM and KIT, within the European FusionDevelopment Agreement. The views presented here are notnecessarily those of the European Commission.

GADE et al.: TOROIDAL FIELD COIL FOR FUSION POWER PLANT 4202705

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