Z. A. Pietrzyk

Behaviour of central plasma relaxation oscillations during localized electron cyclotron heating on the TCV tokamak

During initial studies of ECRH in the TCV tokamak, non-standard central MHD activities, such as humpbacks and saturated and inverted sawteeth, have been observed while changing the heating location, the ECRH power, the plasma shape and the safety factor. For edge safety factors q(alpha) > 4.5, safety factors on-axis q(0) < 1 and small plasmas, complete sawtooth stabilization was achieved with the present 1 MW gyrotron power, and it is likely that sawtooth stabilization can be achieved for all conditions at. higher ECRH power. The conditions under which the various relaxation activities are produced or suppressed are reported, and the origins for such tron-standard behaviour are discussed.

Poloidally asymmetric plasma response with ECH deposition near q=1 in TCV

Note: Proceedings of the IAEA Technical Committee Meeting on ECRH Physics and Technology for Fusion Devices and the 11th Joint Workshop on Electron Cyclotron Resonance Heating (EC-11), Oh-Arai, Japan, October 1999, JAERI-memo 12-041, 275 - 281 (March 2000) Reference CRPP-CONF-2000-031 Record created on 2008-05-13, modified on 2017-12-10

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[1020u200aAu200aW−1u200am−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...

Deconvolution of electron density from lithium beam emission profiles in high edge density plasmas

Active beam emission spectroscopy with high energy Li beams has been shown to be a successful technique for determining electron density profiles of low and medium density plasma edges of a tokamak. For high density plasma edges, however, the reconstruction algorithm necessary to extract the electron density profile from the Li beam emission profile at 671 nm encounters a singularity point at which numerical instabilities can occur. This behaviour is particularly pronounced for noisy input data. An integral method has been developed at JET to replace the standard calculation method in the singularity region of the profile, and the normal deconvolution method is then used for data in the high density region. Using this approach, routine deconvolution of emission profiles extending from low density regions well into high density regions of the plasma has been made possible. The sensitivity of the algorithm to noisy input data and errors on input plasma parameters has been examined for typical conditions expected at JET. The simulations show that even for unfavourable conditions electron density profiles can be recovered with an overall accuracy better than 25%.

Ohmic H-modes in the TCV tokamak

The TCV tokamak has obtained ohmic H-modes in virtually all diverted plasmas with the ion del B drift directed towards an X-point and in several elongated limiter plasmas. Troyon factors (beta(tor)aB/l(p)) up to 2 and line average densities up to 2.2 x 10(20) m(-3), corresponding to the Greenwald limit, have been obtained in diverted ELM-free H-modes. Quasi-stationary H-modes lasting for the entire current flat top (1.5 s) have been obtained in the presence of regular ELMs. The occurrence and magnitude of ELMs have been found to depend on configurational parameters such as the position of the active X-point in unbalanced double-null discharges and the plasma-wall separation in single-null discharges. These dependencies have permitted active control of ELM behaviour in TCV. A continuous spectrum of ELM amplitudes and frequencies has been observed, ranging from clearly identifiable type III ELMs to large, low-frequency ELMs which expel up to 12% of the stored energy and up to 7% of the particle content and are reminiscent of type I ELMs. A previously unknown, benign kind of ELM, with a maximum amplitude in the divertor region, has also been observed.

Plasma-edge Gradients in L-mode and ELM-free H-mode JET Plasmas

Experimental plasma-edge gradients in JET during the edge-localized-mode (ELM) free H-mode are examined for evidence of the presence and location of the transport barrier region inside the magnetic separatrix. High spatial resolution data in electron density is available in- and outside the separatrix from an Li-beam diagnostic, and in electron temperature inside the separatrix from an ECE diagnostic, while outside the separatrix, a reciprocating probe provides electron density and temperature data in the scrape-off layer. Ion temperatures and densities are measured using an edge charge-exchange diagnostic. A comparison of observed widths and gradients of this edge region with each other and with theoretical expectations is made. Measurements show that ions and electrons form different barrier regions. Furthermore, the electron temperature barrier width (3-4 cm) is about twice that of electron density, in conflict with existing scaling laws. Suitable parametrization of the edge data enables an electron pressure gradient to be deduced for the first time at JET. It rises during the ELM-free phase to reach only about half the marginal pressure gradient expected from ballooning stability before the first ELM. Subsequent type I ELMs occur on a pressure gradient contour roughly consistent with both a constant barrier width model and a ballooning mode envelope model.

Effect of plasma shape on confinement and MHD behaviour in TCV

Note: 38th Annual Meeting, APS Division of Plasma Physics, Denver, CO, USA, November 1996, Bull. Amer. Phys. Soc. 41(7), 1513 (1996) Reference CRPP-CONF-1996-037 Record created on 2008-05-13, modified on 2016-08-08

Overview of TCV results

Note: Proc. 18th IAEA Fusion Energy Conference, Sorrento, Italy, 4-10 October 2000, IAEA-CN-77 (OV5/1), p. 37 (2000) Reference CRPP-CONF-2000-087 Record created on 2008-05-13, modified on 2017-05-12

Sustained fully non-inductive scenarios using pressure and current profile control with ECCD

Note: Proceedings of the IAEA Technical Committee Meeting on ECRH Physics and Technology for Fusion Devices and the 11th Joint Workshop on Electron Cyclotron Resonance Heating (EC-11), Oh-Arai, Japan, October 1999, JAERI-memo 12-041, 338 - 349 (March 2000) Reference CRPP-CONF-2000-075View record in Web of Science Record created on 2008-05-13, modified on 2017-05-12