Density pumpout and edge-localized mode (ELM) suppression by applied n=2 magnetic fields in low-collisionality DIII-D plasmas are shown to be correlated with the magnitude of the plasma response ...driven on the high-field side (HFS) of the magnetic axis but not the low-field side (LFS) midplane. These distinct responses are a direct measurement of a multimodal magnetic plasma response, with each structure preferentially excited by a different n=2 applied spectrum and preferentially detected on the LFS or HFS. Ideal and resistive magneto-hydrodynamic (MHD) calculations find that the LFS measurement is primarily sensitive to the excitation of stable kink modes, while the HFS measurement is primarily sensitive to resonant currents (whether fully shielding or partially penetrated). The resonant currents are themselves strongly modified by kink excitation, with the optimal applied field pitch for pumpout and ELM suppression significantly differing from equilibrium field alignment.
Rapid bifurcations in the plasma response to slowly varying n=2 magnetic fields are observed as the plasma transitions into and out of edge-localized mode (ELM) suppression. The rapid transition to ...ELM suppression is characterized by an increase in the toroidal rotation and a reduction in the electron pressure gradient at the top of the pedestal that reduces the perpendicular electron flow there to near zero. These events occur simultaneously with an increase in the inner-wall magnetic response. These observations are consistent with strong resonant field penetration of n=2 fields at the onset of ELM suppression, based on extended MHD simulations using measured plasma profiles. Spontaneous transitions into (and out of) ELM suppression with a static applied n=2 field indicate competing mechanisms of screening and penetration of resonant fields near threshold conditions. Magnetic measurements reveal evidence for the unlocking and rotation of tearinglike structures as the plasma transitions out of ELM suppression.
We observe the formation of a high-pressure staircase pedestal (≈16–20 kPa) in the DIII-D tokamak when large amplitude edge localized modes are suppressed using resonant magnetic perturbations. The ...staircase pedestal is characterized by a flattening of the density and temperature profiles in midpedestal creating a two-step staircase pedestal structure correlated with the appearance of midpedestal broadband fluctuations. The pedestal oscillates between the staircase and single-step structure every 40–60 ms, correlated with oscillations in the heat and particle flux to the divertor. Gyrokinetic analysis using the cgyro code shows that when the heat and particle flux to the divertor decreases, the pedestal broadens and the E×B shear at the midpedestal decreases, triggering a transport bifurcation from the kinetic ballooning mode (KBM) to trapped electron mode (TEM) limited transport that flattens the density and temperature profiles at midpedestal and results in the formation of the staircase pedestal. As the heat flux to the divertor increases, the pedestal narrows and the E×B shear at the midpedestal increases, triggering a back transition from TEM to KBM limited transport. The pedestal pressure increases during the staircase phase, indicating that enhanced midpedestal turbulence can be beneficial for confinement.
Detailed measurements of the main ion ( D + ) and impurity ion ( C 6 + ) evolution during the development of the H-mode pedestal across an L-H transition show significant differences in toroidal ...rotation, density, and temperature profiles in the pedestal region on DIII-D. While both species experience a slow toroidal spin up at constant input neutral beam injected torque, the C 6 + toroidal rotation develops a non monotonic notch feature and lower toroidal rotation near the plasma edge immediately following the L-H transition. This feature is not present in the main ion rotation that instead, depending on plasma parameters, can show a flat or peaked rotation near the separatrix. The D + and C 6 + temperature profiles show a similar evolution; however, the D+ temperature is lower than the C 6 + temperature at the separatrix in both L and H-mode which may be due to cooling of D + via charge exchange with cold edge deuterium neutrals. Local neoclassical predictions of the main ion toroidal rotation based on the impurity properties show good agreement with direct measurements at the pedestal top for a lower power, higher collisionality case but can diverge significantly in the steep gradient region for the two shots studied here. These observations highlight the importance of directly measuring the properties of the main ion species at the plasma edge.
Abstract
Recent dedicated DIII-D experiments in low-torque, ITER-similar-shape (ISS) hydrogen plasmas (at a plasma current
I
p
∼ 1.5 MA and ITER-similar edge safety factor
q
95
∼ 3.6) show that the ...L-H transition power threshold
P
LH
can be reduced substantially (∼30%) with L-mode helium admixtures
n
He
/
n
e
⩽ 25%. In the ensuing H-mode, helium ion fractions
n
He
/
n
H
remain below 25%. H-mode normalized pressure and confinement quality are only slightly affected by helium seeding, and
Z
eff
⩽ 2.15 (including helium and carbon content). The plasmas investigated here are electron-heat dominated, with temperatures
T
e
(0)/
T
i
(0) ⩾ 1 and edge heat flux ratio
Q
e
/
Q
i
(
ρ
= 0.95) ∼ 1.2–1.5. Without mitigation,
P
LH
is higher by a factor of 2–3 in comparison to similar ISS deuterium plasmas. ISS hydrogen plasmas with lower plasma current
I
p
∼ 1 MA (increased edge safety factor
q
95
∼ 5.1) exhibit a substantially lower power threshold. This plasma current dependence, also observed previously on ASDEX-U and in JET, is not accounted for by the commonly used 2008 ITPA multi-machine threshold scaling, but could potentially allow H-mode access at marginal heating power during the initial plasma current ramp-up. Attempts to reduce
P
LH
with low-field- and high-field-side hydrogen pellet injection, using 1.7 mm diameter pellets, have not demonstrated a robust threshold reduction, in contrast to successful earlier experiments with larger 2.7 mm pellets. Techniques for reducing
P
LH
are very important for ITER, in particular for accessing H-mode in hydrogen plasmas during the Pre-Fusion Power Operation-1 (PFPO-1) campaign with marginal auxiliary heating power (20–30 MW of ECH).
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