IT9700689

Associazione EURATOM-ENEA sulla Fusione

FRASCATI 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 resear- ches. FTU is a high-density, high magnetic field tokamak devoted to the study of 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. 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 . 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. On FTU three RF heating1 and current drive systems are installed, corresponding to three different frequency ranges: 11

lower hybrid waves (8 GHz);

electron cyclotron waves (140 GHz);

ion Bernstein waves (433 MHz)

Lower hybrid waves. The lower hybrid experiment was designed in order to test this method at particle density values higher than the other experiments and comparable with those of ITER. It was indeed recognized in the early experiments that above a critical density nc the interaction between lower hybrid waves and electron population was no longer possible. The theoretical explanation, proposed by the Frascati team, [1] led to the conclusion that the critical density should increase approximately as f2, with f being the wave frequency. The explanation, consistent with the data obtained from the lower hybrid experiment at 2.45 GHz on FT (1984), was later confirmed by several other tokamaks and motivated the choice of 8 GHz on FTU, in order to investigate high-density regimes (n=102°m~3).

The choice of such a frequency range has required a major technological effort, in particular promoting the development of new gyrotrons by Thomson Tube Electroniques.

The lower hybrid experiment started on FTU at the end of 1992 with a limited amount of power. During 1993 a discharge was produced with the plasma current entirely driven by lower hybrid waves at a moderate density (n«0.4xl020nr3)[2]. Since then, the system has been upgraded: the present capability (1.4 MW delivered to the plasma for Is) will be further upgraded to 2.8 MW by the end of 1996. So far, the interaction between lower hybrid waves and electrons has been observed up to density in the range n=1020nr3, without appreciable deterioration in the current drive efficiency [3]. It should be stressed that this is the same density value envisaged for ITER. In order to compare the FTU results with the results of other tokamaks, it is convenient to use the conventional definition of the current drive efficiency t]'.

I = n( T\ PRF '

The efficiency T|(T) is expected tp depnd on the plasma temperature, according to general theoretical considerations. The obtained current drive efficiency in various experiment is shown in Fig.2......

It is possible to note that larger devices such as JET and JT-60U, which are able to operate at larger temperature, can reach values for the current drive efficiency a factor about two larger than smaller devices (ASDEX, TORE SUPRA and FTU), The shaded region corresponds to the expected domain which can be explored by FTU when the larger available heating power will allow the achievement of substantially larger temperatures. 12

~n 1—i—i—r 0.30 JT - 60 U Efficiency JET Ti (1020 AW-lm-2) TORE SUPRA 0.20 9 ASDEX

FTU (93) FTU (96) 0.10 ITER

0 0.1 Density m-3)

Fig. 2 - Current drive efficiency obtained in different LH experiments at various densities and wave frequency. The shaded area corresponds to the range which will be explored by FTU. The density value of ITER is shown by the vertical line.

Electron Cyclotron Waves The electron cyclotron system is a joint experiment with the Istituto di Fisica del Plasma del CNR (Milan). Electron cyclotron waves have the features to deposit their energy in a very localized region inside the plasma, thus allowing a good control of the power deposition. Very localized deposition was already observed during the preliminary experiment in 1994[4], as shown in Fig.3 where the electron temperature profile before and after the wave injection is shown.

The capability of localized deposition is particularly interesting in view of the possibility of producing regions inside the plasma column with reduced energy transport, as discussed in the next section. If the deposition region of electron cyclotron waves is chosen in such a way to overlap with the region of reduced transport, a large increase in the local temperature is expected. This scenario might be of interest for ITER, with the a-particle heating playing the role of the electron cyclotron power.

Ion Bernstein Waves The ion Bernstein wave system is the only one on FTU to provide possible direct heating of the ions. With respect to similar heating experiments on other tokamaks, it has the unique feature that the coupling structure is very simple being made by waveguides, instead of the conventional antenna inside the vacuum chamber. The experimental 13

Temperature (keV)

4 i i r BEFORE ECRH

0 0.6 0.8 1.0 1.2 0.6 0.8 1.0 1.2 Major radius (m)

Fig. 3 - Temperature profile before and after the injection of electron cyclotron waves in FTU. The localized power deposition, characteristic of ECRH, produces a large increase of the central temperature. results of the PBX-M tokamak in Princeton have shown that such a system can be of interest for the access to enhanced confinement regimes [5].

Investigation of energy and particle transport.

To confine a plasma means to produce a discharge with a hot core and a cold edge. Such a configuration can be maintained only if the plasma has the capability of retaining heat in the central part of the plasma column. This can be quantified in terms of the so-called energy confinement time which measures the characteristic decay time of the plasma energy if the heating system is swiched-off.

Experimentally, a minimum value for the tokamak confinement is obtained in the so-called low-confinement regime (L-mode). However, most of the tokamak devices which operate in the presence of a magnetic separatrix, exhibit a transition to the so called high-confinement regime (H-mode), characterized by the formation of a transport barrier at the edge of the discharge. The ITER parameters have been chosen in order to operate in this regime. Even though the L-mode confinement plays a minor role in a reactor, its understanding may increase the degree of confidence in our knowledge of tokamak physics.

The energy confinement is mostly .affected by turbulent processes occurring in the plasma, and their correct description is still a subject of active studies. Therefore, in order to describe the confinement in L- and H-mode an empirical approach is used: the experimental data of the existing tdkamaks have been used to determine empirical scaling laws for the confinement 14

10

89-P FTU

other ohmic Tokamaks ITER

0.1 0 4 6 8 Magnetic field (T)

Fig.4 • Energy confinement time, normalized to the 1TER89-P scaling, obtained in different tokamaks (C-MOD, JET, ASDEX, JT-60, TEXTOR and T10) compared to those obtained by FTU. The magnetic field value of ITER is shown by the vertical line. time in the various regimes *. At present, the scaling law which better describes the existing L- mode data is the ITER89-P scaling, obtained using the data of auxiliary heated tokamaks. The comparison with the results of tokamaks with ohmic heating only is particularly interesting since it allows an independent validation of the scaling law. FTU has provided a significant contribution to the validation of such empirical scaling law since it can cover a range in magnetic field which is not covered by other tokamaks [6], as shown in Fig.4, and which includes the magnetic field value of ITER.

In addition to the H-mode, other enhanced confinement regimes have been discovered which can be accessed by proper tayloring the particle density and the current density profiles. Density profile peaking results in improved confinement as shown in several experiment in which pellets of solid deuterium are injected in order to increase the particle density in the central part of a discharge. Good confinement is observed also when the peaking occurs

It should be noted that this approach is not fully empirical: the scaling law is indeed constrained to satisfy the invariance properties under scale transformation of the equations describing the plasma dynamics, as written above. ••..-.,• 15

Fig. 5 - Experimental and theoretical values of the parameter m which measures the relative peakedness of the ion temperature and density profiles. Above a threshold in TJ/, electrostatic turbulence is produced which causes anomalous transport. The figure is taken from Ref.7, by the TFTR group.

spontaneously as in the Frascati Tokamak FT and in ASDEX. More recently, attention has been focused on the enhanced confinement obtained with broad current density profiles (the so called low/reversed magnetic shear scenarios) which has been observed on several tokamaks and may lead to the development of more economic steady-state tokamak reactors.

These experimental observations are consistent with the explanation of tokamak transport based on dynamics of the electrostatic turbulence. In the last ten years the theory of this kind of turbulence has been developed by several groups. Frascati has contributed both in the advance of basic physical understanding of the turbulence dynamics and in the development of semi- empirical transport model. An example is given by the turbulence driven by the ion temperature gradient. Here, the control parameter for the onset of the instability is the so called parameter Tij, which measures the relative peakedness of the ion temperature and density profiles. A correlation between the experimentally measured value of the parameter T|i and the threshold value for the onset of the instability has been found. In the following figure [7], the value of Tji measured on the Princeton tokamak TFTR is compared with the theoretical estimate for the threshold [8] showing a good correlation, 16

The present FTU activity is devoted to the possibility of access to enhanced confinement regimes both with peaked density profiles (using the FTU pellet injector) and the control of the current density profile with lower hybrid waves.

It should be noted that, even though the access to such regimes is not crucial for the success of ITER, it might enlarge significantly the margin for the achievement of the ITER objectives.

Test of first-wall materials.

The particle and energy flux from the plasma core on the wall can extract impurity atoms which penetrate into the plasma. The impurity ions can radiate energy and can replace deuterium or tritium ions inside the plasma, reducing the total amount of fusion power. Thus, impurity penetration must be avoided by a proper choice of the wall materials.

FTU is testing several materials for the wall. Due to the reduced dimension, the power per unit area is comparable with that of lTHR. Even though the configuration is different (on ITER the particles escaping from the plasma column are diverted in regions far from the plasma and accelerated towards a plate, whereas in FTU they hit the inner wall) the obtained results are relevant for the choice of the plate material on ITER.

The results obtained so far on FTU [9], with a limited amount of heating power, show that the use of high atomic number material such as Molybdenum and Tungsten can significantly reduce the plasma contamination as shown in Fig.6, where the impurity concentration is quantified in terms of the so called effective charge Zeff, defined as an appropriate average of the charges of the ions in the plasma (Zefpl for a pure Deuterium plasma). Such a behaviour is due to the fact that, in order to estract high-Z impurities, a plasma ion must have a kinetic energy significantly larger than the energy required to estract low-Z impurities.

Indeed, with low-Z materials such as Carbon, the impurity concentration in FTU is significantly larger. It is important to note the beneficial effect of high-particle density operation: as the density increases the plasma contamination decreases for all the considered materials.

This study will continue at power flux densities comparable with those of ITER.

Theoretical studies on a-particle impact on plasma stability

Present tokamak experiment cannot address the problem of the impact of the a-particle population on plasma stability, since the experimental scenarios in which a significant population of energetic particles is produced are very different from those of ITER. Thus, it is necessary to rely on the theoretical predictions. A strong theoretical effort is being pursued at the ENEA Fusion Division. Part of this effort is carried out in collaboration with other institution such as the University of California at Irvine and the Princeton University. 17

Impurity concentration o Zeff c f "\v FTU 5 - \ (1993 - 96) -

4 — 3 - Mo1\ V 2 -

1

i 0 1 2

Density (10^0 m-3)

Fig. 6 - Plasma contamination vs. density on FTU with different wall materials. High-Z materials produce the lower contamination.

The investigations carried out so far have identified regimes in which the effect of a- particle is beneficial on plasma stability. As an example, in the following figure the stability for the so called internal kink mode is shown [10]. This is an instability which produces periodic relaxation of the central part of the plasma column, with a consequent ejection of particle and heat. In the absence of a-particles the mode becomes unstable above a threshold value of the plasma thermal pressure (or, more exactly, of the ratio between thermal and magnetic pressure). If the effect of a-particle is included, the critical 18

UNSTABLE including Pressure s a. - particle j

y\\\NN^\\ without y^"\\\\\\^ a - particle

STABLE x/y/////// X///////// a - particle concentration

Fig. 7 Domain of instability for the internal-kink mode with and without accounting for the effect of a- particles. For a low concentration of a-particles the stability domain is widened. pressure for instability increases with the a-particle concentration, enlarging the domain free from instability (the blu shaded region in the figure). If the a-particle concentration becomes too large, however, the internal kink becomes again unstable. The theoretical investigation of the effect of this instability on the plasma dynamics is in progress.

Conclusions

As a conclusion it is important to stress that low-scale tokamak experiments can investigate important basic-physics problems as well as reactor relevant issues. As an example, we have shown that:

• FTU can test current drive scheme at density values relevant for ITER; • FTU can investigate enhanced confinement regimes through the control of particle density and current density profiles; • FTU can test the impact of wall materials on plasma behaviour at power flux densities comparable to those of ITER.

The improvement in the understanding of the physics of thermonuclear plasmas is closely related to the existence of a strong programme inside the various Associations. 19

REFERENCES

[1] F. Alladio et al., Nucl. Fusion 24, 725 (1984)

[2] A.A. Tuccillo et al. in "Strong Microwaves in Plasmas", Nizhny Novgorod 1994, Vol. l.p.47

[3] F. Alladio et al., Proc. 16th IAEA Con. on Plasma Phys. and Controlled Fusion Research, Montreal 1996, Paper F1-CN-64/E5

[4] S. Cirant et al. Proc. 15th IAEA, Conference on Plasma Phys. and Controlled Research, Seville 1994, Vol. 2, p. 159-166

[5] M. Ono et al. Proc. 15th IAEA Conf. on Plasma Physics and Controlled Nuclear Fusion Research, Seville 1994, paper CN-60/A-3-I-7

[6] S. Kaye, et al., in preparation

[7] S.D. Scott, et al., Phys. Rev. Lett. 64, 531 (1990)

[8] F. Romanelli, Phys. Fluids B 1, 1018 (1989)

[9] M.L. Apicella et al., to appear on Nucl. Fusion

[10] R.B. White, M.N. Bussac and F. Romanelli, Phys. Rev. Lett. 62, 539 (1989) Edito dall' I Unita Comunicazione e Informazione Lungotevere Grande Ammiraglio Thaon di Revel, 76 • 00196 Roma

Stampa: COM-Centro Stampa Tecnografico - C. R. Frascati

Finito di stampare nel mese di maggio 1997