IT9700689 Associazione EURATOM-ENEA sulla Fusione FRASCATI TOKAMAK UPGRADE (FTU): RESULTS AND DEVELOPMENTS FRANCESCO ROMANELLI ENEA- Oipartimento Energia Centra Ricerche Frascati, Roma rnmBQ/FUB/m/4 £ 9 - 0 2 ENTE PER LE NUOVE TECNOLOGIE, L'ENERGIA E L'AMBIENTE Associazione EURATOM-ENEA sulla Fusione FRASCATI TOKAMAK UPGRADE (FTU): RESULTS AND DEVELOPMENTS FRANCESCO ROMANELLI ENEA- Dipartimento Energia Centro Ricerche Frascati, Roma RT/ERG/FUS/97/4 Paper received in April 1997 This report has been prepared and distributed by: Servizio Edizioni Scientifiche - ENEA Centra Ricerche Frascati, C.P. 65 - 00044 Frascati, Rome, Italy The technical and scientific contents of these reports express the opinion of the authors but not necessarily those of ENEA. SUMMARY In the present note the relation is examined between the FTU experimental pro- gramme and the most important issues in controlled thermonuclear fusion resear- ches. FTU is a high-density, high magnetic field tokamak devoted to the study of plasma heating and current drive, energy and particle confinement and plasma- wall interaction. The most important FTU results and their relevance for ITER will be discussed. FTU, TOKAMAK, CONTROLLED THERMONUCLEAR FUSION, ITER RIASSUNTO In questa nota si esamina il rapporto tra il programma sperimentale di FTU e le problematiche piu' important delle ricerche nell'ambito della fusione nucleare con- trollata. FTU e' un tokamak ad alta densita' ed alto campo il cui programma speri- mentale e' principalmente indirizzato allo studio del conflnamento, allo studio dei metodi di riscaldamento del plasma e di generazione di corrente e allo studio dei meccanismi di interazione plasma-parete in presenza di varie tipologie di materiali. Verranno presentati i risultati piu' importanti ottenuti da FTU in questi campi e ver- ra' discussa la loro rilevanza per ITER INDICE FTU IN THE CONTEXT OF THE FUSION PROGRAMME p. 8 TEST OF HEATING AND CURRENT DRIVE SCHEMES p. 10 INVESTIGATION OF ENERGY AND PARTICLE TRANSPORT. p. 13 TEST OF FIRST-WALL MATERIALS p. 16 THEORETICAL STUDIES ON ALPHA-PARTICLE IMPACT ON PLASMA STABILITY p. 16 CONCLUSIONS p. 18 REFERENCES p. 19 M2XT PAGte(S) ief t El&r.K FRASCATI TOKAMAK UPGRADE (FTU): RESULTS AND DEVELOPMENTS * The tokamak configuration is obtained by superposing on a toroidal magnetic field produced by external windings a poloidal magnetic field produced mainly by the plasma current flowing in the toroidal direction. The simplicity of the tokamak configuration has allowed the achievement of the most promising results, in view of the develoment of a reactor. Tokamaks have obtained the best performances in terms of energy confinement and the understanding of energy and particles transport mechanisms is making continuous progress. As a consequence, we are confident about our capability of being able to design experiments for the investigation of ignition and plasma burn by extrapolating tokamak perfomance through experimentally determined (scaling laws). Furthermore, the tokamak community is continuing its effort, in view of the development of a demonstration reactor, to find the solutions that can minimize its cost. This effort involves both basic physics research and technology R&D. As we will show in the following, relevant studies can be conveniently carried out on small-scale experiments such as FTU (Frascati Tokamak Upgrade). The most important issues addressed by FTU are the following: Test of heating and current drive schemes. In the present experiments the plasma current is basically produced under transient conditions by inducing an electric field in the toroidal direction. The plasma current in a reactor must be driven under steady-state conditions. Even though most of the current will be generated by the plasma itself (bootstrap current), a fraction of the current must be sustained by external means. Therefore efficient current drive methods must be developed. Investigation of energy and particle transport. Even though we are sufficiently confident about our knowledge of confinement to design ignition experiments, the achievement of new regimes with enhanced confinement, recently observed on several tokamaks, makes the investigation of energy transport a field of interest for the development of a tokamak reactor, since the use of enhanced confinement scenarios could make the ignition margin larger, leading to smaller size for the same power and to a more cost effective reactor. Test of first-wall materials. The particle and heat coming out of the plasma ultimately hit the wall surface, releasing impurity atoms. This may have a deleterious effect on plasma dynamics since the impurity atoms can penetrate the plasma column, and, after ionization, replace the Deuterium/Tritium ions, reducing in this way the fusion power. Thus, wall materials Presented during the visit of the Fusion Evaluation Board, September 6th, 1996 able to sustain high particle and heat fluxes, without significant impurity release, must be studied for a reactor. All the above issues are investigated in the context of the FTU experimental programme and will be described in the following, trying to elucidate the results so far obtained by FTU and the most important developments. A brief description of the theoretical activity on the a- particle impact on plasma stability will be also presented. For reference, the parameters of FTU with respect to those of JET (i.e. the largest present experiment) and ITER are listed below: B(T) I(MA) R(m) Paux(MW) FTU 8 1.6 0.93 4.6 JET 3.5 6 3.0 20 ITER 5.7 21 8.1 300 FTU in the context of the Fusion Programme. FTU is a compact, high-magnetic-field experiment constructed along the line of the ALCATOR experiments at MIT and of the FT (Frascati Tokamak) experiment in Frascati. The compact, high magnetic field devices are characterized by a large current density value with B being the toroidal magnetic field intensity and R the major radius. Thus, the combination of large B values and small dimensions allows the achievement of large j values. This, in turn, has several advantages: 1) The power density PQ associated to the ohmic heating is large, with T|, the plasma resistivity, depending on the plasma temperature. Thus, the plasma can be heated up to relatively high temperatures without the need of strong auxiliary heating systems. 2) High particle density values can be achieved. Indeed, the particle density n in tokamaks cannot be arbitrarily increased. Above a critical value nmax. the plasma disrupts and the equilibrium configuration is lost. Such a critical value has been empirically found to scale as the current density nmax Thus, compact, high field experiments can easily reach high values of the particle density. 3) Since the particle density is large, the mean-free-path for the ionization of an impurity atom released from the wall is short and the plasma is efficiently screened from the penetration of impurities. Very pure plasma can indeed be obtained on these devices. 4) In addition, the large value of the particle density allows the study of plasma with moderately high-values of collisionality. In such a situation the equipartition time between electrons and ions is short compared with the energy confinement time and, as a consequence, the electron and ion temperatures are almost equal. It should be stressed that this is indeed the condition typical of a reactor. Upon using conservative assumption on the FTU confinement, at the maximum heating power a central temperature of 6 keV can be reached, at n=1020nr3, allowing the investigation of tokamak physics in a broad parameter range. In order to understand on which extent the parameter range explored by FTU is relevant for reactor operation (and different from the range explored by other devices), it is convenient to consider the dimensionless parameters which describe the plasma dynamics. A tokamak is characterized by several dimensional parameters such as magnetic field B, plasma current Ip, major radius R, minor radius a, plasma density n, temperature T, etc. However, not all these parameters are in fact independent, due to the existence of invariance properties of the equations of plasma physics under scale transformation. Thus, the original set of dimensional parameters can be ultimately reduced to a smaller set of dimensionless parameters. The dimensionless parameters, in turn, can be divided into two groups: the geometrical parameters (aspect ratio R/a, flux surface shape, etc.), which are determined by macroscopic stability considerations and therefore do not significantly change within the existing experiments, and three physical parameters: • the ratio P between thermal and magnetic energy; • the ratio X.* between the electron/ion mean-free-path and the macroscopic device dimensions; • the ratio p* between, the ion Larmor radius and the macroscopic deyice dimensions; It can be shown that the physical dynamics of a device is described by the dimensionless parameters p\ X* and' p». 10 10 o ITER FTU 10 o TFTR 10 TORE SUPRA TEXTOR o 10 0.1 P.(*> 1 Fig 1 - Range in collisionality (A*) and normalized Larmor radius (p*) covered by various experiment with circular cross section, compared with the range of interest for ITER. Therefore it is essential for the programme to have experiments able to cover a dimensionless parameter range close to that of a reactor and sufficiently wider in order to allow a reliable extrapolation. As an example, in Fig. 1 the range in the (A,*,p») plane, at fixed density value n=1.3xl020nr3, typically investigated by several experiments with about the same geometrical parameters (aspect ratio ~ 3, circular flux surfaces), is compared with the parameter range of interest for ITER at the same density value. It is possible to note that FTU potentially covers a range in X,* which is not covered by other devices and close to the range of ITER. Test of heating and current drive schemes Radio frequency (RF) waves can be absorbed by ions or electrons and heat the plasma or can be used to give momentum to the electron population and generate a plasma current.