Frascati Tokamak Upgrade (FTU)

Figure 1:Plasma Physics Laboratory - Frascati (Italy)

The history of FTU
FTU Parameters
FTU Milestones
FTU Objectives

The history of the Frascati Tokamak Upgrade (FTU)

The plasma physics laboratory in Frascati (Italy), which is also called the Ionized Gas Laboratory, was founded in the 1960s and is now part of the National Laboratory of Frascati. It has been undertaking research in the field of fundamental high energy physics since 1956, under the auspices of the National Institute for Nuclear Physics (INFN). Research activities on controlled nuclear fusion began in 1976, in co-operation with EURATOM, CNR (National Research Council) and industry, under the EURATOM Association ENEA.
Over the last 20 years the plasma physics laboratory has undertaken studies and research concerning the production and understanding of the physical properties of dense plasmas with the objective of establishing the possible application of controlled nuclear fusion as a source of clean energy. This activity led to the construction of the FT (Frascati Tokamak) prototype machine in the 1970s and afterwards of the FTU machine (Frascati Tokamak Upgrade).
The FT experiment operated from 1977 to 1989. It was a toroidal device with the following characteristics: very compact, high magnetic field value (10 Tesla), high current and additional heating systems using high power radiofrequency waves.
FTU started operation in 1989. Based on the same principles as FT, it offers a larger surface for plasma access and includes three radiofrequency power injection systems (at 433 MHz, 8 GHz, 140 GHz) that can inject an additional power of up to 5 MW into the plasma. The objective of FTU is to increase the temperature of the high density plasma to between 50 - 100 million degrees.

Figure 2:FTU - Inside

Figure 3:FTU - Plasma

FTU Parameters

Plasma major radius	0.935 m
Plasma minor radius	0.30 m
Magnetic field	8.0 T
Plasma current	1.6 MA
Plasma volume	1.6 m3
Pulse length	1.5 s
Auxiliary heating	5.0 MW

FTU Milestones

1982	EURATOM approval.
1983	Start construction.
1990	First plasma.
1990	First high current and high toroidal field discharge Ip=1MA BT=7.2 T
1993	First lower hybrid (LH) antenna.
1993	Preliminary results of confinement improvement with pellet injection.
1995	Toroidal Limiter Installed.
1996	First observation of reversed shear plasmas with LH current drive.
1997	High central electron temperatures (Te=8keV) and internal transport barrier formation with electron cyclotron (EC) heating on the current ramp.
1997	FTU record value of the triple product in discharges with pellet injection (nτT=0.6x1020m-3skeV).
1998	Record value for the central electron temperature (Te=15keV).
1999	First ion Bernstein wave (IBW) experiments. Preliminary observation of internal transport barrier formation with IBW.
2000	Full ohmic performance Ip=1.6 MA BT=8T.
2000	Quasi steady state enhanced confinement phase and highest neutron production (2x1013n/s) in 8T/1.25MA discharges with multiple pellet injection.
2000	Tearing mode (m=2) stabilization by EC resonance heating followed by a confinement improvement phase.
2000	First observation of absorption of EC waves by the LH produced fast electron tail.
2001	Full LH power (2.1MW at the plasma).
2002	Long duration (170ms, about seven confinement times) internal transport barrier with ECRH and LHCD on the current ramp.

FTU Objectives and Achievements

FTU is a compact, high magnetic field tokamak aimed at studying transport, stability and plasma-wave interaction issues at plasma densities and magnetic field values close to those of ITER. It has been successfully operated up to the design values of B=8T, I=1.6MA due to the recently installed titanization and the boronization systems which, allow a prompt recovery from disruptions and very clean plasma operation. FTU is equipped with three radiofrequency heating systems whose main characteristics and objectives are summarized below. Note that FTU is characterized by dominant electron heating at high density, similar to the situation of a burning plasma, in which the alpha particles produced by the DT reaction are slowed down by the electrons. This is not the case in many of the present tokamak experiments which are characterized by dominant ion heating or by electron heating at low density.

Figure 4:FTU Machine

The Lower Hybrid (LH) system (8GHz, 2.4 MW at the plasma) is characterized by the highest frequency among other tokamak LH systems and close to that foreseen for ITER (5GHz). The system was designed to avoid any direct interaction with the ion population for any value of the plasma density and to demonstrate high current drive efficiency at ITER relevant densities. So far, up to 2.2MW has been successfully injected to the plasma and good coupling conditions are reproducible. Values of the current drive efficiency close to those required for ITER have been achieved on FTU even at high density (in the order of 10²⁰m^-3). Interestingly, at these density values the equipartition time between electron and ions is sufficiently short to allow the observation of collisional ion heating during the LH pulse. In collaboration with CEA a new type of ITER-relevant coupler, the Passive Active Module, is being tested on FTU and will demonstrate the capability of injecting LH waves at sufficiently high power density.
In a joint effort between ENEA and CNR-IFP (Milan), an Electron Cyclotron (EC) heating system (140GHz, 1.6MW at the plasma), has been developed. This is the only system in the world injecting EC waves in the ordinary mode at this frequency, which is also close to that envisaged on ITER. Since under these conditions the cut-off density is high (2.2x10²⁰m ^-3), the effect of collisional ion heating can also be observed. EC waves have also been extensively used to study the formation of internal transport barriers, where very high central electron temperatures (above 10keV) have been obtained close to ITER relevant densities. In particular, the simultaneous injection of LH and EC waves on the plasma current ramp led to the formation of an internal transport barrier at plasma density close to 10²⁰m^-3 for a duration of about seven confinement times. The EC system has already been also used to study local transport properties both in steady state and transient conditions, with particular reference to the so called "profile stiffness" issue, which on FTU can be studied at collisionality values of interest for burning plasmas. Finally, the EC system has also been successfully operated in order to stabilize low toroidal mode number MHD modes,resulting in a significant increase of the central electron temperature being observed after the mode has been stabilized.

Figure 5:FTU Antenna

The third system is the Ion Bernstein Wave (IBW) system, (433Mhz, 0.7MW at the plasma) which has the unique feature of launching the wave using a waveguide array, thus limiting the spurious edge interaction between plasma and antenna. Very high power densities (up to 15MW/m²) have been obtained - the original motivation of the experiment. In addition, evidence has been found of internal transport barrier formation at the radius where the wave is absorbed, in accordance with theoretical predictions of shear flow formation close to the absorption region. If confirmed, this result could open the possibility of controlling the internal transport barrier location in burning plasmas. A correlation between the achievement of reduced transport and changes in the turbulence spectra is in progress (in collaboration with the Kurchatov Institute in Moscow).
The combined use of the LH and the EC system has been the subject of extensive studies. In particular, synergy effects can be used to damp the EC waves on a supra-thermal electron tail produced by LH waves, allowing in this way the use of EC at magnetic field values above that (B=5T) which corresponds to the absorption by thermal electrons. Clear interaction between EC waves and the electron tail has been observed at B=7.2T. The investigation of this scheme, which is envisaged on ITER in order to enhance the flexibility of the EC system, is in progress.
FTU is equipped with a multiple fast pellet injector which has been used in deuterium plasmas which has led to the achievement of quasi steady-state regimes with improved confinement. Up to five pellets injected in sequence on a B=8T I=1.25MA discharge have allowed the achievement of the highest neutron rates (up to 2x10¹³n/s) with very narrow density profiles (central density up to 8x10²⁰m^-3). Vertical pellet injection will be tested in the near future (in collaboration with Consorzio RFX) in order to achieve a deeper penetration.