258 research outputs found
Reduction in benefits of total flux expansion on divertor detachment due to parallel flows
The Super-X divertor (SXD) is an alternative divertor configuration
leveraging total flux expansion at the outer strike point (OSP). Key features
for the attractiveness of the SXD are facilitated detachment access and
control, as predicted by the extended 2-point model (2PM). However, parallel
flows are not consistently included in the 2PM. In this work, the 2PM is
refined to overcome this limitation: the role of total flux expansion on the
pressure balance is made explicit, by including the effect of parallel flows.
In consequence, the effect of total flux expansion on detachment access and
control is weakened, compared to predictions of the 2PM. This new model
partially explains discrepancies between the 2PM and experiments performed on
TCV, in ohmic L-mode scenarios, where in core density ramps in lower
single-null (SN) configuration, the impact of the OSP major radius Rt on the
CIII emission front movement in the divertor outer leg - used as a proxy for
the plasma temperature - is substantially weaker than 2PM predictions; and in
OSP sweeps in lower and upper SN configurations, with a constant core density,
the peak parallel particle flux density at the OSP is almost independent of Rt,
while the 2PM predicts a linear dependence. Finally, analytical and numerical
modelling of parallel flows in the divertor is presented, to support the
argument. It is shown that an increase in total flux expansion can favour
supersonic flows at the OSP. Parallel flows are also shown to be relevant by
analysing SOLPS-ITER simulations of TCV
Results from recent detachment experiments in alternative divertor configurations on TCV
Divertor detachment is explored on the TCV tokamak in alternative magnetic geometries. Starting from typical TCV single-null shapes, the poloidal flux expansion at the outer strikepoint is varied by a factor of 10 to investigate the X-divertor characteristics, and the total flux expansion is varied by 70% to study the properties of the super-X divertor. The effect of an additional X-point near the target is investigated in X-point target divertors. Detachment of the outer target is studied in these plasmas during Ohmic density ramps and with the ion âB drift away from the primary X-point. The detachment threshold, depth of detachment, and the stability of the radiation location are investigated using target measurements from the wall-embedded Langmuir probes and two-dimensional CIII line emissivity profiles across the divertor region, obtained from inverted, toroidally-integrated camera data. It is found that increasing poloidal flux expansion results in a deeper detachment for a given line-averaged density and a reduction in the radiation location sensitivity to core density, while no large effect on the detachment threshold is observed. The total flux expansion, contrary to expectations, does not show a significant influence on any detachment characteristics in these experiments. In X-point target geometries, no evidence is found for a reduced detachment threshold despite a 2-3 fold increase in connection length. A reduced radiation location sensitivity to core plasma density in the vicinity of the target X-point is suggested by the measurements
Comparison of detachment in Ohmic plasmas with positive and negative triangularity
Detachment is investigated using core density ramps for lower single null
Ohmic L-mode plasmas across a wide range of upper, lower, and total
triangularity () in the TCV tokamak. It is universally found that
detachment is more difficult to access with negative triangularity (NT)
shaping. The outer divertor leg of discharges with could
not be cooled below 5 eV using core density ramps alone. The behavior of the
upstream plasma and geometrical divertor effects (e.g. a reduced connection
length at negative lower triangularity) do not fully explain the challenges of
detaching NT plasmas. Langmuir probe measurements of the target heat flux
widths () remained constant within 30% across an upper triangularity
scan, while the spreading factor was found to be lower by up to 50% in NT,
indicating a generally lower integral SOL width. An interesting pattern has
been observed in the particle balance where the line-averaged core density was
typically higher in NT discharges for a given fuelling rate. Conversely, the
divertor neutral pressure and integrated particle content were typically lower
for the same line-averaged density. This indicates that NT plasmas may be
closer to the sheath-limited regime than their PT counterparts, which could
explain why NT is more challenging to detach
Edge localized mode control with an edge resonant magnetic perturbation
A low amplitude (ÎŽbrâBT=1 part in 5000) edge resonantmagnetic field perturbation with toroidalmode number n=3 and poloidal mode numbers between 8 and 15 has been used to suppress most large type I edge localized modes(ELMs) without degrading core plasma confinement. ELMs have been suppressed for periods of up to 8.6 energy confinement times when the edge safety factor q95 is between 3.5 and 4. The large ELMs are replaced by packets of events (possibly type II ELMs) with small amplitude, narrow radial extent, and a higher level of magnetic field and density fluctuations, creating a duty cycle with long âactiveâ intervals of high transport and short âquietâ intervals of low transport. The increased transport associated with these events is less impulsive and slows the recovery of the pedestal profiles to the values reached just before the large ELMs without the n=3 perturbation. Changing the toroidal phase of the perturbation by 60° with respect to the best ELM suppression case reduces the ELM amplitude and frequency by factors of 2â3 in the divertor, produces a more stochastic response in the H-mode pedestal profiles, and displays similar increases in small scale events, although significant numbers of large ELMs survive. In contrast to the best ELM suppression case where the type I ELMs are also suppressed on the outboard midplane, the midplane recycling increases until individual ELMs are no longer discernable. The ELM response depends on the toroidal phase of the applied perturbation because intrinsic error fields make the target plasma nonaxisymmetric, and suggests that at least some of the variation in ELM behavior in a single device or among different devices is due to differences in the intrinsic error fields in these devices. These results indicate that ELMs can be suppressed by small edge resonantmagnetic field perturbations. Extrapolation to next-step burning plasma devices will require extending the regime of operation to lower collisionality and understanding the physical mechanism responsible for the ELM suppression.This work was funded by the U.S. Department of Energy
under Grant Nos. DE-FC02-04ER54698, DE-FG02-
04ER54758, DE-FG03-01ER54615, W-7405-ENG-48, DEFG03-96ER54373,
DE-FG02-89ER53297, DE-AC05-
00OR22725, and DE-AC04-94AL85000
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Resistive Wall Modes and Plasma Rotation in DIII-D
A271 RESISTIVE WALL MODES AND PLASMA ROTATION IN DIII-D. The stabilization of the resistive wall mode (RWM) by toroidal plasma rotation has been demonstrated in neutral beam heated DIII-D discharges for values of {beta} up to 70% above the no-wall stability limit. The stabilizing effect of plasma rotation is explained by assuming some dissipation, which is caused by the rapid plasma flow through a perturbed magnetic field. Sufficient plasma rotation is predicted to extend the operating regime of tokamaks from the conventional no-wall {beta} limit up to the ideal wall {beta} limit. While plasma rotation has a stabilizing effect on the RWM, a finite amplitude RWM also increases the drag on the plasma, which leads to a non-linear interaction between the RWM and the plasma rotation. A good understanding of the underlying dissipation mechanism is crucial for reliable predictions of the plasma rotation which will be required for wall-stabilization in a burning-plasma experiment. In order to measure the stabilizing effect of plasma rotation on the RWM the technique of active MHD spectroscopy, which was previously applied to MHD modes at frequencies above 10 kHz, is extended to frequencies of a few Hz
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