Neutral beam heating data from JET have been analysed in detail to determine what proportion of the current is driven non-inductively. It is found that in low density limiter discharges, currents of the order of 0.5 MA are driven, while in H-mode plasmas currents of the order of 0.7 MA are measured. These measured currents are found to be in reasonable agreement with theoretical predictions based on neoclassical models. In low density plasmas the beam driven current is large while the neoclassical bootstrap current dominates H-mode plasmas.
Equilibrium reconstruction is the essential tool for determining the field configuration and current density in a tokamak discharge. Most equilibrium reconstruction codes use the Grad-Shafranov equation, which relies on the assumption of isotropic pressure. This property is often violated for additionally heated discharges. We report on the implementation of an anisotropic pressure model for the equilibrium reconstruction code EFIT. The anisotropy model exhibits more degrees of freedom and makes the reconstruction more sensitive to experimental errors. We use a regularization technique (L-curve) that attempts to generate an optimal equilibrium. The algorithm is applied to selected highperformance discharges of the tokamaks JET and Tore Supra.
The combination of two regimes of enhanced performance, the H-mode and the pellet enhanced performance (PEP) mode, has been achieved in JET. The strong enhancement of the central plasma parameters, obtained with pellet injection and subsequent auxiliary heating, is found to persist well into the H-mode phase. A characteristic of the PEP regime is that an improvement of the fusion reactivity over non-pellet discharges is obtained under the condition of nearly equal electron and ion temperatures. A maximum neutron production rate of 0.95 × 10l6 s−1 was obtained in a double-null X-point discharge with 2.5 MW of neutral beam heating and 9 MW of ion cyclotron resonance heating, with central ion and electron temperatures of about 10 keV and a central deuterium density of 8.0 × 1019 m−3. The corresponding fusion product nD(0)τETi(0) is between 7.0 and 8.6 × 1020 m−3·s·keV. The enhanced neutron production is predominantly of thermonuclear (Maxwellian) origin. The compatibility of these regimes is an important issue in the context of tokamak ignition strategies. Several technical developments on JET have played a role in the achievement of this result: (1) the use of low voltage plasma breakdown (0.15 V/m) to permit pellet injection in an X-point configuration before the formation of a q = 1 surface; (2) the elimination of ICRH specific impurities with antenna Faraday screens made of solid beryllium; (3) the use of a novel system of plasma radial position control that stabilizes the coupling resistance of the ion cyclotron heating system.
The theoretical basis of the dimensionless parameter scaling technique is derived and the limitations in its application are discussed. The use of the technique is illustrated by the production on JET of a steady-state ITER similarity pulse having the same β and collisionality as the ignited ITER. The key issue of the scaling of the transport with the main dimensionless parameter ρ * is discussed in detail. Finally, possible shortcomings of the technique are examined.
Astract. The change in thermal transport aaoss the L + H transition is studied in detail for those JET high performance H-modes which have a very fast transftion. It is found that in these pulses the transport changes very rapidly (< 4 msecs) over a very large radial region 0.5 < p < 1, and a very large transport barrier is formed. The reasons for the formation of this barrier are discussed.
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