The paper provides a brief introduction to the main aims, overall design philosophy and the planned parameter range of the large tokamak device (major radius R = 2.96 m; horizontal and vertical minor radii, respectively: a= 1.25 m, b = 2.10 m; plasma current, I p = 4.8 MA), the Joint European Torus (JET), situated on the Culham Laboratory site, UK, whose main objective is to obtain and study plasmas in conditions and with dimensions approaching those needed in a fusion reactor. The main emphasis is on initial operation in the Ohmicheating phase, in which results are presented covering a wide range of parameters: plasma currents I p < 3.7 MA; toroidal magnetic fields BT= 1.3-3.4 T; elongation ratios b/a= 1.2-1.7; and safety factor values, q = 2.3-10. Average electron densities n e = (1-4) X 10 19 m~3, with high central electron temperatures (T e up to 5 keV) and ion temperatures (Tj up to 3 keV) have been achieved, although Z e ff was in the range of 2.4-10. Energy confinement times (T E ) of up to 0.8 s have been obtained.' Some problems with metallic and low-Z impurities are noted, causing high radiation levels. Initial experiments, with ion cyclotron resonance frequency (ICRF) heating of H and 3 He minorities in deuterium plasmas at MW levels, are reported. The paper concludes with a description of planned future experiments on impurity control, additional heating (ICRF «» 15 MW, and neutral injection « 10 MW), and preparations for tritium operation.
Results are presented which show the classical anharmonic resonance effects (including hysteresis) on a marginal ion sound instability, when forcing at the fundamental and the subharmonic frequencies. Also, when the instability is well defined it appears to behave as a classical Van der Pol oscillator for drive frequencies near the fundamental and the subharmonics.
Based upon latest results tom JET and other tokamaks and upon model projections, the operating conditions of a fusion reactor are predicted. The technical and scientific issues involved incontinuous operationof such areactor using non-inductive c m n t drive are identified. Candidate techniques include injecting beams of high energy neutralparticles andradio wavesat variousfrequencies (suchasfast.lowerhybridandelectroncyclouonwaves). The basisforacontinuouslvoneratinereactorisnotanoarentandaconvincinedemonstration wouldreaukeither ~ ~~~~ ahighcmnt driveeffici&cy' (abovg 1&dm3w) ai; density above -IP&; or ignition and adeqnate impurity control at a density of 5x 1 Oi9m3 with moderate current drive efficiency (-0.Sx lCroNmzw); or high power operation in a regime with a dominant bootstrap current. Semi-continuous operation with inductive "rent drive offers the only viable alternative for long pulse reactor operation. This could use either forwardcurrent or alternating current operation. provided the centraisolenoid wassufficiently large. A tokamak reactor operating semi-continuously would be simpler in construction. use re-circulating power more efficiently and would likely be more reliable in operation. It is proposed that the Next Step tokamak be based on inductive semi-continuous operation
Results are presented which show that a self-excited ion-sound instability in a plasma behaves in a similar manner to a classical Van der Pol oscillator, both (a) in a single mode régime, and (b) in a multi-mode régime. In the single mode régime various methods are described which enable the linear growth rate α0 to be measured. These are checked by direct measurement and a consistent value is obtained. In the multi-mode régime, values are obtained for mode-mode coupling coefficients, and non-linear saturation coefficients for each mode present in the system, as well as for the parameters associated with the growth rates of these modes. Comparison with a theory developed to describe these two systems, shows good agreement with the experimental measurements.
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