A. Mück

ELM frequency control by continuous small pellet injection in ASDEX Upgrade

Injection of cryogenic deuterium pellets has been successfully applied in ASDEX Upgrade for external edge localized mode (ELM) frequency control in type-I ELMy H-mode discharge scenarios. A pellet velocity of 560 m s−1 and a size of about 6 × 1019 D-atoms was selected for technical reasons, although even lower masses were found sufficient to trigger ELMs. A moderate repetition rate close to 20 Hz was chosen to avoid over-fuelling of the core plasma. Pellet sequences of up to 4 s duration were injected into discharges close to the L–H threshold, intrinsically developing large compound ELMs at a rate of 3 Hz. With pellet injection, these large ELMs were completely replaced by smaller type-I ELMs at the much higher pellet frequency, accompanied by a slight increase of density and even of stored energy. This external ELM control could be repeatedly switched on and off by just interrupting the pellet train. ELMs were triggered in less than 200 µs after pellet arrival at the plasma edge, at which time only a fraction of the pellet has been ablated, forming a rather localized, three-dimensional plasmoid, which drives the edge unstable well before the deposited mass is spread toroidally. The pellet controlled case has also been compared with a discharge at a somewhat lower density, but with otherwise rather similar data, developing spontaneous 20 Hz type-I ELMs. Despite the different trigger mechanisms, the general ELM features turn out to be qualitatively similar, possibly because of the similarity of the two cases in terms of ELM relevant parameters. The scaling with background plasma, heating power, pellet launch parameters, etc over a larger range still remains to be investigated.

Influence of the heating profile on impurity transport in ASDEX Upgrade

The transport of silicon has been investigated for various heating scenarios in ASDEX Upgrade H-mode discharges. Inside of r≈a/4, the diffusion coefficient D is either mainly neoclassical or anomalous depending on the heating method. For all investigated scenarios with NBI-heating and off-axis ECRH or off-axis ICRH, the diffusion coefficient is approximately neoclassical, and the effective heat diffusion coefficient χeff is below the neoclassical ion heat diffusion χi,neo in the plasma core. When central ECRH is added, χeff is above χi,neo, and D strongly increases by a factor of 3–10, i.e. becomes predominantly anomalous. For central ICRH, D is above the neoclassical level by a factor of 2. For radii outside of r≈a/4, D is always anomalous and increases towards the plasma edge. For ra/4, we find a clear scaling of D in terms of χeff, where D is about equal or above χeff. A strong inward drift parameter v/D is only observed in the core and only for cases, when the diffusion coefficient is neoclassical. With central wave heating, the drift parameter decreases to small values.

ELM-free stationary H-mode plasmas in the ASDEX Upgrade tokamak

ELM-free H-mode plasmas with stationary plasma density and radiation level are obtained in the ASDEX Upgrade tokamak with large clearance between the last closed flux surface and the wall, and neutral beam injection in a toroidal direction opposite to that of the plasma current. This behaviour is accompanied by a characteristic narrow-band magnetohydrodynamic (MHD) oscillation with clear harmonics up to n = 11 visible. This mode is localized in the plasma edge region. Conditions and properties of the stationary ELM-free phases and the edge MHD oscillation closely resemble that of the `quiescent H-mode and the `edge harmonic oscillation found in the DIII-D tokamak (Burrell K H et al 2002 Plasma Phys. Control. Fusion 44 A253). In addition, high-frequency MHD oscillations are found with an amplitude correlated with fluctuations of the divertor Dα intensity, suggesting a possible relevance of these modes for particle transport.

Sawtooth control in fusion plasmas

Clear observations of early triggering of neo-classical tearing modes by sawteeth with long quiescent periods have motivated recent efforts to control, and in particular destabilize, sawteeth. One successful approach explored in TCV utilizes electron cyclotron heating in order to locally increase the current penetration time in the core. The latter is also achieved in various machines by depositing electron cyclotron current drive or ion cyclotron current drive close to the q = 1 rational surface. Crucially, localized current drive also succeeds in destabilizing sawteeth which are otherwise stabilized by a co-existing population of energetic trapped ions in the core. In addition, a recent reversed toroidal field campaign at JET demonstrates that counter-neutral beam injection (NBI) results in shorter sawtooth periods than in the Ohmic regime. The clear dependence of the sawtooth period on the NBI heating power and the direction of injection also manifests itself in terms of the toroidal plasma rotation, which consequently requires consideration in the theoretical interpretation of the experiments. Another feature of NBI, expected to be especially evident in the negative ion based neutral beam injection (NNBI) heating planned for ITER, is the parallel velocity asymmetry of the fast ion population. It is predicted that a finite orbit effect of asymmetrically distributed circulating ions could strongly modify sawtooth stability. Furthermore, NNBI driven current with non-monotonic profile could significantly slow down the evolution of the safety factor in the core, thereby delaying sawteeth.

Sawtooth control experiments on ASDEX Upgrade

In a fusion device, the so-called sawtooth instability can lead to the triggering of confinement limiting neoclassical tearing modes. On the other hand, the existence of sawteeth is desirable for the removal of one fusion product, i.e. helium ash from the plasma core. This has led to great interest in the control of sawteeth. The sawtooth period can be changed drastically by local modification of the q-profile. In this paper, the influence of the beam line geometry of the neutral beam injection in the ASDEX Upgrade tokamak will be presented as well as the effect of local electron cyclotron current drive. Systematic scans in the electron cyclotron current deposition from the high-field side to the low-field side resolve areas with sawtooth stabilization and destabilization. These observations will be discussed, including modelling of the main results, with the ASTRA transport code constrained by experimental data.

Recent ECRH results in ASDEX Upgrade

This paper provides an overview on recent experimental results obtained in ASDEX Upgrade using electron cyclotron heating and current drive. The following topics are covered: determination of the power deposition profile, modulated power deposition, studies of the electron heat transport via power balance and heat wave analysis and a comparison with turbulent transport theory, generation of an internal transport barrier for the electron heat flux, impact of electron cyclotron resonance heating (ECRH) on particle and impurity transport, and studies related to neoclassical tearing modes and to sawteeth.

The improved H-mode at ASDEX Upgrade: a candidate for an ITER hybrid scenario

A stationary regime with improved confinement (H98(y, 2) > 1) and, simultaneously, improved stability (βN > 2.5) compared to standard H-mode has been investigated on ASDEX Upgrade for many years. This so-called improved H-mode is characterized by a q-profile with low central magnetic shear and q0 ≥ 1 that is obtained by early heating during the current ramp. Studies of this discharge scenario have been continued. New results are presented concerning the existence domain in the q95 range, the dependence on the normalized Larmor radius and initial experiments showing that high performance improved H-modes can be obtained with strong central ion cyclotron resonance heating (ICRH). In addition, the present status of understanding of the improved H-mode is reviewed. Improved H-mode plasmas are documented for 3.2 1 sawteeth as the main trigger of large amplitude neoclassical tearing modes are avoided. The stability is eventually limited by the occurrence of a mode with poloidal mode number m = 2 and toroidal mode number n = 1 at typically βN ~ 3. As far as the reactor relevance of this regime is concerned, its compatibility with significant central electron heating by ICRH, with high edge densities and low amplitude edge localized modes is of importance. The improved H-mode is, therefore, seen as a candidate for a long pulse ITER hybrid operation.

Active control of MHD instabilities by ECCD in ASDEX Upgrade

The modification of the stability and the behaviour of core MHD with local electron cyclotron current drive (ECCD) is presented. Starting from the innermost resonant surface, the q = 1 surface, the stability and hence the sawtooth period and the size of sawteeth is controlled with local on/off-axis co-/counter-ECCD. The sawteeth themselves can serve as a trigger for neoclassical tearing modes (NTMs) and therefore the excitation of NTMs can be influenced. Once these NTMs get excited they can be fully stabilized at high βN with co-ECCD at the resonant surface. Detailed experiments on the dependence of the stabilization on the ECCD deposition width and the total driven current have shown ways to improve the stabilization efficiency both for the (3/2) and the (2/1)-NTM significantly. In the presence of (3/2)-NTMs the impact on the confinement can be reduced by triggering the so-called frequently interrupted regime (FIR-NTM) with the current drive in the vicinity of the (4/3) surface leading also to a clearer understanding of the FIR-NTM.

Neoclassical tearing modes on ASDEX Upgrade: improved scaling laws, high confinement at high βN and new stabilization experiments

Note: Proc. 19th IAEA Fusion Energy Conference, Lyon, France, October 2002, IAEA-CN-94/EX/S1-4 Reference CRPP-CONF-2002-059 Record created on 2008-05-13, modified on 2017-05-12

Safety factor profile requirements for electron ITB formation in TCV

On the TokamakConfiguration Variable (TCV), electron internal transport barriers (eITBs) can be formed during a gradual evolution from a centrally peaked to a hollow current profile while all external actuators are held constant. The formation occurs rapidly (<τeE) and locally and, according to ASTRA modelling, is consistent with the appearance of a local minimum in the safety factor (q) profile. The eITB is sustained by non-inductively driven currents (including the off-axis bootstrap current) for many current redistribution times while the current in the tokamak transformer is held constant. The maximum duration is limited by the pulse length of the gyrotrons. The transformer coil can be used as a counter (or co-) current source with negligible accompanying input power. In established eITBs the performance can be enhanced (degraded) by altering solely the central current or q-profile. New experiments show that the same stationary eITB performance can be reached starting from discharges with centrally peaked current. A fine scan in surface voltage shows a smooth increase in performance and no sudden improvement with voltage despite the fact that qmin must pass through several low-order rational values. The appearance, in