[Show abstract][Hide abstract] ABSTRACT: The design of a snowflake (SF) equilibrium requires a strong effort on the poloidal field (PF) currents in terms of MAturns and mechanical loads. This has limited the maximum plasma current in SF configurations on Tokamak à Configuration Variable (TCV) to values well below the intrinsic magnetohydrodynamic limits. In this paper the definition of optimized SF configurations in TCV and their experimental tests are illustrated. The PF current optimization procedure proposed in Albanese et al (2014 Plasma Phys. Control. Fusion 56 035008) is adapted and applied to a SF scenario in TCV where the PF currents were close to their operational limits with the aim of reducing the total MAturns in view of higher values of the plasma current. This procedure optimizes the PF currents while fulfilling the machine technological constraints for a given bound on the tolerable plasma shape changes. The method exploits the linearized relation between the plasma–wall gaps and the PF currents. In the investigated TCV scenario the optimization procedure allowed a 20% increase of the plasma current while keeping the plasma shape alignment with respect to the nominal shape within a tolerance of 1 cm. The predicted optimization potential was confirmed in a TCV experiment.
[Show abstract][Hide abstract] ABSTRACT: The nonlinear response of a low-beta tokamak plasma to non-axisymmetric fields offers an alternative to direct measurement of the non-axisymmetric part of the vacuum magnetic fields, often termed 'error fields'. Possible approaches are discussed for determination of error fields and the required current in non-axisymmetric correction coils, with an emphasis on two relatively new methods: measurement of the torque balance on a saturated magnetic island, and measurement of the braking of plasma rotation in the absence of an island. The former is well suited to ohmically heated discharges, while the latter is more appropriate for discharges with a modest amount of neutral beam heating to drive rotation. Both can potentially provide continuous measurements during a discharge, subject to the limitation of a minimum averaging time. The applicability of these methods to ITER is discussed, and an estimate is made of their uncertainties in light of the specifications of ITER's diagnostic systems. The use of plasma response-based techniques in normal ITER operational scenarios may allow identification of the error field contributions by individual central solenoid coils, but identification of the individual contributions by the outer poloidal field coils or other sources is less likely to be feasible.
[Show abstract][Hide abstract] ABSTRACT: One of the approaches to solve the heat load problem in a divertor tokamak is the so called 'snowflake' (SF) configuration, a magnetic equilibrium with two nearby x-points and two additional divertor legs. Here we report on the first EMC3-Eirene simulations of plasma- and neutral particle transport in the scrape-off layer of a series of TCV SF equilibria with different values of σ, the distance between the x-points normalized to the minor radius of the plasma. The constant cross-field transport coefficients were chosen such that the power- and particle deposition profiles at the primary inner strike point (SP) match the Langmuir probe measurements for the σ = 0.1 case. At the secondary SP on the floor, however, a significantly larger power flux than that predicted by the simulation was measured by the probes, indicating an enhanced transport across the primary separatrix. As the ideal SF configuration (σ = 0) is approached, the density as well as the radiation maximum are predicted to move from the target plates upward to the x-point by the simulation.
[Show abstract][Hide abstract] ABSTRACT: Edge-localized modes (ELMs) are instabilities in the edge of tokamak plasmas in the high confinement regime (H-mode). Despite beneficial aspects of ELMs, in a future device the size of the energy loss per ELM must be controlled, in order to avoid intolerable divertor power flux densities. To proceed in understanding how the ELM size is determined and how ELM mitigation methods work it is necessary to characterize the non-linear evolution of ELMs.
[Show abstract][Hide abstract] ABSTRACT: Experimental studies of the novel snowflake divertor concept (D. Ryutov, Phys. Plasmas 14 (2007) 064502) performed in the NSTX and TCV tokamaks are reviewed in this paper. The snowflake divertor enables power sharing between divertor strike points, as well as the divertor plasma-wetted area, effective connection length and divertor volumetric power loss to increase beyond those in the standard divertor, potentially reducing heat flux and plasma temperature at the target. It also enables higher magnetic shear inside the separatrix, potentially affecting pedestal MHD stability. Experimental results from NSTX and TCV confirm the predicted properties of the snowflake divertor. In the NSTX, a large spherical tokamak with a compact divertor and lithium-coated graphite plasma-facing components (PFCs), the snowflake divertor operation led to reduced core and pedestal impurity concentration, as well as re-appearance of Type I ELMs that were suppressed in standard divertor H-mode discharges. In the divertor, an otherwise inaccessible partial detachment of the outer strike point with an up to 50% increase in divertor radiation and a peak divertor heat flux reduction from 3–7 MW/m2 to 0.5–1 MW/m2 was achieved. Impulsive heat fluxes due to Type-I ELMs were significantly dissipated in the high magnetic flux expansion region. In the TCV, a medium-size tokamak with graphite PFCs, several advantageous snowflake divertor features (cf. the standard divertor) have been demonstrated: an unchanged L–H power threshold, enhanced stability of the peeling–ballooning modes in the pedestal region (and generally an extended second stability region), as well as an H-mode pedestal regime with reduced (×2–3) Type I ELM frequency and slightly increased (20–30%) normalized ELM energy, resulting in a favorable average energy loss comparison to the standard divertor. In the divertor, ELM power partitioning between snowflake divertor strike points was demonstrated. The NSTX and TCV experiments are providing support for the snowflake divertor as a viable solution for the outstanding tokamak plasma–material interface issues.
Journal of Nuclear Materials 07/2013; 438:S96–S101. · 2.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Tokamaks with weak to moderate reversed central shear in which the minimum inverse rotational transform (safety factor) qmin is in the neighbourhood of unity can trigger bifurcated magnetohydrodynamic equilibrium states, one of which is similar to a saturated ideal internal kink mode. Peaked prescribed pressure profiles reproduce the ‘snake’ structures observed in many tokamaks which has led to a novel explanation of the snake as a bifurcated equilibrium state. Snake equilibrium structures are computed in simulations of the tokamak `a configuration variable (TCV), DIII-D and mega amp spherical torus (MAST) tokamaks. The internal helical deformations only weakly modulate the plasma–vacuum interface which is more sensitive to ripple and resonant magnetic perturbations. On the other hand, the external perturbations do not alter the helical core deformation in a significant manner. The confinement of fast particles in MAST simulations deteriorate with the amplitude of the helical core distortion. These three-dimensional bifurcated solutions constitute a paradigm shift that motivates the applications of tools developed for stellarator research in tokamak physics investigations.
[Show abstract][Hide abstract] ABSTRACT: Through a diverse research program, TCV addresses physics issues and develops tools for ITER and for the longer-term goals of nuclear fusion, relying especially on its extreme plasma shaping and ECRH launching flexibility and preparing for an ECRH and NBI power upgrade. Localized edge heating was unexpectedly found to decrease the period and relative energy loss of ELMs. Successful ELM pacing has
been demonstrated by following individual ELM detection with an ECRH power cut before turning the power back up to trigger the next ELM, the duration of the cut determining the ELM period. Negative triangularity was also seen to reduce the ELM energy release. H-mode studies have focused on the L–H threshold dependence on the main ion species and on the divertor leg length. Both L– and H–modes have been explored in the snowflake configuration with emphasis on edge measurements, revealing
that the heat flux to the strike points on the secondary separatrix increases as the X–points approach each other, well before they coalesce. In L–mode, a systematic scan of the auxiliary power deposition profile, with no effect on confinement, has ruled it out as the cause of confinement degradation. An ECRH power absorption observer based on transmitted stray radiation was validated for eventual polarization
control. A new profile control methodology was introduced, relying on real-time modeling to supplement diagnostic information; the raptor current transport code in particular has been employed for joint control of the internal inductance and central temperature.
An internal inductance controller using the Ohmic transformer has also been demonstrated.
Fundamental investigations of NTM seed island formation by
sawtooth crashes and of NTM destabilization in the absence of a sawtooth trigger were carried out. Both stabilizing and destabilizing agents (ECCD on or inside the q=1 surface, respectively) were used to pace sawtooth oscillations, permitting precise control of their period. Locking of the sawtooth period to a pre-defined ECRH modulation period was also demonstrated. Sawtooth control has permitted nearly
failsafe NTM prevention when combined with backup NTM stabilization by ECRH.
[Show abstract][Hide abstract] ABSTRACT: The onset and sawtooth triggering mechanisms for Tearing Modes (TMs) has
been extensively investigated during recent experimental campaigns on
the Tokamak a Configuration Variable. The main long-term aim of this
work is to provide understanding of the relation between sawteeth and
TMs so that reliable real-time schemes can be devised for combined
sawtooth and TM control in burning plasma experiments such as ITER.
Hence, our work has focused on studying the dynamical relation between
sawtooth crash and subsequent onset of TMs, sometimes leading to
disruptions, using control techniques for the duration of the sawtooth
period and the TM seeding mechanism via real-time pacing and localized
electron cyclotron heating and current drive.
[Show abstract][Hide abstract] ABSTRACT: Recent DIII-D experiments have shown the effect of off-axis neutral beam
injection (NBI) on the resistive wall mode (RWM) stability, evaluated by
means of active MHD spectroscopy. This work is focused on new modeling
efforts aimed at investigating the role of kinetic damping in the
stabilization of the RWM. In DIII-D experiments, the latter is affected
by the changes in the fast ion distribution, due to varying on- and
off-axis beam combinations used to sustain the plasma current. The MARS
code is used to evaluate the ideal stability and the predicted plasma
response using modeled experimental equilibria, which are compared to
experimental results. Results on pressure, frequency and plasma rotation
scans are presented, comparing equilibria with constant beta, and
different fractions of off-axis NBI power. Comparison of the kinetic
damping model to experiment will shed light on the role of fast ions in
the stability of the RWM at moderate to high beta plasmas.
[Show abstract][Hide abstract] ABSTRACT: Recent DIII-D experiments have investigated the effects of localized
magnetic field perturbations, using coils that approximate the
magnetization of the test blanket modules (TBMs) in one ITER port. In
H-mode discharges, compensation of the TBM field using an applied n=1
field yielded only partial recovery of the plasma rotation, and the
compensation field that maximized plasma rotation differed significantly
from the field that reduced the resonant magnetic response to a very low
value. These results provide insight into the effects of error fields,
and suggest an important role for non-resonant magnetic braking. In
addition, measurements of localized heat deposition with the TBM field
are being compared to orbit following calculations of fast ion loss, and
a new fast ion detector has confirmed earlier observations of reduced 1
MeV triton confinement.
[Show abstract][Hide abstract] ABSTRACT: DIII-D experiments with off-axis NBI yield evidence for the impact of
passing fast ions on resistive wall mode (RWM) stability. The fast ion
radial and pitch angle distribution can be modified in DIII-D by
off-axis neutral beam injection (NBI). Off-axis injection results in an
increased fraction of passing ions relative to on-axis injection. RWM
stability is assessed by measuring the plasma response to a slowly
rotating n=1 perturbation. The plasma response decreases in amplitude as
the fraction of off-axis neutral beam injected ions is increased at
constant normalized beta, implying increased RWM damping. Transport and
stability modeling using a fixed pressure profile also indicate
increased RWM damping with off-axis NBI, due to increased damping from
passing fast ions. While previous investigations have confirmed the
importance of trapped thermal and fast ions, this result is the first
experimental evidence of the significance of passing fast ions for RWM
[Show abstract][Hide abstract] ABSTRACT: Recent studies have determined the scale and likely origins of
limitations to error field correction by using DIII-D's multiple coil
arrays to apply known large amplitude proxy error fields and attempting
correction with additional coils of different structure. It was found
that even with pure n=1 proxy fields and carefully optimized correction
field, the benefits of correction were substantially limited, at the
˜50% level in terms of low density access. This indicates coupling
of residual fields either through higher order resonances and/or through
non-resonant braking of the plasma The interpretation is confirmed by
modeling with the IPEC code, which shows that the correction process
reduces resonant components, but increases non-resonant NTV damping,
thus decreasing rotation and easing penetration of residual resonant
fields. The result is significant, suggesting multiple field components
must be compensated to achieve good correction, and that the best
approach may be to minimize the total field in the plasma by cancelling
error fields close to their source or close to the plasma.
[Show abstract][Hide abstract] ABSTRACT: In this paper, we introduce the first direct perturbed particle transport measurements in resonant magnetic perturbation (RMP) H-mode plasmas. The perturbed particle transport increases as a result of application of RMP deep into the core. In the core, a large reduction in E Multiplication-Sign B shear to a value below the linear growth rate, in conjunction with increasing density fluctuations, is consistent with an increase in turbulent particle transport. In the edge, the changes in turbulent particle transport are less obvious. There is a clear correlation between the linear growth rates and the density fluctuations measured at different scales, but it is uncertain which is the cause and which is the consequence.
Physics of Plasmas 05/2012; 19(5). · 2.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Tokamaks with weak to moderate reversed central shear in which the minimum rotational transform $q_min$ is in the neighbourhood of unity can trigger bifurcated MagnetoHydrodynamic (MHD) equilibrium states, one of which is similar to a saturated ideal internak kink mode. Peaked prescribed pressure profiles reproduce the "snake'' structures observed in many tokamaks which has led to a novel explanation of the snake as a bifurcated equilibrium state.Snake equilibrium structures are computed in simulations of the TCV, DIII-D and MAST tokamaks. The internal helical deformations only weakly modulate the plasma-vacuum interface which is more sensitive to ripple and resonant magnetic perturbations. On the other hand, the external perturbations do not alter the helical core deformation in a significant manner. The confinement of fast particles in MAST simulations deteriorate with the amplitude of the helical core distortion. These three-dimensional bifurcated solutions constitute a paradigm shift that motivates the applications of tools developed for stellarator research in tokamak physics investigations.
Proceedings of the 24th IAEA Fusion Energy Conference; 01/2012
[Show abstract][Hide abstract] ABSTRACT: Edge localized mode (ELM) pacing using modulated n = 3 non-axisymmetric fields has been demonstrated on DIII-D over a wide range of conditions, including significant variations in temperature, βN, density and shape. At low collisionality, the pacing results in a clear reduction in the ELM size and peak heat flux to the divertor, up to a factor of 5–6 for short time windows, although only a factor of two for sustained periods with the present hardware capability. At higher collisionality, although similar increases in the ELM frequency have been demonstrated, no meaningful reduction in the heat flux is typically observed as a direct result of the pacing. However, it appears that the ELM size may be reduced indirectly via changes in the L–H power threshold as a result of density pumpout associated with the application of non-axisymmetric fields. At this stage, it remains unclear whether the failure to reduce the ELM size with modulated fields is a limitation associated with high collisionality, high density, or relative proximity to the L–H power threshold.
[Show abstract][Hide abstract] ABSTRACT: Previous real-time sawtooth control scenarios using EC actuators have attempted to shorten or lengthen the sawtooth period by optimally positioning the EC absorption near the q = 1 surface. In new experiments we demonstrate for the first time that individual sawtooth crashes can be repetitively induced at predictable times by reducing the stabilizing ECCD power after a predetermined time from the preceding crash. Other stabilizing actuators (e.g. ICRF, NBI) are expected to produce similar effects. Armed with these results, we present a new sawtooth / NTM control paradigm for improved performance in burning plasmas. The potential appearance of neo-classical tearing modes, triggered by long period sawtooth crashes even at low beta, becomes predictable and therefore amenable to preemptive ECCD. The ITER Electron Cyclotron Upper Launcher (EC-UL) design incorporates the needed functionalities for this method to be applied. The methodology and associated TCV experiments will be presented.