Abstract
A new coherence imaging spectroscopy (CIS) system has been
characterized and validated for measurements of impurity ion
velocity in the Compact Toroidal Hybrid (CTH) experiment. CIS is an
...interferometric technique providing very high spatial resolution of
line-integrated ion flows; however, these measurements are highly
sensitive to environmental factors such as temperature fluctuations.
A compact interferometer design housed in a thermally regulated
environment yields CIS measurement variations due to thermal
fluctuations of 0.25 km/s. CIS measurements of He
+
flows in
the outer regions of CTH are benchmarked using two optical grating
spectrometers. The deviation between the measurements is typically
less than 2 km/s for multiple lines of sight which is well within
the uncertainties of the diagnostics. Additional experiments
demonstrate a moderate robustness of ion flows in the outer region
of CTH to changing the vacuum transform, direction of the magnetic
field, and plasma current. In contrast, regions of complete flow
reversal are observed when a bias probe is inserted into the plasma
edge.
Results from a joint experimental and computational effort studying the effect of resonant magnetic perturbations (RMPs) on microturbulence levels and their connection to zonal flows in the DIII-D ...tokamak L-mode are presented. Beam emission spectroscopy measurements show a direct increase in density fluctuations at microturbulent scales with increasing RMP amplitude, suggesting that magnetic activity introduced by the RMP affects the regulation of microturbulence on DIII-D. This is analogous to how MHD-scale magnetic fluctuations arising from tearing modes have been observed in simulations to increase microturbulence levels in the reversed-field pinch (RFP). In the RFP, this is attributed to magnetic fluctuations eroding turbulence-limiting zonal flows; this work examines if a similar mechanism is present for DIII-D microturbulence. Gyrokinetic simulations find that the application of an RMP corresponds directly to a decrease in zonal flow levels, producing a similar increase of turbulent fluctuation levels over a range of RMP amplitudes as observed in the experiment.
Tokamak experiments at near-unity aspect ratio A≲1.2 offer new insights into the self-organized H-mode plasma confinement regime. In contrast to conventional A∼3 plasmas, the L-H power threshold ...P_{LH} is ∼15× higher than scaling predictions, and it is insensitive to magnetic topology, consistent with modeling. Edge localized mode (ELM) instabilities shift to lower toroidal mode numbers as A decreases. These ultralow-A operations enable heretofore inaccessible J_{edge}(R,t) measurements through an ELM that show a complex multimodal collapse and the ejection of a current-carrying filament.
Full text
Available for:
CMK, CTK, FMFMET, IJS, NUK, PNG, UM
Abstract In this work, the Coherence Imaging Spectroscopy technique is exploited for active charge exchange radiation measurements to infer high spatial resolution 2D ion temperature ( T i ) maps in ...the core region of Wendelstein 7-X plasmas. A synthetic model of the diagnostic is developed and used for the optimization of the hardware components for the expected ion temperatures ( T i ∼ 2 keV) prioritizing T i measurements while also considering the ion velocity flow resolution. The experimental set-up is shown and the diagnostic calibration procedure for T i measurements is introduced. A combination of both simulations and experimental calibrations enable high fidelity system group delay ( ∂ ϕ ∂ λ ) characterization in the whole visible spectral range. Finally, the signal processing techniques applied to the diagnostic signal are introduced and first measurements of 2D T i maps are presented and validated against standard Charge eXchange Recombination Spectroscopy T i profiles, finding excellent agreement.
Abstract
The edge island domain in Wendelstein 7-X consists of divertor islands whose position coincide with the location of rational values of the rotational transform
-
ι
≈
(
5
/
6
,
5
/
5
,
5
/
4
...)
and surround the main confinement volume (the ‘main plasma’), providing the magnetic topology for an island divertor. The 5/5 edge consists of five individual islands that are unconnected. In contrast, a single island connects onto itself after 6 or 4 toroidal transits in the 5/6 or 5/4 edge, respectively. Many interesting phenomena are related to these islands and diagnostic analyses require a mapping from ‘laboratory’ or real space coordinates to the island coordinate system (ICS). A procedure is described here to calculate several scalar and vector quantities for closed island structures which can be utilized in fast interpolation schemes for inverse maps. For the 5/5 edge, a fixed-boundary vacuum (zero beta) magneto-hydrodynamic solution of the 5/5 island is found with the Variational Moments Equilibrium Code, VMEC. The solution is compatible with already existing routines which determine the radial and binormal vectors of VMEC solutions at arbitrary laboratory coordinates. VMEC does not support solutions for the 5/4 and 5/6 islands, but the radial and binormal vectors are available in a local 2D Fourier ICS.
H-mode is obtained at A∼1.2 in the Pegasus Toroidal Experiment via Ohmic heating, high-field-side fueling, and low edge recycling in both limited and diverted magnetic topologies. These H-mode ...plasmas show the formation of edge current and pressure pedestals and a doubling of the energy confinement time to H98y,2∼1. The L-H power threshold PLH increases with density, and there is no PLH minimum observed in the attainable density space. The power threshold is equivalent in limited and diverted plasmas, consistent with the FM3 model. However, the measured PLH is ∼15 × higher than that predicted by conventional International Tokamak Physics Activity (ITPA) scalings, and PLH/PITPA08 increases as A→1. Small ELMs are present at low input power PIN∼PLH, with toroidal mode number n 4. At PIN≫PLH, they transition to large ELMs with intermediate 5<n<15. The dominant-n component of a large ELM grows exponentially, while other components evolve nonlinearly and can damp prior to the crash. Direct measurements of the current profile in the pedestal region show that both ELM types exhibit a generation of a current-hole, followed by a pedestal recovery. Large ELMs are shown to further expel a current-carrying filament. Small ELM suppression via injection of low levels of helical current into the edge plasma region is also indicated.
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