Poloidal Field Design and Plasma Scenarios for FIRE Page: 2 of 8
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DYNAMIC PLASMA DISCHARGE SIMULATIONS
TSC was used to develop full discharge simulations including plasma current rampup, heating to
burn, flattop burn, burn termination, and plasma current rampdown. TSC is a two-dimensional time
dependent free-boundary simulation code that advances the MHD equations describing the transport
time-scale evolution of an axisymmetric magnetized tokamak plasma. For the present simulations the
plasma density is prescribed by
n(z, t) = no (t) [(1 - 2.0)05 + 0.3 (1)
leading to a peak to average density of 1.3, intended to represent an ELMing H-mode. The simulation
includes neoclassical resistivity, bootstrap current, a time-averaged sawtooth model, a modified Coppi-
Tang energy transport model, radiation, and alpha and fusion effects. Passive structures are included
to obtain accurate estimates of flux consumption and response of feedback control systems. The passive
structure for FIRE is a double walled vacuum vessel and copper passive stabilizer plates (both inboard
and outboard). Since TSC must begin its simulation with a plasma present, a constant voltage is applied
as an initial condition over the computational grid. This causes finite structure currents to exist at the
beginning of the simulation, as would be expected after the plasma breakdown. Feedback systems for
the plasma current and radial position were used (simulations were up-down symmetric), with all the PF
coils contributing to current control, and PF4 providing radial position control.
Several plasma characteristics are assumed; the impurity is 3% beryllium, the effective particle con-
finement time is 5 times the energy confinement time, the energy confinement time is about 0.5 s, the
Harris[2] bootstrap model is used, 100% of the ICRF heating goes into ions, and the plasma edge temper-
ature is 500 eV. The vacuum toroidal field is ramped up over 21 s to its flattop value of 10.0 T (reaching
this value at SOF), and begins dropping at the end of the flattop (EOB) with its L/R time scale (about
20 s).
Plasma current rampup extends from 0(SOD) to 6(SOF) s. The plasma starts as a circular 100 kA
plasma limited on the inboard wall, and is grown over the rampup to full size and shape. The plasma
current is ramped up linearly from 100 kA to 6.44 MA in 6.0 s. The plasma is diverted at about 3.2
s, and the full 30 MW of ICRF heating is applied at 4.8 s (Ip = 5.5 MA) causing the plasma to enter
the H-mode. The heating during rampup was found to provide a robust entry into H-mode that could
be maintained during the fast density rise that occurs later. The plasma consumes 31.3 V-s giving a
flux state of 11.2 Wb at SOF. The plasma startup trajectory remains in the stable region of the li-q95
diagram. However, q95 briefly drops below 3.0 at SOF. The Ejima coefficient is 0.35, due to the heating
during current rampup. The sawtooth radius reaches about 0.17, which is slowed down by the heating.
At the end of the plasma current rampup phase the peak electron density is 1.75x 1020 /m3, the peak
electron temperature is 17 keV and ion temperature is 24 keV, #N reaches 0.75, Pa is 10 MW, and Paux
is 30 MW.
The approach to burn and flattop burn extend from 6 to 27(EOB) s. Since heating begins in the
plasma current rampup phase, there is no well defined heating to burn phase. From 6 to 8 s, the peak
electron density is increased to 4x 1020 /m3. The density is further ramped more slowly to 5x 1020 /m3
from 8 to 14 s, to control the fusion power overshoot. At 7.5 s the heating power is dropped to 22 MW,
and the alpha power rises to 50 MW by 9 s. Q (Pfusion/Paux) is greater than 10 from 8.5(SOB) s to 27
s, which is 37 energy confinement times. The helium density remains constant from 14 to 27 s, which
is 5 effective particle confinement times. The helium ash reaches 3% of the electron density, with Zeff
becoming 1.4. The plasma internal inductance drops to 0.85 as the bootstrap current develops (reaching
1.6 MA) and the higher edge temperature associated with the H-mode allows more current to flow at
the plasma edge. The poloidal and toroidal # values rise to 1.2 and 3%, respectively, corresponding to2
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Kessel, C. E. & Bulmer, R. H. Poloidal Field Design and Plasma Scenarios for FIRE, report, October 1, 1999; Princeton, New Jersey. (https://digital.library.unt.edu/ark:/67531/metadc622217/m1/2/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.