The Pumped Divertor THE NEW PHASE OF JET B.E. Keen and M.L. Watkins and the JET Team JET Joint Undertaking, Abingdon, Oxford, UK Associate Member of EPS The pumped divertor experiment, in demonstrating before full deuterium-tritium operation an effective method of impurity control, aims to provide essential design data for a Next Step tokamak fusion device. The basic principle of the fusion pro­ Switzerland and Sweden. By mid-1983, by careful design of the targets and speci­ cess is the fusing of light nuclei to form the construction of JET, its power supplies fic operational techniques, but is limited, heavier ones and the accompanying re­ and buildings were completed on sche­ ultimately, by an unacceptably high influx lease of substantial energy. For a fusion dule and broadly to budget and the prog­ of impurities. The fourth area of work had reactor, there are several possible fusion ramme started. been started by earlier studies of energetic reactions, but the one that is easiest to JET is the largest project in the coordi­ particles produced as fusion products or achieve is that between the deuterium and nated programme of EURATOM, whose by ion cyclotron resonance heating tritium isotopes of hydrogen. This D-T fusion programme is designed to lead ulti­ (ICRH). It has now been addressed further reaction is : mately to the construction of an energy by the first tokamak plasma experiments D + T → 4He + neutrons + 17.6 MeV. producing reactor. Its strategy is based on in D-T mixtures. These results are presen­ At the temperatures needed for this reac­ the sequential construction of major ap­ ted briefly in the following sections. tion to occur, the D-T fuel is in the plasma paratus such as JET, a Next Step device state, comprising a mixture of charged and a demonstration reactor (which Plasma Performance and Impurity particles (nuclei and electrons), which can should be a full ignition, high power de­ Control be contained by magnetic fields. The most vice), supported by medium sized specia­ JET is in the second half of its experi­ effective magnetic configuration is the to­ lised tokamaks. mental programme. The technical design roidal tokamak device, of which the Joint The objective of JET is to obtain and specifications of JET have been achieved European Torus is the largest in operation. study a plasma in conditions and dimen­ in all parameters and exceeded in several For a DT fusion reactor, the triple fusion sions approaching those needed in a ther­ cases (see Table 1). The plasma current of product of the temperature Ti, the density monuclear reactor [1]. This involves four 7 MA and the current duration of up to ni and energy confinement time τE must main areas of work to study: 60 s are world records and are more than exceed the value niτETi of 5 x 1021 m-3 - various methods of heating plasmas to twice the values achieved in any other skeV. This measures the performance of a the thermonuclear regime; fusion experiment. The neutral beam in­ fusion device and shows how close condi­ - the scaling of plasma behaviour as jection heating system has been brought tions are to those of a reactor. Typically, parameters approach the reactor range; up to full power (≈ 21 MW) and the ion for a reactor based on magnetic confine­ - the interaction of plasma with the ves­ cyclotron resonance frequency (ICRF) ment concepts, the following are required : sel walls and how to continuously fuel and heating power has been increased to - central ion temperature, T i: 10-20 keV exhaust the plasma; ≈ 22 MW in the plasma. In combination, - central ion density, ni: 2.5 x 1020 m-3 - the production of alpha-particles gene­ these heating systems have provided over - global energy confinement time, τE: 1-2 s rated in the fusion of D and T atoms and 35 MW of power to the plasma. During the early 1970's, it was clear the consequent heating of plasma by Since the start of operation in 1983 the that the achievement of near-reactor con­ these alpha-particles. study of plasma-wall interactions under ditions required much larger experiments, The first and second areas of work have such high power conditions and the con­ which were likely to be beyond the re­ now been well covered and the third area trol of impurities have always been con­ sources of any individual country. In 1973, has been well studied in the limiter confi­ sidered as key scientific and technical it was decided in Europe that a large guration for which JET was originally de­ issues to which particular attention has device, the Joint European Torus (JET), signed. However, the highest performance been paid. Impurity production has been should be built as a joint venture. The for­ JET discharges have been mal organization of the Project — the JET obtained with a "magnetic Table 1 — JET parameters Joint Undertaking — was set up near limiter", that is, in the so- Abingdon, UK, in 1978. The Project Team called X-point configura­ Parameters Design Achieved is drawn from EURATOM and the fourteen tion with a magnetic sepa- member nations — the twelve European values values ratrix inside the vacuum plasma major radius, R0 2.96 m 2.5-3.4 m Community countries, together with vessel, with plasma con­ tacting localised areas of plasma minor radius - horizontal 1.25 m 0.8-1.2 m B.E. Keen leads the Publications Group at the JET - vertical 2.1 m 0.8-2.1 m Joint Undertaking, Abingdon, Oxon 0X14 3EA, wall (the X-point targets) UK. He received a Ph.D. in low temperature phy­ and detached from the limi­ toroidal field at R0 3.45 T 3.45 T sics from Oxford in 1962 and then worked at ters (the first points of con­ Oxford, Yale University, Sussex Univ., and Cul- plasma current - limiter mode 4.8 MA 7.1 MA ham Laboratory (from 1967) before joining JET. tact with the plasma) ex­ - single X-point not foreseen 5.1 MA M.L. Watkins is a Scientific Assistant to the cept during the formation - double X-point not foreseen 4.5 MA Director of JET. After receiving a Ph.D. in plasma of the discharge. The du­ neutral beam power - 80 kV, D 20 MW 21 MW physics from Imperial College, London, in 1971 he ration of the high perfor­ joined the UKAEA Culham Laboratory where he - 140 kV, D 15 MW 15 MW worked on the JET project before being assigned mance phase of these dis­ ICRH power to plasma 15 MW 22 MW to JET in 1979. charges can exceed 1.5 s Europhys. News 23 (1992) 123 reduced by both passive methods of im­ sient, lasting for less than 1 s. It could not by the proper selection of target materials purity control (such as the proper choice be sustained in the steady state: the impu­ and by reducing the plasma temperature of plasma facing components — beryl­ rity influx observed with carbon walls also at the targets as far as possible. Although lium or beryllium carbide) and active me­ occurs with Be and causes a degradation the principal source of impurities is well thods (such as sweeping the magnetic of plasma parameters. This emphasizes removed from the main plasma, sputtered configuration, and hence the plasma, the need for improved methods of impu­ impurities cannot be eliminated comple­ across the targets at which the plasma- rity control in fusion devices. tely and these have then to be retained in wall interaction is often localised). The the divertor region in order to avoid conta­ resulting significant improvements in plas­ The New Phase of JET and the Pumped minating the main plasma. In principle, ma performance have brought JET to Divertor this can be achieved by a strong flow of equivalent "breakeven" conditions and to The aim of the New Phase is to de­ deuterium directed towards the targets, within a factor of 5 of the fusion triple pro­ monstrate, prior to full D-T operation in preventing the back diffusion of impurities duct needed for a fusion reactor. JET, effective methods of impurity control under the influence of frictional forces. Up to 1988, JET operated with a carbon in operating conditions close to those of a The X-point should be well separated from first wall (carbon tiles and wall carbonisa­ Next Step tokamak, with a stationary the targets, allowing a long connection tion). A fusion triple product of 2.5 x plasma of "thermonuclear grade" in an length (≈ 5-10 m) along the open mag­ 1020 m-3 keVs was achieved [2]. The at­ axisymmetric pumped divertor configura­ netic field lines, between the X-point re­ tainment of higher plasma parameters tion. The New Phase should demonstrate gion and the targets. This allows the plas­ was limited by impurity influxes, mostly a concept of impurity control; determine ma temperature near the targets to fall to carbon and oxygen, from the walls. The the size and geometry needed to realise acceptable levels and the effective screen­ impurities diluted the plasma fuel, thereby this concept in a Next Step tokamak; ing of impurities to occur. decreasing the fusion reactivity and in­ allow a choice of suitable plasma facing The general features of the JET pumped creasing the radiative energy losses. Ex­ components; and demonstrate the opera­ divertor are illustrated in Fig. 1b-d. It is of cessively high impurity influxes (called the tional domain for such a device. The New the "open" type with Be-clad, copper tar­ "carbon bloom") were observed during Phase for JET started in 1992 [4], with get plates based on so-called Hypervapo- high power heating and led to a rapid dete­ first results becoming available in 1993; trons which are water cooled elements for rioration of plasma parameters and fusion the programme will then continue to the transferring large quantities of heat from a performance [3].