Proceedings of ITC/ISHW2007

Internal Energy Dump for Superconducting Magnet Protection ofthe LHD-Type Fusion Reactor FFHR
Kazuya Takahata, Toshiyuki Mito, Hitoshi Tamura, Shinsaku Imagawa, and Akio Sagara
National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, JapanThe superconducting magnet systems of LHD-type reactors have large magnetic energies of over 100 GJ. Whena quench occurs, this magnetic energy must be absorbed quickly with a resistor. A conventional energy dump withan external resistor generates a high terminal voltage, which may cause a serious dielectric breakdown. This paper
presents the internal energy dump concept for quench protection of the LHD-type reactor FFHR. In an internalenergy dump, a shorted secondary circuit acts as a heater and induces a normal transition of the whole coil. Most of
the magnetic energy can be absorbed inside the coil while the temperature of the coil rises. In the case of the FFHR,
the temperature rises up to 200 K. We designed and analyzed a secondary circuit using a copper winding. The
designed circuit can reduce the terminal voltage by a factor of ten and induce a normal transition of the adjacentsuperconductor within 1 s. The results show that the internal energy dump concept is appropriate for design ofsuperconducting magnets for fusion reactors.
Keywords: fusion reactor, heliotron reactor, large helical device, superconducting magnet, quench protection,
magnetic energy, internal energy dump

1. Inroduction
Heliotron power reactors have competitive
advantages for steady-state operation due to the fact thatthey employ currentless plasma. These advantages havebeen demonstrated by the Large Helical Device (LHD)with a superconducting magnet system since the start ofexperiments in 1998 [1]. Engineering and physics resultsfrom studies of the LHD confirm that LHD-type heliotron
reactors are well suited for steady-state power plants.
Based on the outputs from the LHD, we have performed
design studies of LHD-type demonstration reactors (theFFHR series). The detailed design of the blanket, operating
scenario, superconducting magnet, coil-supporting
structure and maintenance procedures has been publishedpreviously [2, 3]. In the design studies, we applied realistic
technologies that are expected to be developed in the nearfuture.
Heliotron power reactors also have an important
advantage in the superconducting magnet design.
Superconductors do not generate heat loss due to achanging magnetic field, known as ac loss. Therefore, large
cooling capability is not required, in contrast to Tokamak
reactors. We therefore propose an indirect cooling method,which is commonly used in accelerator magnets, as analternative to pool cooling and forced-flow cooling [4, 5].
The use of indirect cooling allows a simple coil structure toauthor’s e-mail: takahata@LHD.nifs.ac.jp

be used.
An important issue for the design of superconducting
magnets is the dumping of magnetic energy when a quench
occurs. A heliotron reactor magnet consists of continuous
windings with large diameter. Therefore, the magnet haslarge magnetic energy of over 100 GJ. Because an
unexpected quench may cause burn-out of asuperconductor, a protection system must be developed toreduce the coil current rapidly and to dump the magneticenergy. The conventional protection system has an external
resistor near a power supply, which absorbs most of themagnetic energy. This method is called the external energy
dump. When employing an external energy dump in large
magnets, there are two important engineering issues. First,
high voltages over 10 kV are applied across a coil, which
may cause a dielectric breakdown. Second, the external
resistor must have a large mass of heat-absorbing material.
When using water as heat-absorbing material, a large
amount of water must be heated and evaporated. To absorb
a magnetic energy of 100 GJ, 40,000 kg of water isrequired.
To resolve the issues of the external energy dump, we
have investigated an internal energy dump concept [6, 7].
In this concept, the magnetic energy is absorbed into thecoil using the normal resistance of the superconductor.
When a quench occurs, an embedded heater induces thenormal transition of the whole of the superconductor. A

Proceedings of ITC/ISHW2007

Fig. 1

Quench protection circuit with an external resistor.

Fig. 2

Quench protection circuit with a secondary circuit.

conventional resistive heater needs a power supply, andhence we employ a shorted secondary circuit as a heater.
The current of the secondary circuit is induced by thedecay of the coil current. The secondary circuit, therefore,
does not need a power supply. In this paper, wedemonstrate the applicability of the internal energy dump
as a protection system for the FFHR. The design of thesecondary circuit is presented and its performance isinvestigated.

become normal. Therefore, the decay time constant isassumed to be L1 / Rn . The maximum terminal voltage canbe reduced because R1 can be determined independentlyof the quench protection. Therefore, the coil current canpotentially be reduced. In addition, the external resistor
does not require a large amount of heat-absorbing material
as most of the heat is absorbed by the coil itself.

2. Comparison between External and Internal
Energy Dumps

The internal energy dump causes a uniform
temperature rise of the whole coil. Here we investigate
whether the magnetic energy can be absorbed into the coil.
The balance equation between the magnetic energy E
and the heat capacity of the coil can be written as:

Figure 1 shows a typical quench protection systemwith an external resistor. When a quench is detected, thebreaker is opened. The current then shifts from the power
supply to the external resistor. The resistance of the resistor
R1 is usually much higher than that of the superconductorin the normal state Rn . The coil current I1 decays with atime constant τ of L1 / R1 while the resistor absorbs themagnetic energy. The maximum terminal voltage is I 0 R1 ,where I 0 is the initial current. The maximum voltage
Vm can be rewritten as:

Vm =2EI 0τ(1)where E is the stored magnetic energy of the coil. The
maximum voltage is proportional to the energy. Note thatthe coil current must be increased to reduce the voltage.
For the FFHR with the coil current of 100 kA, the decay
time constant should be less than 18 s to avoid burn-out of
the superconductor [5]. The maximum terminal voltage
then reaches 60 kV, which is much higher than thedesigned voltage for typical superconducting magnets. To
reduce the voltage, the coil must be divided into several
subdivisions.
In an internal energy dump, the resistance Rn ishigher than R1 because the whole of the superconductor

3. Temperature Rise of the FFHR Magnet during
an Internal Energy Dump

E = ∑ Vi ∫ Ci (T )dTTmiT0(2)where i is the material number, Vi is the volume ofeach material, and Ci (T ) is the specific heat per unit
volume. T0 and Tm are the initial and maximum
temperatures. We applied this equation to the helical coil ofthe reactor design FFHR2m1. The coil structure ispresented in detail in a previous paper [5]. The materials
include superconductors, insulators, and cooling panels.
The maximum temperature Tm was calculated to be 210
K for a magnetic energy of one helical coil of 66 GJ, which
is within acceptable levels, indicating that the internalenergy dump can be successfully applied.

4. Quench Protection with a Secondary Circuit
Figure 2 shows a quench protection circuit with asecondary circuit. The secondary circuit is a normal coilthat is closely coupled to the superconducting coil. Whenthe current of the superconducting coil starts to decay as
the breaker is opened, the current shifts from thesuperconducting circuit to the secondary circuit. The

Proceedings of ITC/ISHW2007
estimated from the heat balance equation:

C1 (T )

dT= λρ1 (T ) J12 (T )dt

(4)where C1 is the average specific heat per unit volume inthe coil, λ is the ratio of coil volume to copper volume,ρ1 is the resistivity of copper, and J1 is the overall
current density. In addition, the temperature of thesecondary coil can be obtained by

C2 (T )

Fig. 3 Cross-sectional structure of the helical coil andarrangement of the secondary coils.

normal secondary circuit then heats the adjacentsuperconductor by Joule heating. Finally, the secondarycircuit causes the whole of the superconducting coil tobecome normal. This process is referred to as quench back[6, 7]. This concept can induce a normal region morereliably than a resistive heater. The switch in the secondary
circuit is closed only during a quench. A current shiftduring an excitation of the magnet is inhibited by opening
the switch.

5. Quench Protection Circuit for the FFHR
Figure 3 shows the structure of the helical coil of theFFHR and the arrangement of the secondary coil [5]. The
superconductor is wound by a layer winding method (36turns/layer, 12 layers). Four cooling panels with coolingchannels are embedded in the coil. The superconductor iscooled indirectly by the cooling panels. The secondary
coils consist of copper wires and are arranged between setsof two superconductor layers. There are thus six layers ofsecondary coil. We determine the thickness of each layer tobe 0.5 mm.
The shift of current from the primary circuit I1 tothe secondary circuit I 2 can be obtained by the following
circuit equations:dI1dI+ M 2 + I1 ( R1 + Rn ) = 0dtdt
dI
dI
L2 2 + M 1 + I 2 R2 = 0
dt
dt
L1(3)

where L2 is the inductance of the secondary circuit, M
is the mutual inductance between the two circuits, and R2
is the resistance of the secondary coil. Note that theresistance of the normal zone Rn and the resistance of thesecondary coil R2 depend on temperature. Thetemperature rise of the superconducting coil can bedT= ρ 2 (T ) J 2 2 (T )dt

(5)where C2 is the specific heat per unit volume of copper,
ρ 2 is the resistivity of copper, and J 2 is the currentdensity. Even though the heat transfer between thesuperconducting and secondary coils is neglected, thecalculations can be the first step in investigating theperformance of the secondary circuit.
By solving Eqs. (3) and (5), we find that thetemperature rise is independent of the turn number of thesecondary coil if the thickness of the coil is fixed.
Therefore, the cross-section of the wire and the turnnumber need not be considered further. The self inductanceof the secondary coil also affects the temperature rise,
which increases with increasing inductance. Theinductance must then be adjusted to half the inductance ofan ideal close winding to avoid a burn-out of the secondary
coil itself. This adjustment of inductance is technicallypossible using a non-inductive winding method.
A key issue for this protection method is whether we
can increase the initial decay time constant of thesuperconducting coil ( L1 / R1 ) and reduce the maximum
terminal voltage ( I 0 R1 ). We analyzed quench protection
for a time constant of 200 s, about ten times larger than themaximum time constant for external energy dump
protection of 18 s. Figure 4 shows the calculations of thecurrent shift and the temperature rise of the secondary coil
before quench back. In this calculation, the resistance ofthe superconducting coil Rn is zero. The currents arenormalized by the initial current of the superconducting
coil and the turn number. The results show that thetemperature of the secondary coil increases to about 35 K
within 1 s. This temperature rise can certainly induce anormal transition of an adjacent superconductor because
the critical temperature in a magnetic field of 13 T is about
10 K in the case of a Nb3Sn superconductor.
We next analyzed the temperature rise of the coils
after quench back. Figure 5 shows the calculations of thecurrent shift and the temperature rise. The quench back is
assumed to occur with a time delay of 1 s from the start ofthe energy dump. After the quench back, the resistance
Rn is calculated using the normal resistance of the wholesuperconducting coil. The current of the superconductingProceedings of ITC/ISHW2007

Fig. 4Current shift (solid lines) and temperature rise(broken line) of the secondary coil before quench
back. The currents are normalized by the initial
current of the superconducting coil I 0 and theturn numbers. N1 and N 2 are the turn numbersof the superconducting and the secondary coils,respectively.

coil decays rapidly with increasing temperature because
the resistance of the coil increases with increasing
temperature. The maximum temperatures of both thesuperconducting and secondary coils are about 200 K.
The results demonstrate that the designed circuit canprotect the coil safely even though the initial decay time
constant increases by a factor of ten. The circuit also
reduces the terminal voltage.

Fig. 5Current shift (solid lines) and temperature rise of thesuperconducting and secondary coils during theenergy dump. Quench back occurs with a timedelay of 1 s. The dashed and the broken lines
represent the temperatures of the superconducting
and secondary coils.

The results demonstrate that the internal energy dump can
be a key technology for large superconducting magnets forfusion reactors.

Acknowledgements
This work was funded by MEXT of the Japanese
government under grant no. NIFS07ULAA107 and by theJapan Society for the Promotion of Science, Grant-in-Aidfor Scientific Research (C) no. 18560793.

6. Conclusions
An internal energy dump concept has been
demonstrated by applying it to the LHD-type fusion reactor
FFHR. The analysis confirmed that the coil of the FFHR
can be protected from a quench using an internal energy
dump. The magnetic energy can be absorbed into thesuperconducting coil while the coil temperature increases
to about 200 K. Quench back, which is necessary for theinternal energy dump, can be induced using a shorted
secondary coil consisting of copper wires. The maximum
terminal voltage can be reduced by a factor of ten
compared with the conventional external energy dump.

References
[1]
[2]
[3]
[4]
[5]
[6]
[7]

O. Motojima et al., Fusion Eng. Des. 81, 2277 (2006).
A. Sagara et al., Nucl. Fusion 45, 258 (2005).
A. Sagara et al., Fusion Eng. Des. 81, 2703 (2006).
T. Mito et al., Fusion Eng. Des. 81, 2389 (2006).
K. Takahata et al, Fusion Eng. Des. (2007),
doi:10.1016/j.fusengdes.2007.04.050
M.A. Green, Cryogenics 24, 3 (1984).
M.A. Green, Cryogenics 24, 659 (1984).