809 research outputs found
Nonlinear stabilization of tokamak microturbulence by fast ions
Nonlinear electromagnetic stabilization by suprathermal pressure gradients
found in specific regimes is shown to be a key factor in reducing tokamak
microturbulence, augmenting significantly the thermal pressure electromagnetic
stabilization. Based on nonlinear gyrokinetic simulations investigating a set
of ion heat transport experiments on the JET tokamak, described by Mantica et
al. [Phys. Rev. Lett. 107 135004 (2011)], this result explains the
experimentally observed ion heat flux and stiffness reduction. These findings
are expected to improve the extrapolation of advanced tokamak scenarios to
reactor relevant regimes.Comment: 5 pages, 5 figure
Electromagnetic stabilization of tokamak microturbulence in a high- regime
The impact of electromagnetic stabilization and flow shear stabilization on
ITG turbulence is investigated. Analysis of a low- JET L-mode discharge
illustrates the relation between ITG stabilization, and proximity to the
electromagnetic instability threshold. This threshold is reduced by
suprathermal pressure gradients, highlighting the effectiveness of fast ions in
ITG stabilization. Extensive linear and nonlinear gyrokinetic simulations are
then carried out for the high- JET hybrid discharge 75225, at two
separate locations at inner and outer radii. It is found that at the inner
radius, nonlinear electromagnetic stabilization is dominant, and is critical
for achieving simulated heat fluxes in agreement with the experiment. The
enhancement of this effect by suprathermal pressure also remains significant.
It is also found that flow shear stabilization is not effective at the inner
radii. However, at outer radii the situation is reversed. Electromagnetic
stabilization is negligible while the flow shear stabilization is significant.
These results constitute the high- generalization of comparable
observations found at low- at JET. This is encouraging for the
extrapolation of electromagnetic ITG stabilization to future devices. An
estimation of the impact of this effect on the ITER hybrid scenario leads to a
20% fusion power improvement.Comment: 10 pages, 13 figures. Paper coupled to invited talk at the 41st EPS
conference, Berlin, 201
Control of the tokamak safety factor profile with time-varying constraints using MPC
A controller is designed for the tokamak safety factor profile that takes real-time-varying operational and physics limits into account. This so-called model predictive controller (MPC) employs a prediction model in order to compute optimal control inputs that satisfy the given limits. The use of linearized models around a reference trajectory results in a quadratic programming problem that can easily be solved online. The performance of the controller is analysed in a set of ITER L-mode scenarios simulated with the non-linear plasma transport code RAPTOR. It is shown that the controller can reduce the tracking error due to an overestimation or underestimation of the modelled transport, while making a trade-off between residual error and amount of controller action. It is also shown that the controller can account for a sudden decrease in the available actuator power, while providing warnings ahead of time about expected violations of operational and physics limits. This controller can be extended and implemented in existing tokamaks in the near future.</p
Magnetic Barriers and their q95 dependence at DIII-D
It is well known that externally generated resonant magnetic perturbations
(RMPs) can form islands in the plasma edge. In turn, large overlapping islands
generate stochastic fields, which are believed to play a role in the avoidance
and suppression of edge localized modes (ELMs) at DIII-D. However, large
coalescing islands can also generate, in the middle of these stochastic
regions, KAM surfaces effectively acting as "barriers" against field-line
dispersion and, indirectly, particle diffusion. It was predicted in [H. Ali and
A. Punjabi, Plasma Phys. Control. Fusion 49 (2007), 1565-1582] that such
magnetic barriers can form in piecewise analytic DIII-D plasma equilibria. In
the present work, the formation of magnetic barriers at DIII-D is corroborated
by field-line tracing calculations using experimentally constrained EFIT [L.
Lao, et al., Nucl. Fusion 25, 1611 (1985)] DIII-D equilibria perturbed to
include the vacuum field from the internal coils utilized in the experiments.
According to these calculations, the occurrence and location of magnetic
barriers depends on the edge safety factor q95. It was thus suggested that
magnetic barriers might contribute to narrowing the edge stochastic layer and
play an indirect role in the RMPs failing to control ELMs for certain values of
q95. The analysis of DIII-D discharges where q95 was varied, however, does not
show anti-correlation between barrier formation and ELM suppression
ITER-like current ramps in JET with ILW: experiments, modelling and consequences for ITER
Since the ITER-like wall in JET (JET-ILW) came into operation, dedicated ITER-like plasma current ( I p ) ramp-up (RU) and ramp-down (RD) experiments have been performed and matched to similar discharges with the carbon wall (JET-C). The experiments show that access to H-mode early in the I p</p
ITER-like current ramps in JET with ILW: experiments, modelling and consequences for ITER
Since the ITER-like wall in JET (JET-ILW) came into operation, dedicated ITER-like plasma current ( I p ) ramp-up (RU) and ramp-down (RD) experiments have been performed and matched to similar discharges with the carbon wall (JET-C). The experiments show that access to H-mode early in the I p</p
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