TCV: A short presentation
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The TCV device Picture of TCV & surrounding systems General presentation on CRPP (Lausanne site) |
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last updated August 2007 by H. Weisen
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The TCV device Picture of TCV & surrounding systems General presentation on CRPP (Lausanne site) |
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The TCV tokamak (Tokamak à Configuration Variable), which came into operation in November 1992 at the Lausanne site of CRPP, is the largest experimental facility at the Swiss Federal Institute of Technology in Lausanne (EPFL). The purpose of this device is to controlled thermonuclear fusion research by exploring new territory in tokamak operation. It's flexible design allows strong plasma shaping and extreme elongation. Both magnetohydrodynamic (MHD) stability and plasma confinement are sensitively dependent on plasma shaping. Discharges with elongations up to 2.8, triangularities in the range -0.7 to +1 in limiter, single null and double null diverted configurations have been investigated (figure 1). Plasma currents up to 1MA have been obtained at extreme elongation. Vertical stabilization at high elongation is achieved by means of fast (0.1ms response time) internal feedback coils. Since 1996 the available auxiliary electron cyclotron heating (ECH) power has gradually been increased to its present value of 4.5MW. The ECH system now comprises six 0.5 MW gyrotron sources at 82.7 GHz (2nd harmonic) and three 0.5 MW sources at 118 GHz (3rd harmonic), which deliver their power using a set of orientable launchers. The second harmonic microwave launchers can be steered both poloidally, for highly localized deposition at any radial position, and toroidally, for electron cyclotron current drive (ECCD). The rationale for the 3rd harmonic sources is to allow heating at densities exceeding second harmonic cut-off density (4×1019m-3).
The inside wall of the TCV vessel is fully protected by carbon tiles, except for diagnostics and heating ports. Although designed for pulse durations of 2 seconds, TCV has produced fully EC driven discharges for a duration of 4 seconds. The typical pulse repetition time is 15 minutes. The large variety of diagnostics installed was designed to provide coverage irrespectively of plasma shape and position. The systems currently in operation include magnetic probes (some 250 probes and flux loops), a Thomson scattering system (35 channels), a Far Infrared Interferometer (14 channels), an X-ray tomography system (200 channels in 10 cameras), a 64-channel high resolution multiwire proportional X-ray camera, 5 metal foil bolometer cameras (64 channels), 3 absolute UV diode bolometer cameras (48 channels), an ECE radiometer (3 selectable receivers, 24 channels), arrays of tile embedded Langmuir probes, a reciprocating Langmuir probe and a variety of spectrometers ranging from the visible domain to X-rays, as well as a dedicated 52keV diagnostic hydrogen beam for charge exchange spectroscopy.
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Figure 1: Examples of plasma configurations created on TCV, demonstrating the extreme shaping capability of the device, allowing for elongations up to 2.8 (left). back to top ^
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Experience gained on TCV contributes to the optimization of the tokamak as a controlled nuclear fusion reactor concept. One of the main attractions of operating at high edge elongation κa stems from the possibility of raising the plasma current, since Ip ~ (1 + κa2)/2, for a fixed value of the edge safety factor. High plasma current is expected to result in an increased beta limit and to improve energy confinement.
Normalized currents, Ip/aBT, of up to 3.6 MAm-1T-1 have been obtained at the highest elongations (κa ≤2.8), well above those achieved in other conventional aspect ratio tokamaks. A direct improvement of confinement with elongation has also been found, leading to the introduction of a shape enhancement factor, which predicts confinement time scaling with plasma shaping. Experiments on TCV have also shown that at extreme elongation (κa>2.2), confinement ceases to increase with elongation and the beta limit falls below the Troyon limit prediction.
Research on TCV has established ECCD as a current drive method and a tool for current and pressure profile control in advanced tokamak regimes by demonstrating, for the first time in 2000, fully EC current driven, steady state discharges, with currents up to 210 kA. Fully EC driven, bootstrap dominated reversed shear electron ITB discharges have been produced in 2002, also a world first. A variety of ECH/ECCD scenarios, conventional (ELMy H-modes) and advanced, are currently under development.
The TCV group vigorously pursues the development of its diagnostics, plasma control systems and interpretational codes, with emphasis on tools for the study of wave-electron interactions and transport. The versatility of the device, together with the powerful and flexible ECH/ECCD system, have provided the TCV team with an excellent position for a long term programme focused on electron physics, transport and MHD stability as part of the international effort to develop fusion as a source of energy for the future, as well as for investigating fundamental processes in fusion plasmas. A round of upgrades is currently under consideration, including provisions for ion heating and ELM control.
1976 : First proposal to build an elongated tokamak by the new Swiss Association
1985 : Second proposal to build a highly elongated tokamak
1986 : Acceptance for preferential support for `Tokamak à Configuration Variable'
Aims:
1) Creation and control of highly elongated tokamak discharges
2) Determination of operational boundaries for highly shaped discharges
3) Investigation of confinement and MHD properties of highly shaped discharges
Milestones to be achieved included discharges with normalized plasma currents and elongations well above those achieved in other devices, up to the maximum allowed by the vessel dimensions, which has an elongation of 2.9.
1992 : Acceptance of application for preferential support for `Electron Cyclotron Resonance Heating in TCV' as a tool for achieving the device scientific and technical aims, as well as for the development of ECH and ECCD as a tool for scenario development, current drive and current profile control. The approved system is to include 6 gyrotrons at 82.7GHz (2nd harmonic) and 3 gyrotrons at 118 GHz (3rd harmonic), with a unit power of 0.5 MW.
1992 : First plasma discharge in TCV
1994 : First Ohmic H-mode in TCV
1996 : Commissioning of in-vessel vertical feedback coils in TCV for operation at extreme elongation
1996 : First TCV plasma with Ip>1MA
1996 : First gyrotron source at 82.7 GHz delivers 0.45 MW to the plasma
1997 : First plasma with elongation κa>2.5, setting a new world record for plasma elongation at conventional aspect ratio.
1998 : Three gyrotrons at 82.7 GHz deliver 1.4 MW to plasma.
1999 : Six gyrotrons at 82.7 GHz deliver 2.7 MW to plasma; world highest EC power density.
1999 : First steady-state fully EC driven discharge at 123 kA (world record) using 1.4 MW of EC power
1999 : Creation of first internal transport barriers on TCV with Te(0)≥10 keV, using ECCD.
2000 : First steady-state fully EC driven discharge at 210 kA (world record) using 2.7 MW of EC power from all 6 gyrotron sources at 82.7 MHz.
2000 : TCV record for elongation κa rises to 2.8, for a normalized current IN=3.6MAm-1T-1. Both figures are world records for conventional aspect ratio tokamaks.
2000 : First of the third harmonic gyrotrons delivers 0.42 MW to plasma, demonstrating full absorption in combination with second harmonic current drive.
2001 : World longest fully EC driven plasma (4s), using two sets of gyrotrons consecutively.
2002 : 1.5 MW third harmonic power to plasma from three sources; together with the second harmonic sources this is the world highest available ECH power.
This
milestone marks the completion and commissioning of the heating systems on TCV,
as granted by preferential support in 1992
2002 : World first fully EC driven reversed shear steady state bootstrap-dominated discharge with ITB.
2003: World first fully current driven ITB plasmas (created by off-axis ECCD on steady state Ohmic target)
2005: Stationary ELMy and ELM-free H-modes heated by ECH (third harmonic) with bN~2
2006: First achievement of Electron Bernstein heating in a tokamak
After completion of the ECH heating systems in 2002, the focus of the programme has increasingly shifted towards physics issues in the areas of energy and particle transport, internal transport barriers, boundary physics and plasma control. These investigations also make use of continuously improved diagnostic capabilities, which aretoo detailed to be enumerated here.
updated H.W., August 2007