Chapter 2 196 FUSION Issues and Achievements

neutrons are produced, or in deuterium, for which the neutron production rate is Magnetic Confinement Fusion almost two orders of magnitude lower than in a deuterium-tritium mixture. To achieve the conditions necessary for David Campbell ‘ignition’, where the α-particle power pro­ The NET Team, Max-Planck-Institut für Plasmaphysik, Garching, Germany duced by fusion reactions exactly balances the heat loss due to transport processes, the plasma must be heated to a tempera­ ture of approximately 108 K (nearly a fac­ tor of 10 greater than the temperature at the centre of the sun) at a particle density Left A schematic of the Wendelstein in the region of 1020 ions per cubic metre 7-X optimized stellarator, under (about 10-6 the density of air), while main­ In the course of the construction at Greifswald in taining an energy replacement time of Northern Germany. The last 50 years 'modular coil' about 5 seconds. The energy replacement research on mag­ arrangement which time is a measure of the confinement netically confined produces the helical quality of the plasma system and it is plasmas has magnetic field defined by the ratio of the thermal energy brought magnetic geometry is evident and stored in the plasma to the power lost by confinement a helical magnetic field conduction and convection. This should fusion to the thresh­ surface can be seen within not be confused with plasma lifetime, old of net power pro­ the coils.This device has a major radius of 5.5 meters and ar which is usually much longer. How the duction and has revealed 'effective'minor radius of 0.55 meters required plasma conditions are attained, much of the physics underlying the prospects for maintaining them in the astonishingly complex behaviour of the kinetic energy of the neutron, which steady-state, and the processes that affect hot plasmas immersed in a magnetic field. would be absorbed in the structure of a confinement quality, plasma stability, and The simplest approach to magnetic power plant and provide most of the ener­ plasma-wall interactions are outlined in confinement uses the longitudinal field gy for steam generation. The α-particle the following sections. inside a long straight solenoid. would be trapped in the plasma where its Unfortunately, the rapid transport of par­ energy would heat the plasma and main­ Toroidal magnetic confinement ticles along the field lines leads to unsus­ tain the conditions required for fusion Three principal toroidal confinement sys­ tainable losses from the ends of the con­ reactions to occur. Since tritium is a tems have emerged as potential routes to a figuration. If the solenoid is bent into a radioactive gas and since a high flux of fusion power plant. In the tokamak, doughnut, or torus, the problem of end 14.1 MeV neutrons would induce signifi­ figure lb, a strong toroidal field of several losses immediately disappears. However, a cant radioactivity in the structure sur­ Tesla is produced by a set of discrete coils. toroidal magnetic field is curved and its rounding the plasma, current experiments Transformer coupling between a vertical magnitude falls off with increasing radius on magnetically confined plasmas are usu­ solenoid which is wound inside the central from the centre of the torus. Both these ally carried out in hydrogen, so that no hole of the torus and the toroidal ring of properties cause charged particles to drift up or down across the field lines. Since ions and electrons drift in opposite direc­ tions a large electric field develops further enhancing the loss of plasma across field lines. As shown in figure la, which defines key parameters of toroidal geometry, a poloidal magnetic field, acting at right angles to the toroidal field, can be added. The resultant field consists of a set of nest­ ed helical field lines, and as the particles follow these helical trajectories around the torus, the drifts are cancelled producing a well confined magnetoplasma. The focus of contemporary fusion research is the deuterium-tritium reaction 1D2 + 1T3 → 2He4 (3.5 MeV) + 0n1 (14.1 MeV) Fig 1a Schematic of a toroidal magnetic confinement 1b Spatial variation of the toroidal and poloidal fields system illustrating the principal parameters and the across the poloidal cross-section of the torus in a which is the fusion reaction with the magnetic geometry.The distance from the central axis of tokamak. Note the difference in magnitude of the two symmetry to the centre of the toroidal ring is the major while the poloidal field, Bpol, is directed around the short largest cross-section at the temperatures radius, R, and the distance from the centre of the toroidal circumference. The field line corresponding to the total which are likely to be achieved in labora­ ring to its edge is the minor radius, a. The ratio R/a is field then forms a helix around the torus, and the tory experiments (several 108 K). Eighty known as the 'aspect ratio'. The toroidal field, Btor, Is magnetic configuration consists of a set of nested helices per cent of the reaction energy appears as directed around the long circumference of the torus with different pitch angles see colour illustration above 2.1 Magnetic Confinement Fusion Campbell Europhysics News November/December 1998 November/December News Europhysics FUSION 197

have important consequences for the tokamak following the announcement in growth of instabilities in the plasma ring. 1968, by a group at the Kurchatov Institute The third class of toroidal confinement in Moscow, that plasmas in the T-3 toka­ devices is the stellarator (figure 1d) which mak had reached the then unheard of differs in an essential way from tokamaks temperature of 1 keV. Since 1968 tokamak and RFPs in that the helical fields are cre­ dimensions have increased from major ated entirely by coils external to the plas­ and minor radii of 1 metre and 0.12 meters ma, with no net toroidal current flowing in T-3 to the region of 3 and 1 meters within the plasma (see Alejaldre, chapter respectively in the current generation of 5.1). In a stellarator, the magnitudes of the large tokamaks, represented by JET in the toroidal and poloidal fields are similar to UK and the Japanese JT-60U experiment those of a tokamak and thus the range of in Naka. The plasma volume has increased q values in the plasma is of the same order from ~o.3 m3 to ~100 m3 and the current in the two devices. However, in the stel­ carrying capability, which has an impor­ Fig 1 c Spatial variation of the toroidal and poloidal fields larator q has a weak radial variation and tant bearing on plasma confinement in in a reversed field pinch. Note that the two are of similar falls towards the plasma edge. tokamaks, from 60 kA to 7 MA. The paral­ magnitude and that the toroidal field changes sign close In terms of power plant constraints the lel development of high power plasma to the plasma edge (courtesy D.E. Baker and W.E. Quinn, absence of a net toroidal current is a par­ heating systems has allowed the central Fusion, edited by E Teller, Academic Press, New York, 1981, ticularly attractive feature of stellarators. plasma electron temperature to be 437-475) However, the stellarator plasma forms a increased from 1 keV in T-3 to above plasma induces a toroidal current. This toroidally periodic, three-dimensional 10 keV in JET, JT-60U, and the US TFTR produces the poloidal component of the structure (as shown in the colour illustra­ device in Princeton. The increase in cen­ magnetic field and also raises the plasma tion on the previous page) whereas toka­ tral ion temperatures into the range 30- temperature, to perhaps several keV, by mak and RFP plasmas consist of a toroidal 40 keV has been even more spectacular. Joule (or ohmic) heating (1 keV = ring of circular or elliptical poloidal cross- Until the first plasmas were produced 1.16X107 K is the conventional unit of tem­ section within which helical magnetic early in 1998 in the new Japanese LHD perature in fusion research). Additional field lines are embedded, as in figure 1a. stellarator in Toki the largest stellarators poloidal field components can then be This additional complexity degrades the had a plasma volume of ~1 m3, compara­ applied by external coils to shape the plas­ plasma confinement properties of a stel­ ble to that of the generation of tokamaks ma and control its position. larator, but the identification, in the 1980s, constructed in the late 1970s. The advent The reversed field pinch (RFP) is a of a class of helical geometries which of LHD, and the Wendelstein 7-X (W7-X) closely related configuration, since the approximate the intrinsic confinement experiment, which is under construction plasma formation and ohmic heating are properties of tokamaks revitalized stel­ at Greifswald, Germany (see Europhysics essentially identical to the tokamak. larator research (see Nührenberg, chapter News 26 3 1995), both with plasma vol­ However, in the tokamak the average 3.4). An additional and significant advan­ umes of ~30 m3, will bring stellarators into poloidal field is limited by stability tage of these ‘advanced stellarators’ is that line with the largest of the current genera­ requirements to approximately an order of the magnetic configuration can be estab­ tion of medium-sized tokamaks, such as magnitude smaller than the toroidal field, lished by a toroidally distributed set of ASDEX Upgrade in Garching, Germany, whereas the two are of similar magnitude ‘modular’ coils (see previous page). Tore Supra in Cadarache, France, and in the RFP, both being typically less than At present the development of plasma DIII-D in San Diego. It is not surprising, 1 Tesla (figure 1c). Moreover, to provide parameters required for power production therefore, that the highest ion and electron stability the toroidal field must reverse is most advanced in the tokamak. This is temperatures obtained in stellarators are sign at the edge of an RFP (hence the due, in part, to the increased focus on the 1.5 keV and 5.8 keV respectively, values name) and this occurs as a result of which were achieved in Wendelstein 7-AS poloidal currents which develop inside the (W7-AS) in Garching. plasma due to a magnetic relaxation Although RFPs such as MST in Madison process. This is referred to as the RFP and RFX in Padua, Italy, have plasma vol­ dynamo and it leads to a minimum energy umes of 6-8 m3 and plasma current capa­ state for the configuration. bilities of 1-2 MA, and are therefore equiv­ The helical field in toroidal configura­ alent in these respects to a medium-sized tions can be described by the ‘safety fac­ tokamak, the maximum ion temperature tor’ q (or by the rotational transform 1 = achieved to date has been ~1 keV. This 2π/q). This is most simply thought of as a reflects the poorer confinement quality of ‘magnetic winding number’ which gives RFP plasmas, which is due in part to the the number of times a helical field line lower magnetic field and in part to the must circle the torus toroidally before it magnetic relaxation (dynamo) processes returns to the same position in the associated with the formation of the poloidal plane. Since the poloidal and reversed field equilibrium. toroidal fields are of the same order in the RFP, q < 1 everywhere and falls towards Plasma heating and confinement Fig 1d Spatial variation of the toroidal and poloidal To attain the necessary plasma parameters the plasma edge, whereas in the tokamak fields in a stellarator. Values at two toroidal positions are q is of order unity at the plasma centre shown (ϕ=0° and ϕ=36°) to illustrate the toroidally for ignition, where α-particle heating is and rises to 3-5 near the plasma edge. The asymmetric nature of a stellarator plasma—the sufficient to maintain the plasma tempera­ absolute value of q and the way in which it tokamak and RFP profiles are the same for any toroidal ture, the plasma must be heated in some varies across the plasma cross-section position way. In tokamaks and RFPs, ohmic heating1998 November/December News Europhysics 198 FUSION

is unlikely to be adequate to reach igni­ toroidal periodicity of the plasma gives In the absence of a quantitative under­ tion, while stellarators lack even this ele­ rise to helically trapped particles, whose standing of the processes determining mentary form of heating. Four auxiliary orbits deviate significantly from surfaces transport, a more empirical approach to plasma-heating techniques have therefore formed by the magnetic field lines and can the characterization of plasma confine­ been developed to an advanced level—the even intersect the wall of the plasma ment has been developed. This relates the diversity of approaches illustrates the chamber. The configuration optimization global-energy-confinement time of a plas­ links that fusion research has established approach exploited in ‘advanced stellara­ ma configuration to macroscopic plasma with many other areas of physics. tors’, which will be implemented in the parameters, and has the merit that it offers Intense beams of neutral particles with W7-X experiment is expected to minimize a route to determining how large a device energies in the 100 keV range and total the loss of such particles. is required to be in order to achieve the powers of up to 40 MW can be injected The actual transport losses measured in energy-confinement time required for across the magnetic field so that they col­ tokamaks and stellarators turn out to be ignition. Confinement in the majority of lide with the plasma particles, are ionized, similar to each other, but much worse tokamak plasmas can be described by an confined, and slow down, transferring than originally expected. In particular, in energy-confinement time scaling which their energy to the background plasma. tokamaks the rate of heat loss through the depends linearly on the plasma current, Plasmas can also be heated by RF waves electrons is approximately two orders of the 1.5 power of the characteristic length using the variety of resonant absorption magnitude greater than predicted on the scale (eg the major radius), and inversely processes which occur. Ion cyclotron heat­ basis of‘neoclassical theory’, which on the total input power raised to a power ing operates at frequencies in the range derives heat and particle transport rates between 1/2 and 2/3. There are also other, 30-120 MHz; lower hybrid wave heating, from the underlying Coulomb collisions, less significant, dependences. Intriguingly, named after a coupled resonance of the taking the effects of toroidal geometry this type of confinement behaviour, ions and electrons, operates in the region touched on in the previous paragraph into known as low mode, or L-mode, finds a of 1-8 GHz; and electron cyclotron reso­ account. It is believed that the enhanced parallel in the confinement behaviour in nance absorption involves frequencies in rates of transport losses are due to small- stellarators when the plasma current is the region of 30-140 GHz. RF systems scale instabilities with wavelengths very replaced by the magnetic field. In RFPs the capable of launching up to 20 MW high level of magnetic fluctuations in for many seconds are operational. the plasma interior associated with the The most significant application of sustainment of the configuration lower hybrid wave heating in mod­ means that the plasma thermal insula­ ern tokamak experiments is in the tion is provided by a thin layer at the driving of plasma current, since a plasma edge. As a result, energy-con­ toroidally asymmetric wave spec­ finement times in the largest RFPs are trum can be launched, which only a few milliseconds, or 1-2 orders of imparts a net momentum to the magnitude smaller than that in toka­ electron population. The other maks of equivalent size and current. three heating schemes also have a While L-mode confinement repre­ current drive capability, but their sents the ‘lowest common denomina­ efficiency (the ratio of driven cur­ tor’ of tokamak and stellarator confine rent to input power) is lower than ment, many plasma regimes have been that of lower hybrid waves. identified in which the global energy For ignition it is necessary not confinement exceeds the predictions of only to achieve high temperatures, L-mode scaling by a factor of 2 or but to retain energy within the more. The most reproducible of these plasma for sufficiently long that the is the high confinement regime, or H- rate of α-particle heating balances mode, which was first observed in 1982 the power lost by conduction, con­ in the ASDEX divertor tokamak in vection and radiation. In toroidal Garching. Energy confinement is typi­ confinement systems magnetic field cally twice as high as L-mode, though lines close-up within the torus and power Fig 2 Energy-confinement time plotted against the the scaling with plasma parameters is is then lost by transport across field lines. input power for a series of L- and H-mode plasmas in the broadly similar in the two regimes (figure The irreducible minimum transport rate is JET tokamak. The lower curve (dashed) Is derived from 2). In recent years this regime has also then set by Coulomb collisions between an empirical energy-confinement time scaling in which been observed in the W7-AS stellarator the confinement time varies inversely with square root plasma particles. Toroidal geometry can of the input power, while the upper line corresponds to and, although the improvement in energy play a significant role in these processes twice this scaling. This illustrates that H-mode energy confinement is not as great as in toka­ since a fraction of the particles can be confinement is approximately a factor of 2 higher than L- maks, the details of the transition and trapped by magnetic mirrors within the mode energy confinement, but that the scaling with subsequent plasma behaviour appear very plasma. Indeed, a particle following a heli­ plasma parameters is broadly similar for the two regimes similar in the two systems. cal field line sees a magnetic field of vary­ (courtesy JET Joint Undertaking) The physics processes which produce ing intensity and will be reflected when the H-mode transition are not yet fully reaching a region of stronger magnetic much smaller than the plasma minor understood (see Romanelli and Hidalgo, field if it has insufficient velocity parallel radius. The proposed instabilities are dri­ chapter 3.1), but it is clear that the transi­ to the field line. These ‘trapped particles’ ven by the free energy available in the tion originates at the plasma edge. Here are well confined in a tokamak, but their plasma pressure and magnetic field pro­ the temperature and density profiles existence can be inferred from macroscop­ files (see Romanelli and Hidalgo, chapter become very steep in a narrow layer, per­

Europhysics News November/December 1998 November/December News Europhysics ic measurements. In stellarators the 3.1 and Garbet chapter 3.5). haps only several centimetres wide, and FUSION 199 strong poloidal and toroidal plasma flows tion of the field lines. The theory of MHD radius which increases with p. As a result, can occur. A widely accepted explanation stability of toroidal plasmas is highly the maximum β achieved in stellarators is of the H-mode transition is that in the nar­ developed and provides an excellent quali­ -2.2%, obtained in CHS at Nagoya. The β- row edge region the development of a gra­ tative, and in many cases quantitative, induced equilibrium shift can be mini­ dient in the radial electric field, whose ori­ description of plasma behaviour. mized by a process which is closely related gin is not understood, produces sheared For fusion power plants the attainable to the optimization technique discussed plasma flows as a result of the E x B drift. plasma beta, β, is a central issue (see previously. Thus β values as high as 5% These flows can suppress the growth of Villard, chapter 3.2). This is a normalized are considered possible in the W7-X stel- electrostatic instabilities thought to be measure of the plasma pressure, defined larator. The theoretical limit in the TJ-II responsible for the anomalous transport in as the ratio of the average plasma pressure stellarator at Madrid is even higher (see L-mode plasmas (see Romanelli and to the magnetic field pressure. Beta values Alejaldre, chapter 5.1). Hidalgo, chapter 3.1). Recent measurements of several per cent or greater, correspond­ Exceeding pressure, density, or current in RFX have shown that a strongly sheared ing to a plasma pressure of ~3X105 Pascal limits in tokamak plasmas can give rise to plasma flow can occur in the edge region (or several times atmospheric pressure), various MHD instabilities of which the of an RFP plasma, suggesting that turbu­ must be achieved if magnetic fusion most serious is the major disruption. This lent transport can be quenched by a simi­ power plants are to be economically event, which can be triggered by resistive lar mechanism to that observed in toka- viable. In the DIII-D tokamak in San or ideal instabilities, typically quenches maks and stellarators. Diego, values of β as high as 12.5% have the plasma thermal energy in less than In fusion power plants 3.5 MeV α-parti- been achieved for periods much shorter 1 ms and the plasma poloidal magnetic cles produced by the DT reactions give than 1 second and values of 8% for peri­ energy in less than 10 ms. Although these rise to self-heating of the plasma—an ods of ~1 second, while the ‘spherical toka timescales may be up to an order of mag­ effect observed for the first time in mag­ mak’ START at the Culham Laboratory nitude longer in tokamak power plants, netically confined plasmas in recent DT (see Morris, chapter 5.2) has reached β val­ the total energy involved in each stage will experiments in TFTR and JET (see ues of up to 40% for about 1 ms. Beta can be ~1 GJ, so that some areas of the first Keilhacker and Watkins, chapter 4.4). be limited by either ideal or resistive wall of the chamber will be exposed to Alpha particles must therefore be well modes, depending on the current and high energy loads ~100 MJm-2. In addi­ confined to ensure efficient plasma heat­ pressure profiles. tion, inductive effects could give rise to ing and to protect the first wall of the con­ In RFPs, β ~ 10% is typical because the high current pulses, of order 10 MA, in the finement chamber from bombardment by relaxation process sustaining the field metallic structure of the plasma chamber. high energy particles. The identification reversal acts to maintain stability against Large electromechanical forces, ~104 and analysis of possible sources of anom­ β-limiting MHD modes. This attractive tonnes, would result. Experiments are alous losses of α-particles have been par­ feature of RFPs has sustained the long­ underway to develop techniques which ticularly fruitful areas of interaction term interest in the development of this can dissipate the thermal and magnetic between experiment and theory. configuration. In stellarators the β-limit is energy of the plasma in a short timescale Experimental evidence has come from the set by constraints on the global equilibri­ to avoid the most severe disruptive effects. many devices in which high energy ion um rather than by MHD instabilities. The It is regarded as an important advantage populations produced by additional heat­ force balance between plasma pressure of stellarators and RFPs that neither suf­ ing systems have been used to simulate α- and magnetic field in a toroidal system fers from this form of instability. particle populations, as well as from the produces currents parallel and perpendic­ TFTR and JET DT experiments. This has ular to the magnetic fields lines, though Power and particle exhaust provided extensive benchmarking of theo­ no net toroidal current results. The paral­ In a fusion power plant the several hun­ retical predictions and, although some lel current component causes a change in dred MW of power deposited by α-parti- areas of theoretical uncertainty remain, the plasma equilibrium, essentially a shift cles must be exhausted in a way which extrapolation to experiments such as ITER of the plasma column along the major does not exceed the power handling capa- (see Parker, chapter 4.3) indicates that a- particle confinement will be acceptable in Fig 3 Poloidal cross-section ignited plasmas. of the 'island divertor' geometry in the Wendelstein Magnetohydrodynamic stability 7-AS stellarator in Garching Magnetohydrodynamic (MHD) theory in Germany, showing the describes the stability properties of a plas­ magnetic configuration with ma fluid immersed in a magnetic field. a ring of magnetic islands at Two general classes of instabilities can be the plasma edge which act identified. Ideal instabilities would occur as localized divertors when even if the plasma were perfectly conduct­ they interact with a material ing and they involve motions in which the target plasma and magnetic field are essentially ‘frozen’ together. Resistive instabilities involve the small but finite plasma resis­ tivity—typical plasmas in modern fusion experiments have a resistivity of the order of that of room temperature copper (~2X10-8 Ωm). These instabilities can involve rearrangement of the magnetic field topology by tearing and reconnec­ Europhysics News November/December 1998 200 FUSION

bility of the first wall, -5-10 MWm-2, and LHD. An alternative concept, the ‘island advantage in the context of steady-state which does not lead to excessive impurity divertor’, which is formed when an appro­ operation since there is no requirement production which would contaminate the priate value of the edge rotational trans­ for an externally driven current in the plasma and quench the thermonuclear form is selected, has been demonstrated plasma. The superconducting coils of the burn. Impurity production also causes successfully on the W7-AS stellarator (fig­ LHD and W-7X experiments will allow a erosion of first wall materials and this ure 3) and is planned for W7-X. Control of steady-state magnetic configuration to be must be limited to ensure an adequate the plasma-wall interaction in RFPs is less established for the first time, so that the lifetime for the surfaces in contact with well developed, though it is now receiving remaining physics issues required to the plasma edge. The thermalized α-parti- increased attention. While the majority of establish the credibility of these systems cles, or ‘helium ash’, must in addition be RFPs operate with a limiter, a divertor as power plants can be addressed—ade­ pumped away from the plasma edge to configuration has been studied in TPE-2M quate confinement scaling to keep the size avoid their poisoning the plasma and in Tsukuba in Japan. of ignition experiments within practical slowing the reaction rate. Exhaust of helium ash from the plasma limits, acceptable divertor operation to In present experiments, where there is must be rapid enough so that the resi­ provide the requisite power handling and little thermonuclear power, two solutions dence time of helium in the vacuum particle control, and the demonstration of to the power handling problem have been chamber is at most 5-10 times the energy- the required plasma β for economic power developed. In the ‘limiter’ concept a spe­ confinement time. To achieve this helium production. cially prepared surface intersects the heli­ ions must diffuse sufficiently rapidly Both RFPs and tokamaks use a toroidal cal field lines just inside the bore of the across the plasma and then the neutral­ plasma current to establish the plasma toroidal vacuum chamber and defines the ized helium must be pumped away. It has equilibrium and a source of current other plasma edge. In the ‘divertor’ concept the been demonstrated on several tokamaks than that produced by transformer action plasma boundary is defined by a magnetic that helium injected into the plasma by is required for steady-state operation. In surface known as a ‘separatrix’, which is neutral beams or by gas puffing is trans­ RFPs, moreover, the magnetic turbulence formed by creating a position of zero field ported across the plasma at the same rate associated with the dynamo responsible (a ‘null’ or ‘X-point’) in the for field reversal is thought to con­ poloidal field distribution by tribute significantly to the poor means of external coils. Field energy confinement observed. lines outside the separatrix then Reducing the level of this turbu­ intersect a ‘target plate’ (see lence is therefore the major chal­ Schneider, chapter3.3). lenge facing research on RFPs. Power handling limitations External poloidal current drive by are similar for divertors and pulsing of the toroidal field coils, limiters and an additional fac­ called pulsed poloidal current tor is needed. The solution, drive (PPCD), has been studied on devised initially in the TEXTOR the MST and RFX devices. These tokamak in Jülich in Germany, experiments have shown that the is to radiate as much of the magnetic fluctuations measured at plasma power as possible, vir­ the plasma edge are strongly tually too per cent in some reduced, MHD activity is sup­ experiments, by‘seeding’ the pressed, and energy confinement plasma with small quantities of can improve by up to a factor of 5. noble gases such as neon. Such Figure 4 illustrates the associated impurities enhance the radia­ improvement in central electron tion primarily from the plasma temperature observed in RFX due edge, which limits their contri­ to the reduction in core magnetic bution to transport in the plas­ fluctuations. Fig 4 Radial profiles of electron temperature measured ma core. In this way the power is effective­ in the RFX reversed field pinch at the Istituto Gas Providing a steady-state current drive ly redistributed over the entire wall of the lonizzati del CNR in Padua. Temperature profiles in capability is one of the most significant vacuum chamber, reducing the power normal operation (squares) and during pulsed poloidal issues facing tokamak research and this handling problem to a manageable level. current drive operation (triangles) are compared. This has been addressed by exploiting the cur­ Nevertheless, major challenges remain in illustrates the improvement in core plasma confinement rent drive capability of additional heating reducing the impact of the radiation on which is obtained during pulsed poloidal current drive systems. For example, externally driven plasma confinement and in minimizing operation as a result of the reduction in magnetic plasma currents of up to 3.6 MA for sever­ the contamination of the plasma by the fluctuations al seconds have been obtained in JT-60U radiating impurities. using about 10 MW of lower hybrid waves. Stellarator and RFP research has as other particles and that sufficiently This technique has also been used to pro­ focussed more on the development of core high neutral pressure can be obtained at duce a 2 hour pulse at 20 kA in the plasma performance and is only now the plasma edge to sustain the helium TRIAM-1 tokamak in Kyushu and a 120 addressing the problem of power exhaust. pumping speed (1.5 Pa m3 per GW of second pulse at 800 kA in Tore Supra. In one class of stellarator, the heliotron, a fusion power) required for tokamak Nevertheless, at the current drive efficien­ ‘helical divertor’ structure can be formed power plants. cies expected in tokamak power plants, naturally in the magnetic field at the plas­ several 100 MW of additional heating ma edge. This configuration has been Steady-State Operation power would be required to achieve full investigated in the Heliotron-E experi­ Among the three principal magnetic con­ non-inductive current drive, which is unre­

Europhysics News November/December 1998 ment in Nagoya and is being exploited in finement systems stellarators have a clear alistic from an economic point of view. FUSION 201

Fig 5 Progress in the fusion performance parameter, nτET, during the last 40 years of magnetic confinement research— n is the central plasma density, τE the energy-confinement time, and T the central ion temperature. This parameter must reach a value of ~5x1021 nr3skeV for a plasma to achieve ignition. The largest experiments, JET, JT-60U and TFTR have achieved values of ~1x1021 nv3skeV.The apparent saturation of the trend in recent years reflects the fact that these large experiments are essentially at the limit of their capability and that a new device, such as ITER (for which the predicted value is shown), is required to produce the parameters necessary for an ignited plasma (courtesy J.A. Wesson Tokamaks second edition, Oxford University Press, 1997)

In tokamaks, therefore, the challenge of yet been fully characterized. Nevertheless, steady-state tokamak operation, and the providing a steady-state current drive its existence points the way to the develop­ realization of a variety of new tools for capability would be daunting were it not ment of a plasma regime (the ‘advanced enhancing plasma performance in RFPs. for an important plasma-induced effect: tokamak scenario’) with better confine­ Finally, given the production of over the ‘bootstrap current’. This is a toroidal ment than is currently achieved together 10 MW of DT fusion power in TFTR and current, driven by the radial pressure gra­ with good MHD stability and a high frac­ 16 MW in JET, plasma performance in dient, which arises from the exchange of tion of bootstrap current. tokamaks has advanced to the point where momentum between trapped and ‘passing’ the construction of a DT-burning plasma (ie non-trapped) particles in a toroidal Status experiment such as ITER would be a time­ confinement system. Its existence is a fur­ The progress in magnetic fusion research ly next step. ther experimental confirmation of the is shown in figure 5, which shows the The key issues and achievements in neoclassical theory of plasma behaviour steady increase in the fusion performance magnetic confinement research have been discussed earlier (see plasma heating and parameter, nτET (the product of plasma reviewed briefly here, and it has been pos­ confinement in this chapter). In specially density, energy-confinement time, and ion sible to touch on only a very few of the prepared discharges in the largest toka- temperature) during the last 40 years. This impressive achievements in experimental, mak experiments up to 80 per cent of the must reach a value of ~5X1021 m-3skeV for theoretical, and computational physics, as plasma current has been driven in this a plasma to achieve ignition. The increas­ well as in engineering, which have accom­ way. Steady-state current drive could thus ing scale of magnetic confinement experi­ panied the development of toroidal con­ be achieved in a tokamak power plant by ments, together with the accompanying finement systems to their present level of driving the remaining fraction of the cur­ improvements in the understanding of the sophistication. In particular it has not rent using one or more of the additional physics of magnetoplasmas, has raised the been possible to illustrate adequately that heating systems at a power level of values attained experimentally by 7 orders magnetic confinement fusion research has ~ 100 MW, which would be both practical of magnitude and has brought the field to proved an extremely fertile meeting and economically viable. the present point, where the largest exper­ ground for many branches of physics and Significant progress has been made iments are within a factor of 5 of the engineering. recently in developing a tokamak plasma required value. On this basis it can be regime which combines important ele­ expected that the parameters of the ITER The author would like to thank Rosario ments required for steady-state operation. tokamak (see Parker, chapter 4.3) are ade­ Bartiromo of the Istituto Gas Ionizzati del Plasmas have been established in which the quate to ensure that ignition will be CNR in Padua, Fritz Wagner of the Max- q-profile has a minimum at approximately achieved. Planck-Institut für Plasmaphysik in 1/3 to 1/2 of the minor radius and then Three principal conclusions can be Garching, R. Koch of École Royale Militaire remains flat, or increases, towards the plas­ drawn about the present status of magnet­ in Brussels, and also L. Giannone, A. ma centre. The transport losses at the plas­ ic confinement fusion. Firstly, there is now Loarte, and E. Righi for preparing several ma centre then reduce substantially and the substantial, though still incomplete, of the figures for this review. MHD stability improves. There is some evi­ understanding of plasma behaviour in the dence that the reduced transport is associ­ principal toroidal confinement configura­ Further reading ated with the development of an increased tions, and there is a much deeper appreci­ R.J. Goldston, P.H. Rutherford Introduction to electric field gradient near the plasma cen­ ation of the complexity of the physics of Plasma Physics (Institute of Physics Publishing, Bristol, tre which gives rise to an ‘internal trans­ high temperature magnetoplasmas. 1995) port barrier’. The high confinement then Secondly, new opportunities for further R.O. Dendy editor Plasma Physics: An Introductory leads to the development of a high pressure improvement in plasma performance are Course (Cambridge University Press, 1993) gradient, on which the bootstrap current opening with the advent of a new genera­ J.A. Wesson Tokamaks second edition (Oxford depends. In present experiments this plas­ tion of large stellarators such as LHD, the University Press, 1997) J. Sheffield The Physics of Magnetic Fusion Reactors ma regime is usually transient, lasting only development of ‘advanced tokamak sce­ Rev.Mod.Phys. 6 6(1994) 1015

1 second or so, and its properties have not narios’, which may offer a viable route to Europhysics News Novem ber/December 1998