911 research outputs found
Comparison of BES measurements of ion-scale turbulence with direct, gyrokinetic simulations of MAST L-mode plasmas
Observations of ion-scale (k_y*rho_i <= 1) density turbulence of relative
amplitude dn_e/n_e <= 0.2% are available on the Mega Amp Spherical Tokamak
(MAST) using a 2D (8 radial x 4 poloidal channel) imaging Beam Emission
Spectroscopy (BES) diagnostic. Spatial and temporal characteristics of this
turbulence, i.e., amplitudes, correlation times, radial and perpendicular
correlation lengths and apparent phase velocities of the density contours, are
determined by means of correlation analysis. For a low-density, L-mode
discharge with strong equilibrium flow shear exhibiting an internal transport
barrier (ITB) in the ion channel, the observed turbulence characteristics are
compared with synthetic density turbulence data generated from global,
non-linear, gyro-kinetic simulations using the particle-in-cell (PIC) code
NEMORB. This validation exercise highlights the need to include increasingly
sophisticated physics, e.g., kinetic treatment of trapped electrons,
equilibrium flow shear and collisions, to reproduce most of the characteristics
of the observed turbulence. Even so, significant discrepancies remain: an
underprediction by the simulations of the turbulence amplituide and heat flux
at plasma periphery and the finding that the correlation times of the
numerically simulated turbulence are typically two orders of magnitude longer
than those measured in MAST. Comparison of these correlation times with various
linear timescales suggests that, while the measured turbulence is strong and
may be `critically balanced', the simulated turbulence is weak.Comment: 27 pages, 11 figure
Experimental Signatures of Critically Balanced Turbulence in MAST
Beam Emission Spectroscopy (BES) measurements of ion-scale density
fluctuations in the MAST tokamak are used to show that the turbulence
correlation time, the drift time associated with ion temperature or density
gradients, the particle (ion) streaming time along the magnetic field and the
magnetic drift time are consistently comparable, suggesting a "critically
balanced" turbulence determined by the local equilibrium. The resulting
scalings of the poloidal and radial correlation lengths are derived and tested.
The nonlinear time inferred from the density fluctuations is longer than the
other times; its ratio to the correlation time scales as
, where ion collision rate/streaming rate.
This is consistent with turbulent decorrelation being controlled by a zonal
component, invisible to the BES, with an amplitude exceeding the drift waves'
by .Comment: 6 pages, 4 figures, submitted to PR
Special behavior of alkali beam emission spectroscopy in low-ion-temperature plasma
Beam emission spectroscopy (BES) is a powerful plasma diagnostic method
especially suited for the measurement of plasma density and its fluctuations.
As such, synthetic BES codes are regularly used to aid the design or
utilization of these diagnostic systems. However, synthetic diagnostics can
also be used to study the method in previously not yet explored operational
conditions. This paper presents such an analysis utilizing the RENATE-OD
synthetic diagnostic code for a hypothetical alkali BES system on the HSX
stellarator. HSX is a device featuring an unusual operating regime in the world
of fusion devices due to the low ion temperature and low plasma density. It was
found that BES shows unusual tendencies in these conditions. The relation
between beam energy and plasma penetration in low-ion-temperature plasma,
together with unique emission features facilitated by low-density plasma, and
the underlying reasons behind these features are explored in this paper
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