Proposal for

Engineering Study

of

Single Turn Toroidal Field Coil and Power Supply System\
for the Spherical Torus

C. Neumeyer, R. Woolley

January 2003

*Summary*

The spherical torus (ST) concept ultimately relies on the implementation
of a new type of toroidal field (TF) coil and power supply system which
consists of a single turn driven to ultra-high currents at very low
voltages. A study of the engineering aspects of the single turn TF coil
and power supply system is proposed. This study will address the options
for implementation of the power supply, single turn center post,
electrical joints, and outer return leg. In addition it will consider
the impact of the large conducting structures on plasma shape and
position control.

The outcome of the study will be a report along with a summary paper
suitable for journal publication. This will include recommendations for
follow-on R&D activities, possibly including the construction of a small
test device which would provide an actual demonstration of single turn
TF operation. This device would probably be limited to engineering
systems and would not include a plasma. Besides identifying future
development tasks the report would serve to dispel concerns about the
feasibility of the ST configuration in the long term.

Support for six man-weeks of engineering effort is hereby requested.

*Background*

The spherical torus (ST) is one of the most promising of the alternate
concepts being developed in parallel with the standard tokamak approach
to magnetic fusion energy. The National Spherical Torus Experiment
(NSTX) at PPPL, along with the Mega Ampere Spherical Tokamak (MAST) at
Culham, are "Proof of Principle" ST Experiments which are well into
their research programs and have met with considerable success to date.
Plans are being formulated for follow-on devices which link the NSTX and
MAST research programs with the overall fusion development path toward
DEMO. These future devices include the Next Step ST (NSST) device and
the Component Test Facility (CTF). The NSST would extend the physics
basis of the ST to Proof of Performance reactor relevant levels, but in
a pulsed mode. The CTF would operate with a plasma very similar in
performance to NSST but in steady state, providing a high neutron flux
nuclear test facility for blanket and divertor components operating at
realistic fusion reactor conditions, a key prerequiste for DEMO.
Depending on the outcome of the tokamak burning plasma program
(International Thermonuclear Experimental Reactor (ITER) or Fusion
Ignition Reactor Experiment (FIRE)) and the experience with NSST and
CTF, DEMO itself could well take the form of an ST.

The ST differs from the standard tokamak line of development in that its
aspect ratio (major radius divided by minor radius, A=R0/a) is much
smaller, typically of order 1.5 or lower compared to 3.0 or higher for
tokamaks. Thus for devices of similar scale (major radius) the region
within the major radius tends to be filled with plasma more so in the ST
than the tokamak, i.e. like an apple (ST) compared to a doughnut
(tokamak). This difference leads to many physics advantages which arise
mainly due to the fact that more of the plasma volume is closer to the
magnetic axis of the toroidal field (TF) which falls off according to
distance (1/r) from the axis. With this more efficient usage of the
magnetic field, the required field magnitude for stable confinement
tends to be lower, i.e. the ratio of plasma pressure to magnetic field
(beta) can be higher.

In the ST, compared to the tokamak, more of the region within the major
radius is given up to the plasma at the expense of the solenoid and
shielding found in traditional tokamak reactor designs which utilize
superconducting magnets. As a result the two major challenges of the ST
are to 1) initiate and sustain plasma currents using means which do not
require a central solenoid, and 2) to implement a compact toroidal field
center post which can survive in the nuclear radiation environment. The
latter requirement most likely means that multi-turn insulated TF coil
conductors cannot be used. And it is highly unlikely that a
superconducting solution exists. Instead, a single turn copper alloy TF
center post has been envisioned in various design studies of future ST
reactors (e.g. Volumetric Neutron Source (VNS)[^1]^,^[^2] ^,^[^3]
^,^[^4], and ARIES-ST[^5]).

While solenoid-less start-up and sustainment are primary elements of the
physics research missions of NSTX, MAST, and NSST, there are no plans at
present for any R&D or experimental devices which would target the
technology challenges related to the single turn TF center post. The
existing NSTX and MAST devices use multi-turn insulated TF coil
conductors connected in series in the same way that all tokamaks have
been built in the past. The same is true for NSST, since the effective
utilization of the PPPL infrastructure (mainly the modular 1kV/24kA
AC/DC converters) dictates the use of a multi-turn TF system.

The transition from multi-turn to single turn TF technology will not be
trivial. It is essential to perform sufficient R&D and experimentation
at a suitable scale prior to undertaking the CTF which, if implemented
using the ST approach, must utilize the single turn TF technology.

Reactor scale devices such at CTF require a TF current of the same order
as the plasma current. Recent design point studies[^6] indicate that a
current of order 20MA has to be driven through the center post, outer
return conductor, associated joints, and bus bar at a voltage drop in
the range of 5 to 10 volts. There are significant electrical engineering
challenges associated with this concept which need to be addressed.

The following is a list of the major issues foreseen at this time.

1)  Ultra-high current power supply

> Although two brief studies[^7]^,^[^8] are known to have examined the
> possibility of building power supplies in the range of 20MA at 5-10
> volts based on extension of today's silicon power semiconductor
> technology, there are major challenges with this approach. The largest
> continuous duty AC/DC converters built today (in the electrochemical
> and aluminum refining industries)[^9] are in the 200kA range, which
> indicates an extrapolation of about 100 in current to the ST
> requriement. And nearly all high power recitifiers operate at 600V or
> above, around 100 times higher that the single turn ST TF voltage
> requirement. The intrinsic voltage drop of semiconductor diodes
> themselves, of order 1.5 volts for high power diodes operating at
> rated current, places a rather severe upper limit on the efficiency,
> even for converter configurations which employ only one series device.
> In addition, because of the relatively low current density of an AC/DC
> converter the physical dimensions of such a power supply would be
> extremely large, increasing the length of the current paths and
> further reducing efficiency. In addition to such difficulties, the
> parts count (number of diodes, fuses, etc.) would be extremely large,
> implying reliability/availability/maintainability (RAM) challenges.
> Other types of semiconductors could be considered[^10] such as MOSFETs
> which have lower voltage drop but such devices are not manufactured at
> relevant current ratings (the highest current ratings available are
> about 1/10 of silicon based semiconductors). It appears that the
> homopolar generator may offer an attractive solution to this problem.
> Based on the principle of a conductor (typically a disc or drum)
> rotating in a magnetic field, such devices tend to produce very high
> currents but at very low voltage. These are exactly the
> characteristics desirable for the ST power supply. While some fusion
> multi-turn tokamak and electromagnetic launcher experiments have
> utilized homopolar generators for high energy multi-MA discharges at
> 100's of volts[^11]^,^[^12]^,^[^13] such devices have never been
> applied to steady state applications such as those of the single turn
> ST. Here the required voltage is very low, which would significantly
> relax the design requirements of a homopolar generator compared to
> prior applications. A homopolar generator driven by a large vertical
> shaft synchronous machine is envisioned. The homopolar generator would
> be physically integrated with the rotor of the synchronous machine to
> the extent feasible. Current collection off of the homopolar rotor
> would be accomplished using liquid metal techniques. The homopolar
> generator would be located immediately below the torus so as to
> minimize the electrical losses in the bus work.

2)  Center Post Design

> Prior studies of the single turn center post have chosen monolithic
> copper alloy structures with coolant passages running the full length
> of the structure. It is not clear that this approach is optimal. An
> alternate cooling scheme would use a large central passage acting as a
> manifold to deliver coolant to radial cooling passages which would
> empty underneath the first wall shield material (perhaps a tungsten
> jacket). Two coolant systems would exist, one above and one below the
> midplane. The density of the coolant passages would be matched to the
> power dissipation, so that there would be a larger number of passages
> per unit volume near the midplane where the electrical and nuclear
> dissipation is the highest. This cooling scheme should be much more
> effective than the prior concepts, and much easier to manufacture. An
> additional design option to consider is to employ two different
> materials, a radiation resistant one on the outside, and pure copper
> on the inside. Since the copper alloy materials tend to be less
> electrically conductive than pure copper, the use of pure copper on
> the inside would enhance the overall conductivity of the system.

3)  Electrical Joints

> One end of the TF center post would be electrically insulated from the
> outer return leg but physically attached to it. The other end needs to
> provide an ultra-high current connection but allow for the
> differential thermal expansion of the center post and the outer return
> leg. In addition, to preserve the excellent remote handling
> characteristics of the ST with the demountable center post and
> removable top lid, the joint must accommodate repeated assembly and
> disassembly. A liquid metal joint is envisioned. Other ultra-high
> current demountable joints are required in the bus work and outer
> return legs. The use of multi-lam type joint technology is envisioned.

4)  Outer return leg

> The outer return leg is envisioned to consist of a thick walled
> aluminum or copper cylinder with lids on each end. Since the current
> density in the lids decreases by 1/r it may be advantageous to vary
> the thickness accordingly. Some schemes also envision the outer return
> leg structure serving as the primary or seconday vacuum containment.
> The outer return leg must be cooled, and designed to accommodate the
> mechanical stresses due to electromagnetic and thermal loads. Openings
> in the structure which will be required for ports will tend to cause
> field errors and stress concentrators which need to be evaluated. The
> thick outer shell will have implications in terms of plasma control
> due to field penetration time from outer PF coils. So, it may be
> necessary to introduce slits to increase the toroidal and/or
> resistance to poloidal and/or toroidal eddy current flow. It is
> possible that such slits or other means could be employed to direct
> the outer leg return current to have a toroidal component. This could
> be used to introduce a background field which would lessen the
> requirements on the poloidal field coils.

5)  Plasma position control

> The extensive conducting structure will limit the effectiveness of
> external PF coils ability to control plasma shape and position.
> Internal PF coils may be difficult to implement because of the nuclear
> environment within the outer return leg, close to the plasma. In
> addition, magnetic sensors may need to be positioned inside the outer
> return leg structure in order to obtain satisfactory time response.

[^1]: "Spherical Torus VNS", T. Hender et al, Fusion Engineering and
    Design, 45 (1999), p. 265

[^2]: Ibid, "Mechanical Design Considerations of the Centerpost for a ST
    VNS", I. Sviatoslavsky et al, p. 281

[^3]: Ibid, "The Prospects of Copper Alloys for VNS Applications", V.
    Belyakov et al, p. 295

[^4]: Ibid, "The Centerpost Alternate Design Version for a ST based
    VNS", V. Belyakov et al, p. 317

[^5]: "Toroidal Field Coil for ARIES-ST", W Reiersen et al, 17^th^ SOFE

[^6]: "CTF Design Point Studies", C. Neumeyer, 9/5/2002

[^7]: "Scheme for Multi-Megamp Power Supply", C Neumeyer 6/5/97

[^8]: Excerpt from Culham memo by J. Hicks, private communication from
    G. Voss

[^9]: "AC to DC Power Conversion Now and in the Future", A. Siebert et
    al, IEEE Transactions on Industry Applications, Vol. 38, No. 4,
    7/2002

[^10]: "Operational Loss Reduction using Power MOSFET for Magnetic Field
    Coil Power Supplies of Nuclear Fusion Experimental Machine", T.
    Matsukawa et al, 17^th^ Int'l Conf on Magnet Technology, 9/2001

[^11]: "Applying a Homopolar Power Supply to a Tokamak", P. Wildi et al,
    1^st^ IEEE Int'l Pulsed Power Conference, 1976

[^12]: Ibid, "Pulsed Power from Inertial Storage with Homopolar Machines
    for Conversion", H. Woodson et al

[^13]: "Final Manufacture and Assembly of a Modular 60MJ Pulsed
    Homopolar Power Supply", W. Noble et al, IEEE Transactions on
    Magnetics, Vol. MAG-22, No. 6, 11/86