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Spherical Torus Center Stack Design
C. Neumeyer, P. Heitzenroeder, C. Kessel, M. Ono, M. Peng, J. Schmidt, R. Woolley, I. Zatz,
Princeton University Plasma Physics Laboratory*, 'Oak Ridge National LaboratoryAbstract-- The low aspect ratio spherical torus (ST)
configuration requires that the center stack design be optimized
within a limited available space, using materials within their
established allowables. This paper presents center stack design
methods developed by the National Spherical Torus Experiment
(NSTX) Project Team during the initial design of NSTX, and
more recently for studies of a possible next step ST (NSST)
device.
1. INTRODUCTION
Design point selection requires parametric study relying on
simple analytic solutions to characterize performance in
terms of plasma and engineering quantities, short of detailed
analysis using, e.g. finite element methods. Many excellent
papers have been written on this subject for conventional
tokamaks [1] and also for STs [2,3]. This paper presents the
method used by the NSTX team which includes some perhaps
interesting new ideas being considered for NSST. The focus is
on engineering issues related to pulsed copper/copper alloy
magnets and, further, to the Toroidal Field (TF) and Ohmic
Heating (OH) coils associated with the "center stack" of the
ST device.Methods for estimating these factors are presented herein
(except for the TF insulation shear, which is highly dependant
on the torque reaction scheme). However, it is emphasized that
careful judgment is required in choosing allowables and
margins. The challenge is to select, based on simple analysis,
a design point which will withstand the scrutiny of detailed
analysis with, in the end, satisfactory margins.
III. METHODOLOGY
Methods presented here are in use to study options for a
next step ST. However, they are derived from earlier work on
NSTX [4] and MAST [2] and can be applied more generally.
A. OH Coil
To rough-out the thermal and mechanical conditions, the
following simplified OH waveform is assumed.Ioh
[1
II. DESIGN ISSUES
A. Requirements
Performance requirements for the TF system are the toroidal
field Bt to be produced at radius Ro, and flat top duration. For
the OH system the primary requirement is to produce a
specified flux swing over the time of plasma current duration.
Secondary requirements relate to the coil height, which
determines the fraction of OH flux coupled to the plasma, and
the stray field. Since the OH coil surrounds the inner legs of
the TF coil in an ST, the OH coil also sets the TF height.
B. Engineering Constraints
While the complete design of the TF and OH coils and the
integrated center stack requires detailed analysis of many
aspects, the main design drivers are as follows.
- OH Coil
- Conductor temperature rise and mechanical stress
" TF Coil Inner Legs
- Conductor temperature rise and mechanical stress
- Insulation shear stress
" Integrated Center Stack
- Peak power, typically at start of flat top (SOFT)
- Peak energy, typically at end of flat top (EOFT)
- Overall heightUnder USDOE Contract # DE-AC02-76CH03073
12
Start of
Plasma
(SOP)
A3It4
End of
t5 Plasma
(EOP)
0 tti l t2
Start of End of
Flat Top flat Top
(SOFT) (EOFT)Fig 1. Simplified OH Current Waveform
OH conductor materials will be cooled, in the case the NSST,
to liquid nitrogen (LN2) temperature (80K) prior to the pulse
and allowed to heat to 373K (100C) at the end of the pulse.
Optimum performance is realized when materials are operated
at their low temperature stress limit at SOP and their high
temperature stress limit at EOFT, and at their thermal limit at
EOP. Toward this end, for the OH waveform shown in Fig.
1, an asymmetry in the OH current waveform can be chosen to
optimized performance. Assuming that the EM stress is due to
Ioh only, then...Ia
2 'cold
'2 G"ot asm(1)
A second benefit of asymmetric OH operation is reduced
dissipation during the relatively long plasma flat top time. A
third benefit is that the peak OH power which coincides with
the peak TF power at SOFT is reduced, such that the total
composite power peak imposed on the facility is reduced.
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Neumeyer, C.; Heitzenroeder, P.; Kessel, C.; Ono, M.; Peng, M.; Schmidt, J. et al. Spherical Torus Center Stack Design, report, January 18, 2002; Princeton, New Jersey. (https://digital.library.unt.edu/ark:/67531/metadc739304/m1/3/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.