56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014;
New Orleans, Louisiana
Session YI1: Fast Ions, Momentum Input, RF, and Current Drive
9:30 AM–12:30 PM,
Friday, October 31, 2014
Room: Acadia
Chair: Zhihong Lin, University of California, Irvine
Abstract ID: BAPS.2014.DPP.YI1.2
Abstract: YI1.00002 : Measurements of LHCD current profile and efficiency for simulation validation on Alcator C-Mod
10:00 AM–10:30 AM
Preview Abstract
Abstract
Author:
Robert T. Mumgaard
(MIT PSFC)
Lower hybrid current drive (LHCD) is an effective tool to significantly
modify the magnetic equilibrium by driving off-axis, non-inductive current.
On Alcator C-Mod, an upgraded Motional Stark Effect (MSE) diagnostic enables
the current profile to be accurately reconstructed during plasmas with
strong LHCD and a hard X-ray camera measures the fast electron
Bremsstrahlung profile. LHCD is applied for \textgreater 4 current
relaxation times, producing fully-relaxed magnetic equilibria in plasmas
with non-inductive current fraction up to unity at currents up to 1.0 MA.
C-Mod has developed an extensive database of LHCD performance, spanning a
wide range in plasma current, launched n$_{\vert \vert }$, LHCD power,
T$_{e}$ and plasma density. This dataset provides a unique platform for
validation of LHCD current drive simulations with the plasma shape, density,
field and LH frequency range envisioned for ITER and future reactors. In
these conditions the measured current drive efficiencies are similar to that
assumed for ITER with values up to 0.4*10$^{20}$A/Wm$^{2}$ despite being in
a weak single-pass absorption regime. The driven current is observed to be
off-axis, broadening the current profile, raising q$_{0}$ above 1,
suppressing sawteeth, decreasing/reversing the magnetic shear and sometimes
destabilizing MHD modes and/or triggering internal transport barriers.
Measurements indicate increased efficiency at increased temperature and
plasma current but with a complicated dependence on launched n$_{\vert \vert
}$. The MSE-constrained reconstructions show a loss in current drive
efficiency as the plasma density is increased above \textless
n$_{e}$\textgreater $=$1.0x10$^{20}$ m$^{-3}$ consistent with previous
observations of a precipitous drop in hard x-ray emission. Additionally, the
measured driven current profile moves radially outward as the density is
increased. Ray tracing simulations using GENRAY-CQL3D qualitatively
reproduce these trends showing the rays make many passes through the plasma
at high density and predicting a narrower current and HXR profile with than
that observed in the experiment.
This work is supported by USDoE awards DE-FC02-99ER54512 and
DE-AC02-09CH11466.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2014.DPP.YI1.2