High-bootstrap, noninductively sustained electron internal transport barriers in the Tokamak à Configuration Variable

Coda, S.; Goodman, T. P.; Henderson, M.; Sauter, O.; Behn, R.; Bottino, A.; Camenen, Y.; Fable, E.; Martynov, An.; Nikkola, P.; Scarabosio, A.; Zhuang, G.; Zucca, C.

doi:10.1063/1.1896953

Cited by 17 publications

(17 citation statements)

References 36 publications

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“…While rational q surfaces do not appear to play a role in the formation of eITBs, as discussed in section 2, the q profile does affect the MHD stability of the discharge, and strong internal modes develop in some cases which can significantly degrade the confinement [5].…”

Section: Mhd Activity In Eitbsmentioning

confidence: 99%

“…Fully noninductive scenarios involve an appropriate distribution of current drive (ECCD) sources sustaining a hollow current profile, further enhanced by the bootstrap current centred in the high gradient barrier region [4][5][6][7]. Depending on the details of the discharge parameters and conditions, these scenarios may or may not evolve to a true steady state, whose duration is limited merely by equipment constraints and can equal several current redistribution times and up to hundreds of electron energy confinement times.…”

Section: Introductionmentioning

confidence: 99%

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The physics of electron internal transport barriers in the TCV tokamak

et al. 2007

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Electron internal transport barriers (eITBs) are generated in the TCV tokamak with strong electron cyclotron resonance heating in a variety of conditions, ranging from steady-state fully noninductive scenarios to stationary discharges with a finite inductive component and finally to transient current ramps without current drive. The confinement improvement over L-mode ranges from 3 to 6; the bootstrap current fraction is invariably large and is above 70% in the highest confinement cases, with good current profile alignment permitting the attainment of steady state. Barriers are observed both in the electron temperature and density profiles, with a strong correlation both in location and in steepness. The dominant role of the current profile in the formation and properties of eITBs has been conclusively proven in a TCV experiment exploiting the large current drive efficiency of the Ohmic transformer: small current perturbations accompanied by negligible energy transfer dramatically alter the confinement. The crucial element in the formation of the barrier is the appearance of a central region of negative magnetic shear, with the barrier strength improving with increasingly steep shear. This connection has also been corroborated by transport modelling assisted by gyrofluid simulations. Rational safety-factor (q) values do not appear to play a role in the barrier formation, at least in the q range 1.3-2.3, as evidenced by the smooth dependence of the confinement enhancement on the loop voltage over a broad eITB database. MHD mode activity is however influenced by rational q values and results in a complex, sometimes cyclic, dynamic evolution.

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Section: Mhd Activity In Eitbsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

The physics of electron internal transport barriers in the TCV tokamak

et al. 2007

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“…In previous experiments, many different q profiles have been studied for use in advanced tokamak discharges with a high bootstrap current fraction [3,[12][13][14][15][16][17][18][19]. These reports focused primarily on specific q profiles where high-performance discharges were successfully produced.…”

Section: Introductionmentioning

confidence: 99%

Optimization of the safety factor profile for high noninductive current fraction discharges in DIII-D

et al. 2011

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Abstract. In order to assess the optimum q profile for discharges in DIII-D with 100% of the current driven noninductively (f NI = 1), the self-consistent response of the plasma profiles to changes in the q profile was studied in high f NI , high N discharges through a scan of q min and q 95 at two values of N . As expected, both the bootstrap current fraction, f BS , and f NI increased with q 95 . The temperature and density profiles were found to broaden as either q min or N is increased.A consequence is that f BS does not continue to increase at the highest values of q min . A scaling function that depends on q min , q 95 , and the peaking factor for the thermal pressure was found to represent well the f BS / N inferred from the experimental profiles. The changes in the shapes of the 2 density and temperature profiles as N is increased modify the bootstrap current density (J BS ) profile from peaked close to the axis to relatively flat in the region between the axis and the H-mode pedestal. Therefore, significant externally-driven current density in the region inside the H-mode pedestal is required in addition to J BS in order to match the profiles of the noninductive current density (J NI ) to the desired total current density (J). In this experiment, the additional current density was provided mostly by neutral beam current drive with the neutral-beam-driven current fraction 40%-90% of f BS . The profiles of J NI and J were most similar at q min 1.35-1.65, q 95 6.8, where f BS is also maximum, establishing this q profile as the optimal choice for f NI = 1 operation in DIII-D with the existing set of external current drive sources.3

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“…Auxiliary heating is provided by Electron Cyclotron Resonance Heating (ECRH), supplied by nine 0.5-MW gyrotrons (six at the 2 nd (X2) and three at the 3 rd (X3) harmonic, respectively 82.7 and 118.0 GHz) to seven launchers with independent real-time steering capabilities. Fully non-inductive operation with Electron Cyclotron Current Drive (ECCD) is routinely performed up to a current of 210 kA [6], and a wide range of regimes is accessible, including internal transport barriers with reverse magnetic shear [7] and quiescent ELM-free H-modes [8], spanning a ratio of electron to ion temperature from 1 to 20. The dominant electron heating is an effective simulator of aspects of ITER [9] alpha-particle heating, while the overall flexibility encourages investigations of alternative paths to commercial fusion.…”

Section: Ov/4-4mentioning

confidence: 99%

Overview of recent and current research on the TCV tokamak

Coda

2013

Nucl. Fusion

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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 frequency. 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.

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High-bootstrap, noninductively sustained electron internal transport barriers in the Tokamak à Configuration Variable

Cited by 17 publications

References 36 publications

The physics of electron internal transport barriers in the TCV tokamak

The physics of electron internal transport barriers in the TCV tokamak

Optimization of the safety factor profile for high noninductive current fraction discharges in DIII-D

Overview of recent and current research on the TCV tokamak

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