TCV

Figure 1:TCV at CRPP in Lausanne (Switzerland)

The TCV Device
TCV Parameters
TCV Milestones
TCV Achievements and Outlook

The TCV Device

The TCV tokamak (Tokamak à Configuration Variable), which came into operation in November 1992 at the Lausanne (CH) site of Association EURATOM-CRPP (Centre de Recherches en Physique des Plasmas), is the largest experimental facility at the Swiss Federal Institute of Technology in Lausanne (EPFL). The purpose of this device is to explore new territory in tokamak operation using a flexible design, which allows strong plasma shaping and extreme elongation. Both 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; below). 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 (2^nd harmonic) and three 0.5 MW sources at 118 GHz (3^rd 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 3^rdharmonic sources is to allow heating at densities exceeding second harmonic cut-off density (4 x 10¹⁹ m^-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.

Figure 2:Examples of plasma configurations created on TCV, demonstrating the
extreme shaping capability of the device, allowing for elongations up to 2.8.

TCV Parameters

Major radius	0.88 m
Minor radius	0.24 m
Vessel height	1.39 m
Maximum design plasma current Ip	1.2 MA
Maximum design elongation	2.9
Triangularity	-0.7 to 1
Aspect ratio	3.6
Toroidal Magnetic field	1.43 T
ECRH power: At 83 GHz (2*fce) At 118 GHz (3*fce)	3 MW 1.5 MW
Plasma duration (max.)	2 s
Transformer flux	3.4 Vs
V loop (max.)	10 V

Figure 3:TCV Machine

TCV Milestones

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 EURATOM preferential support for "Tokamak à Configuration Variable" Aims: Creation and control of highly elongated tokamak discharges. Determination of operational boundaries for highly shaped discharges. 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 EURATOM 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.
2002	World first fully EC driven reversed shear steady state bootstrap-dominated discharge with ITB.

Figure 4:TCV Inside

Figure 5:TCV Plasma

Figure 6:TCV Gyrotrons

TCV Achievements and Outlook

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 elongation stems from the possibility of raising the plasma current, since I_p ∞ (1 + Κ²)/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, I_p/aB_T, of up to 3.6 MAm^-1T^-1 have been obtained at the highest elongations (Κ_a ≤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 (Κ_a>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.