F. Santini

High magnetic field tokamaks

A review of existing high magnetic field tokamaks is presented. The results obtained, especially in the research areas concerning confinement, impurity control and heating, are discussed and compared with those of tokamaks at lower toroidal field. New devices, either proposed or under construction, are surveyed. Advantages and difficulties of such machines in exploring the approach to ignition are examined.

Progress in LHCD : a tool for advanced regimes on ITER

The recent success in coupling lower hybrid (LH) waves in high performance plasmas at JET together with the first demonstration on FTU of the coupling capability of the new passive active multijunction launcher removed major concerns on the possibility of using LH on ITER. LH exhibits the highest experimental current drive (CD) efficiency at low plasma temperature thus making it the natural candidate for off-axis CD on ITER where current profile control will help in maintaining burning performance on a long-time scale. We review recent LH results: long internal transport barrier obtained in JET with current profile sustained and controlled by LH acting under real time feedback together with first LH control of flat q-profile in a hybrid regime with T e ∼ T i . Minutes long fully non-inductive LH driven discharges on Tore Supra (TS). High CD efficiency with electron cyclotron in synergy with LH obtained in FTU and TS opening the possibility of interesting scenarii on ITER for MHD stabilization. Preliminary results of LH modelling for ITER are also reported. A brief overview of ITER LH system is reported together with some indication of new coming LH experiments, in particular KSTAR where CW klystrons at the foreseen ITER frequency of 5 GHz are being developed.

Effect of low magnetic shear induced by lower hybrid current drive on high performance internal transport barriers in the Joint European Torus (JET)

~Received 18 February 2002; accepted 12 April 2002!A low/negative magnetic shear profile is maintained in the Joint European Torus in a plasma targetwith plasma current of 2.4 MA using 2.2 MW of lower hybrid ~LH! power combined with neutralbeam injection and ion cyclotron radiofrequency heating. In this scenario, an internal transportbarrier ~ITB! with time duration up to about4sisproduced. The fraction of LH driven current isabout 25% the total plasma current. During LH power application, the layer with reversed shearq-profile can be maintained in a suitable radial position to inhibit the onset of turbulence, whichmight otherwise drive the ITB to collapse. LH power could be used as a tool to drive moderateamounts of noninductive off-axis current and sustain high performance ITBs at high plasma current.© 2002 American Institute of Physics. @DOI: 10.1063/1.1488951#Internal transport barriers ~ITBs! are produced in manytokamaks as a result of auxiliary heating power injection inconditions of reversed shear q profile.

Chapter 11: The Heating and Current Drive Systems of the FTU

Abstract High-frequency wave systems with high-power density launching capability have been the preferred choice to heat the Frascati Tokamak Upgrade (FTU) because of physics arguments (electron heating at very high density) and space constraints from the compactness of the machine design (8-cm-wide port). They do include an 8-GHz lower hybrid current drive (LHCD) system, a 140-GHz electron cyclotron resonance heating (ECRH) system, and a 433-MHz ion Bernstein waves system (IBW). The technical aspects of these systems will be reviewed in this article. The main features of the design include the following: (a) a very compact conventional LHCD grill with a compact window to keep the vacuum on 48 (12 columns, 4 rows) individual waveguides allowing the maximum flexibility in spectra generation to be achieved; power handling up to [approximately equal to]10 kW/cm2 has been achieved, (b) ECRH launchers designed as a quasi-optical system (implementing ITER relevant solutions) retaining the maximum flexibility in the equatorial launcher (poloidal/toroidal steerability) to exploit a variety of scenarios, (c) a two-waveguides launching array making the IBW experiment on FTU unique. Other technical aspects (sources, transmission lines, etc.) are also reviewed. The development of a new ITER relevant lower hybrid launcher, the passive active multijunction, is described.

MHD activity in FTU plasmas with reversed magnetic shear

The MHD activity of plasma configurations with reversed magnetic shear has been investigated on the FTU tokamak. In the presence of pairs of surfaces with the same rational value q = m/n of the safety factor, double-tearing modes are excited which give rise in most cases to bursts of sawtooth-like profile rearrangements. More stable regimes have also been found, in which the activity is dominated by rotating saturated modes. In a particular case with and a discharge without any detectable MHD activity during the current flat-top has been obtained. In high-temperature regimes ( at ), an irregular activity has been detected near the plasma centre which could be due to the excitation of resistive interchange modes.

Steady improved confinement in FTU high field plasmas sustained by deep pellet injection

High density plasmas (n0 ≈ 8 × 1020m-3) featuring steady improved core confinement have been obtained in FTU up to the maximum nominal toroidal field (8 T) by deep multiple pellet injection. These plasmas also feature high purity efficient electron-ion coupling and peaked density profiles sustained for several confinement times. Neutron yields in excess of 1 × 1013 n/s are measured, consistent with the reduction of the ion transport to neoclassical levels.

Heating experiments on the FT Tokamak in the lower hybrid frequency range

In this work we describe the heating results in the LH frequency range (f = 2.45 GHz, Prf lt=250 kW, Ps lt= 6 kW/cm2) both in the electron and ion regimes. Efficient electron heating was observed for n lt= 5.10 13 cm-3. Increases of the peak electron temperature up to 700 eV were measured. At higher densities the interaction with electrons disappears. In the ion regime fast neutral tails and neutron enhancement were observed. The influence of plasma boundary conditions on the penetration of the wave is demonstrated. The principal physical problems are pointed out and some possible conclusions are given.

Enhanced confinement regimes with strong electron heating in the presence of flat or inverted safety factor profiles

The role of magnetic shear in affecting electron transport is discussed on the basis of the Frascati tokamak upgrade (FTU) data with central electron cyclotron resonance heating (ECRH) on the current ramp phase and with pellet injection. The results point out that strongly negative magnetic shear is not a necessary condition in order to have good electron transport and that magnetohydrodynamic (MHD) activity plays a crucial role in affecting the electron transport in the region with low/negative magnetic shear. The theoretical arguments for the dependence of transport on magnetic shear are reviewed and compared with the experimental evidence.

Energy confinement and plasma heating during lower hybrid experiments

RF power up to 450 kW has been injected into the plasma of FT in the electron heating regime (ne<or=5*1013 cm-3) producing electron and ion temperature increases of about 1 keV and .5 keV respectively without significant enhancement of Zeff. A density increase is observed due to an improvement of particle confinement time. An energy balance at intermediate power levels (PRF=300 kW) is carried out for two different types of discharges, one without sawteeth the other with sawteeth. The balance shows that in order to account for the total injected power one has to assume equal electron thermal conductivity for the OH and the OH+RF phase. The energy confinement time does not vary from its ohmic value for both discharges. Finally an investigation, at ne above the density limit, of the radial source of the fast ion tails and of the characteristics of the parametric decay instability is presented.

Non-thermal fusion in a beam plasma system

The problem of producing fusion power with low neutron emission has been debated in the past in the framework of the magnetic confinement fusion research. Proposals are still being renewed to use advanced fuels in various plasma systems. Since todays toroidal devices cannot support plasma conditions suitable for a large fusion production with such fuels, new concepts and configurations have been studied, where the plasma components are not in a thermal equilibrium. Here, a system of a neutral beam injected into a confined plasma is considered where fusion is produced only between the beam and plasma ions. The collisional slowing down of the beam into the plasma is described by a fluid model. General considerations in this model allow conditions to be found for the fusion-produced power to breakeven against the power needed to sustain the system itself. These conditions are only necessary since the nuclear power is maximized in the present analysis by using favourable assumptions. Nevertheless, the results for different advanced fuels indicate again the very high difficulty of getting a net power produced by the fusion of such fuels, unless the plasma target temperature reaches very high and unrealistic values.