Y. Martin

Magnetic triggering of ELMs in TCV

Keywords: LRP 753 Reference CRPP-REPORT-2003-011 Record created on 2008-04-18, modified on 2017-05-12

The control of tokamak configuration variable plasmas

AbstractThe general control of tokamak plasmas has evolved considerably over the past few years with an increase in the plasma pulse length, an increase in the control of additional heating and fueling, and an increase in the degree to which the shape of the plasma can be varied. The Tokamak Configuration Variable (TCV) is specifically designed to explore the operational benefits of plasma shaping over a wide variety of plasma shapes. Consequently, considerable attention has been given to the control of the poloidal field coil currents that impose the desired shape. All aspects of the control of TCV plasmas, from the diagnostic measurements to the power supplies, via particular control algorithms and overall supervision are discussed.

Plasma dynamics with second and third-harmonic ECRH and access to quasi-stationary ELM-free H-mode on TCV

Intense electron cyclotron resonance heating (ECRH) and electron cyclotron current drive (ECCD) are employed on the Tokamak a Configuration Variable (TCV) both in second- and third-harmonic X-mode (X2 and X3). The plasma behaviour under such conditions is driven largely by the electron dynamics, motivating extensive studies of the heating and relaxation phenomena governing both the thermal and suprathermal electron populations. In particular, the dynamics of suprathermal electrons are intimately tied to the physics of X2 ECCD. ECRH is also a useful tool for manipulating the electron distribution function in both physical and velocity space. Fundamental studies of the energetic electron dynamics have been performed using periodic, low-duty-cycle bursts of ECRH, with negligible average power injection, and with electron cyclotron emission (ECE). The characteristic times of the dynamical evolution are clearly revealed. Suprathermal electrons have also been shown to affect the absorption of X3 radiation. Thermal electrons play a crucial role in high density plasmas where indirect ion heating can be achieved through ion-electron collisions. In recent experiments approximate to 1.35 MW of vertically launched X3 ECRH was coupled to a diverted ELMy H-mode plasma. In cases where >= 1.1 MW of ECRH power was coupled, the discharge was able to transition into a quasi-stationary ELM-free H-mode regime. These H-modes operated at beta(N) approximate to 2, (n) over bar (e)/n(G) approximate to 0.25 and had high energy confinement, H-IPB98(y,H-2) up to approximate to 1.6. Despite being purely electron heated and having no net particle source these discharges maintained a density peaking factor (n(e,o)/ approximate to 1.6). They also exhibited spontaneous toroidal momentum production in the co-current direction. The momentum production is due to a transport process as there is no external momentum input. This process supports little or no radial gradient of the toroidal velocity.

Edge-localized mode control by electron cyclotron waves in a tokamak plasma

Electron cyclotron resonance heating is applied to the edge of a high-confinement (H-mode) plasma featuring type I edge-localized modes (ELMs) in the TCV tokamak. As the deposition location is shifted gradually in a highly controlled manner towards the plasma pressure pedestal, an increase in the ELM frequency by a factor 2 and a decrease in the energy loss per ELM by the same factor are observed, even though the power absorption efficiency is reduced. This unexpected and, as yet, unexplained phenomenon, observed for the first time, runs contrary to the intrinsic type I ELM power dependence and provides a new approach for ELM mitigation.

Recent results from the electron cyclotron heated plasmas in Tokamak à Configuration Variable (TCV)

In noninductively driven discharges, 0.9 MW second harmonic (X2) off-axis co-electron cyclotron current drive deposition is combined with 0.45 MW X2 central heating to create an electron internal transport barrier (eITB) in steady plasma conditions resulting in a 1.6-fold increase of the confinement time (τEe) over ITER-98L-mode scaling. The eITB is associated with a reversed shear current profile enhanced by a large bootstrap current fraction (up to 80%) and is sustained for up to 10 current redistribution times. A linear dependence of the confinement improvement on the product of the global shear reversal factor (q0/qmin) and the reversed shear volume (ρq-min2) is shown. In other discharges heated with X2 the sawteeth are destabilized (respectively stabilized) when heating just inside (respectively outside) the q=1 surface. Control of the sawteeth may allow the avoidance of neoclassical tearing modes that can be seeded by the sawtooth instability. Results on H-mode and highly elongated plasmas using the...

An overview of results from the TCV tokamak

The Tokamak Configuration Variable (TCV) tokamak (R = 0.88 m, a < 0.25 m, B < 1.54 T) programme is based on flexible plasma shaping and heating for studies of confinement, transport, control and power exhaust. Recent advances in fully sustained off-axis electron cyclotron current drive (ECCD) scenarios have allowed the creation of plasmas with high bootstrap fraction, steady-state reversed central shear and an electron internal transport barrier. High elongation plasmas, kappa = 2.5, are produced at low normalized current using far off-axis electron cyclotron heating and ECCD to broaden the current profile. Third harmonic heating is used to heat the plasma centre where the second harmonic is in cut-off. Both second and third harmonic heating are used to heat H-mode plasmas, at the edge and centre, respectively. The ELM frequency is decreased by the additional power. In separate experiments, the ELM frequency can be affected by locking to an external perturbation current in the internal coils of TCV. Spatially resolved current profiles are measured at the inner and outer divertor targets by Langmuir probe arrays during ELMs. The strong, reasonably balanced currents are thought to be thermoelectric in origin.

Dynamics of edge localized modes in the TCV tokamak

Keywords: LRP 699 Reference CRPP-REPORT-2001-007 Record created on 2008-04-18, modified on 2017-05-12

Recent results from the TCV Tokamak

During the first two years of operation, the TCV tokamak has produced a large variety of plasma shapes and magnetic configurations, with 1.0≤Btor≤1.46T,Ip≤800kA,k≤2.05, −0.7≤δ≤1. A new shape control algorithm, based on a finite element reconstruction of the plasma current in real time, has been implemented. Vertical growth rates up to 1000s−1 have been stabilized using the external coil system. Ohmic H-modes with Troyon factors (βtoraB/Ip) up to two and densities up to 2.25×1020m−3, corresponding to the Greenwald limit, have been obtained in diverted discharges. Limiter H-modes with line averaged electron densities up to 1.7×1020m−3 have been obtained in elongated D-shaped plasmas with 360 kA≤IP≤600 kA.

Steady-state fully noninductive operation with electron cyclotron current drive and current profile control in the tokamak à configuration variable (TCV)

Fully noninductive, steady-state electron cyclotron current drive (ECCD) has been demonstrated for the first time in experiments carried out in the tokamak a configuration variable (TCV) [O. Sauter et al., Phys. Rev. Lett. 84, 3322 (2000)]. By appropriately distributing six 0.45 MW ECCD sources over the discharge cross section, fully noninductive, stable, and stationary plasmas with Ip up to 210 kA were obtained for the full discharge duration of 1.9 s, corresponding to more than 900 energy confinement times and more than 10 current redistribution times at an average current drive efficiency η20CD=0.01[1020 A W−1 m−2]. These experiments have also demonstrated for the first time the steady recharging of the ohmic transformer using ECCD only. The effect of localized off-axis electron cyclotron heating (ECH) and EC current drive (ECCD) (co- and counter-) is investigated showing that locally driven currents amounting to only 1% of Ip significantly alter sawtooth periods and crash amplitudes. An improved quasi...

Third-harmonic, top-launch, ECRH experiments on TCV tokamak

In the moderate magnetic field of TCV (1.5 T), the recently installed X3 system (3 gyrotrons, 118 GHz, 0.45 MW each, 2 s) broadens the operational space with the possibility of heating plasmas at high density, well above the cutoff density of the X2 system. To compensate for the significantly weaker absorption coefficient compared to the absorption of X2, the top-launch injection allows the ray path to maximize along the resonance layer thereby maximizing the optical depth. To maintain the maximum absorption in plasma discharges with a dynamic variation in both density (refraction) and temperature (relativistic shift) a real-time control system on the mirror injection angle, based on a mirror modulation technique (synchronous demodulation), has been developed and successfully tested on TCV. Comparisons of the absorption calculated with the TORAY-GA ray-tracing code and the ECWGB beam-tracing code, which includes diffraction effects, are presented. An experimental study of the X3 absorption versus plasma density in an L-mode plasma shows that with a total injected power of 1.35 MW full single-pass absorption is reached with a significant fraction of the absorbed power associated with the presence of suprathermal electrons. Compared with ohmic/low-power-heating of ELMy H-modes, it has been possible to enter into a different ELMy regime with an injected power of 1.35 MW.