| 1. |
- Hawkes, N. C., et al.
(author)
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Observation of zero current density in the core of JET discharges with lower hybrid heating and current drive
- 2001
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 8711:11, s. art. no.-115001
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Journal article (peer-reviewed)abstract
- Simultaneous current ramping and application of lower hybrid heating and current drive (LHCD) have produced a region with zero current density within measurement errors in the core (r/a less than or equal to 0.2) of JET tokamak optimized shear discharges. The reduction of core current density is consistent with a simple physical explanation and numerical simulations of radial current diffusion including the effects of LHCD. However, the core current density is clamped at zero, indicating the existence of a physical mechanism which prevents it from becoming negative.
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| 2. |
- Hawkes, N. C., et al.
(author)
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The formation and evolution of extreme shear reversal in JET and its influence on local thermal transport
- 2002
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In: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 0741-3335 .- 1361-6587. ; 44:7, s. 1105-1125
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Journal article (peer-reviewed)abstract
- In JET discharges where lower hybrid heating and current drive (LHCD) is applied early during the current ramp, a region of the plasma with zero current density is formed near the axis. At the boundary of this region the current density is large and B-theta increases rapidly over a small distance. In the central region the safety factor, q, is effectively infinite, but this falls steeply in the boundary region. Outside the boundary region q reaches a minimum, where the magnetic shears equivalent to r/q (dq/dr) becomes zero. The formation of this region of zero current is dependent on both the heating and the current drive effects of the LHCD. When LHCD is switched off the current profile begins to relax towards the resistive peaked current distribution of fully inductive tokamak operation. If LHCD is not used in the current rise then these current profiles are not established. Although the physical mechanism exists to drive the central plasma current below zero, in most cases it appears to be prevented from going negative. At least one MHD mechanism has been identified which could be responsible for this. The presence of the zero central current is closely linked to the periodic relaxation events seen in these discharges. In these discharges, internal transport barriers have been observed with additional heating powers substantially below the values required to obtain barriers in monotonic q profile cases.
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| 3. |
- Kuldkepp, Matias, et al.
(author)
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First mirror contamination studies for polarimetry motional Stark effect measurements for ITER
- 2004
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In: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 75, s. 3446-3448
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Journal article (peer-reviewed)abstract
- The motional Stark effect (MSE) diagnostic on the International Thermonuclear Experimental Reactor will need to guide the light through a labyrinth of mirrors to provide neutron shielding. Knowledge of how the mirrors change the polarization is essential for accurate determination of the q profile. The optical properties of the plasma facing mirror are also expected to change with time due to deposition/erosion. For the purpose of examining this experimentally a detector system, identical to the JET MSE system, using twin photoelastic modulators was constructed. Measurements have been performed on freshly prepared mirrors, on mirrors after exposure to plasmas in Tore Supra, and labyrinth designs. The result shows a significant effect on the optical properties and demonstrate the need for in situ monitoring. The measured properties of the labyrinth closely follow the Mueller matrix formalism. With a correct choice of material the angle change introduced by the four mirrors furthest away from the plasma will be below 1 degrees.
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| 4. |
- Mantsinen, M. J., et al.
(author)
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On the role of different phasings of the ICRF antennas in optimized shear discharges in JET
- 2000
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In: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 40:10, s. 1773-1789
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Journal article (peer-reviewed)abstract
- In discharges with internal transport barriers produced by combined NBI and ICRF heating using the hydrogen minority scheme in JET, confinement and fusion performance are strongly affected by the direction of propagation of the ICRF waves. When the waves propagate along the plasma current, the formation of an internal transport barrier is prompter and the neutron yield is up to a factor of two higher than that for propagation against the current. An ICRF induced pinch of resonating trapped ions is put forward as a candidate for explaining the observations. Simulation results are presented which show that this effect is strong enough to provide a credible explanation for the experimental results.
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| 5. |
- von Hellermann, M, et al.
(author)
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Pilot experiments for the International Thermonuclear Experimental Reactor active beam spectroscopy diagnostic
- 2004
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In: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 75:10, s. 3458-3461
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Journal article (peer-reviewed)abstract
- Supporting pilot experiments and activities which are currently considered or already performed for the development of the International Thermonuclear Experiment Reactor active beam spectroscopy diagnostic are addressed in this article. Four key issues are presented including optimization of spectral instrumentation, feasibility of a motional Stark effect (MSE) evaluation based on line ratios, "first-mirror" test-bed experiments at the tokamak TEXTOR, and finally the role of integrated data analysis for the conceptual layout of the change exchange recombination spectroscopy and MSE diagnostic.
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