Destabilization of fast particle stabilized sawteeth in ASDEX Upgrade with electron cyclotron current drive

Igochine, V.; Chapman, I. T.; Bobkov, V.; Günter, S.; Maraschek, M.; Moseev, D.; Pereversev, G.; Reich, M.; Stober, J.

doi:10.1088/0741-3335/53/2/022002

Cited by 12 publications

(14 citation statements)

References 19 publications

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“…The detailed distribution of NBI generated fast ions around the close q = 1 surface plays a crucial role there. The application of ECCD is able to destabilize these artificially stabilized large sawteeth again and consequently remove the large sawteeth, which potentially trigger NTMs [93]. An comprehensive overview over the control of sawteeth is given in [94].…”

Section: Avoidance Of Ntm Triggering Mhdmentioning

confidence: 99%

Control of neoclassical tearing modes

Maraschek

2012

Nucl. Fusion

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Abstract. Neoclassically driven tearing modes (NTM) are a major problem for tokamaks operating in a conventional ELMy H-mode scenario. Depending on the mode numbers these pressure driven perturbations cause a mild reduction of the maximum achievable β N = β t /() before the onset of the NTM, or can even lead to disruptions at low edge safety factor, q 95 . A control of this type of modes in high β N plasmas is therefore of vital interest for magnetically confined fusion plasmas. The control consists of two major approaches, namely the control of the excitation of these modes and the removal, or at least mitigation, of these modes, once an excitation could not be avoided. For both routes examples will be given and the applicability of these approaches to ITER will be discussed.

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Section: Avoidance Of Ntm Triggering Mhdmentioning

confidence: 99%

Control of neoclassical tearing modes

Maraschek

2012

Nucl. Fusion

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“…Of course δW will also evolve during the long quiescent ramp phase of a sawtooth but, in what follows, we assume that δW may have reached a quasi steady state and that the resistive evolution of q 0 , or, of r 1 q ′ (r 1 ), may have a crucial influence in triggering the next sawtooth collapse. Evidence in support of this picture appeared in the, very effective, triggering of a sawtooth collapse using ECCD in ASDEX [6,7].…”

Section: Introductionmentioning

confidence: 93%

Improved criterion for sawtooth trigger and modelling

Zocco¹,

Connor²,

Gimblett³

et al. 2013

Plasma Phys. Control. Fusion

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We discuss the role of neoclassical resistivity and local magnetic shear in the triggering of the sawtooth in tokamaks. When collisional detrapping of electrons is considered the value of the safety factor on axis, q(0, t), evolves on a new time scale, τ * = την * /(8 √ ǫ), where τη = 4πa 2 /[c 2 η (0)] is the resistive diffusion time, ν * = νe/(ǫ 3/2 ωte) the electron collision frequency normalised to the transit frequency and ǫ = a/R0 the tokamak inverse aspect ratio. Such evolution is characterised by the formation of a structure of size δ * ∼ ν 2/3 * a around the magnetic axis, which can drive rapid evolution of the magnetic shear and decrease of q(0, t). We investigate two possible trigger mechanisms for a sawtooth collapse corresponding to crossing the linear threshold for the m = 1, n = 1 instability and non-linear triggering of this mode by a core resonant mode near the magnetic axis. The sawtooth period in each case is determined by the time for the resistive evolution of the q-profile to reach the relevant stability threshold; in the latter case it can be strongly affected by ν * .

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“…Sawtooth destabilisation of long period sawteeth induced by ICRH generated core fast ions with energies ≥ 0.5MeV was achieved in Tore Supra, even with modest levels of ECCD power [47,48]. Similarly, ECCD destabilisation has also been achieved in the presence of ICRH accelerated neutral beam injection (NBI) ions in ASDEX Upgrade [49] as well as with normal NBI fast ions in ASDEX Upgrade [50] and JT-60U [51]. More recently sawtooth control using ECCD has even been demonstrated in ITER-like plasmas with a large fast ion fraction, wide q = 1 radius and long uncontrolled sawtooth periods in DIII-D [52].…”

Section: Experimental Evidence Of Sawtooth Control In the Presence Ofmentioning

confidence: 99%

Power requirements for electron cyclotron current drive and ion cyclotron resonance heating for sawtooth control in ITER

et al. 2013

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Abstract.13MW of electron cyclotron current drive (ECCD) power deposited inside the q = 1 surface is likely to reduce the sawtooth period in ITER baseline scenario below the level empirically predicted to trigger neo-classical tearing modes (NTMs). However, since the ECCD control scheme is solely predicated upon changing the local magnetic shear, it is prudent to plan to use a complementary scheme which directly decreases the potential energy of the kink mode in order to reduce the sawtooth period. In the event that the natural sawtooth period is longer than expected, due to enhanced α particle stabilisation for instance, this ancillary sawtooth control can be provided from > 10MW of ion cyclotron resonance heating (ICRH) power with a resonance just inside the q = 1 surface. Both ECCD and ICRH control schemes would benefit greatly from active feedback of the deposition with respect to the rational surface. If the q = 1 surface can be maintained closer to the magnetic axis, the efficacy of ECCD and ICRH schemes significantly increases, the negative effect on the fusion gain is reduced, and off-axis negative-ion neutral beam injection (NNBI) can also be considered for sawtooth control. Consequently, schemes to reduce the q = 1 radius are highly desirable, such as early heating to delay the current penetration and, of course, active sawtooth destabilisation to mediate small frequent sawteeth and retain a small q = 1 radius. Finally, there remains a residual risk that the ECCD+ICRH control actuators cannot keep the sawtooth period below the threshold for triggering NTMs (since this is derived only from empirical scaling and the control modelling Power requirements for sawtooth control in ITER 2 has numerous caveats). If this is the case, a secondary control scheme of sawtooth stabilisation via ECCD+ICRH+NNBI, interspersed with deliberate triggering of a crash through auxilliary power reduction and simultaneous pre-emptive NTM control by off-axis ECCD has been considered, permitting long transient periods with high fusion gain. The power requirements for the necessary degree of sawtooth control using either destabilisation or stabilisation schemes are expected to be within the specification of anticipated ICRH and ECRH heating in ITER, provided the requisite power can be dedicated to sawtooth control.

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Destabilization of fast particle stabilized sawteeth in ASDEX Upgrade with electron cyclotron current drive

Cited by 12 publications

References 19 publications

Control of neoclassical tearing modes

Control of neoclassical tearing modes

Improved criterion for sawtooth trigger and modelling

Power requirements for electron cyclotron current drive and ion cyclotron resonance heating for sawtooth control in ITER

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