| 1. |
- Hellsten, Torbjörn, et al.
(författare)
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Fast wave current drive in JET ITB-plasma
- 2005
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Ingår i: AIP Conference Proceedings. - : AIP. - 0094-243X. ; , s. 273-278
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Konferensbidrag (refereegranskat)abstract
- Fast wave current drive has been performed in JET plasmas with internal transport barriers, ITBs, and strongly reversed magnetic shear. Although the current drive efficiency of the power absorbed on the electrons is fairly high, only small effects are seen in the central current density. The main reasons are the parasitic absorption of RF power, the strongly inductive nature of the plasma and the interplay between the fast wave driven current and bootstrap current. The direct electron heating in the FWCD experiments is found to be strongly degraded compared to that with the dipole phasing.
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| 2. |
- Litaudon, X., et al.
(författare)
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Prospects for steady-state scenarios on JET
- 2007
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 47:9, s. 1285-1292
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Tidskriftsartikel (refereegranskat)abstract
- In the 2006 experimental campaign, progress has been made on JET to operate non-inductive scenarios at higher applied powers (31 MW) and density (n(1) similar to 4 x 10(19) m(-3)), with ITER-relevant safety factor (q(95) similar to 5) and plasma shaping, taking advantage of the new divertor capabilities. The extrapolation of the performance using transport modelling benchmarked on the experimental database indicates that the foreseen power upgrade (similar to 45 MW) will allow the development of non-inductive scenarios where the bootstrap current is maximized together with the fusion yield and not, as in present-day experiments, at its expense. The tools for the long-term JET programme are the new ITER-like ICRH antenna (similar to 15 MW), an upgrade of the NB power (35 MW/20s or 17.5 MW/40s), a new ITER-like first wall, a new pellet injector for edge localized mode control together with improved diagnostic and control capability. Operation with the new wall will set new constraints on non-inductive scenarios that are already addressed experimentally and in the modelling. The fusion performance and driven current that could be reached at high density and power have been estimated using either 0D or 1-1/2D validated transport models. In the high power case (45 MW), the calculations indicate the potential for the operational space of the non-inductive regime to be extended in terms of current (similar to 2.5 MA) and density (n(1) > 5 x 10(19) m(-3)), with high beta(N) (beta(N) > 3.0) and a fraction of the bootstrap current within 60-70% at high toroidal field (similar to 3.5 T).
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| 3. |
- Stork, D., et al.
(författare)
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Overview of transport, fast particle and heating and current drive physics using tritium in JET plasmas
- 2005
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Ingår i: Nuclear Fusion. - 0029-5515 .- 1741-4326. ; 45:10, s. S181-S194
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Tidskriftsartikel (refereegranskat)abstract
- Results are presented from the JET Trace Tritium Experimental (TTE) campaign using minority tritium (T) plasmas (n(T)/n(D) < 3%). Thermal tritium particle transport coefficients (D-T, nu(T)) are found to exceed neo-classical values in all regimes, except in ELMy H-modes at high densities and in the region of internal transport barriers (ITBs) in reversed shear plasmas. In ELMy H-mode dimensionless parameter scans, at q(95) 2.8 and triangularity delta = 0.2, the T particle transport scales in a gyro-Bohm manner in the inner plasma (r/a < 0.4), whilst the outer plasma particle transport scaling is more Bohm-like. Dimensionless parameter scans show contrasting behaviour for the trace particle confinement (increases with collisionality, nu* and beta) and bulk energy confinement (decreases with nu* and is independent of beta). In an extended ELMy H-mode data set, with rho*, nu*, and q varied but with neo-classical tearing modes (NTMs) either absent or limited to weak, benign core modes (4/3 or above), the multiparameter fit to the normalized diffusion coefficient in the outer plasma (0.65 < r/a < 0.8) gives D-T/B-phi similar to rho*(2.46) nu*(-0.23) beta(-1.01) q(2.03). In hybrid scenarios (q(min) similar to 1, low positive shear, no sawteeth), the T particle confinement is found to scale with increasing triangularity and plasma current. Comparing regimes (ELMy H-mode, ITB plasma and hybrid scenarios) in the outer plasma region, a correlation of high values of D-T with high values Of nu(T) is seen. The normalized diffusion coefficients for the hybrid and ITB scenarios do not fit the scaling derived for ELMy H-modes. The normalized tritium diffusion scales with normalized poloidal Larmor radius (rho(theta)* = q rho*) in a manner close to gyro-Bohm (similar to rho(sigma)*(3)), with an added inverse P dependence. The effects of ELMs, sawteeth and NTMs on the T particle transport are described. Fast-ion confinement in current-hole (CH) plasmas was tested in TTE by tritium neutral beam injection into JET CH plasmas. gamma-rays from the reactions of fusion alpha and beryllium impurities (Be-9(alpha, n gamma)C-12) characterized the fast fusion-alpha population evolution. The gamma-decay times are consistent with classical alpha plus parent fast triton slowing down times (tau(Ts) + tau(alpha s)) for high plasma currents (I-p > 2 MA) and monotonic q-profiles. In CH discharges the gamma-ray emission decay times are much lower than classical (tau(Ts) + tau(alpha s)), indicating alpha confinement degradation, due to the orbit losses and particle orbit drift predicted by a 3-D Fokker-Planck numerical code and modelled using TRANSP.
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| 4. |
- Rantamäki, K., et al.
(författare)
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LH wave coupling over ITER-like distances at JET
- 2007
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Ingår i: Radio Frequency Power in Plasmas. - : American Institute of Physics (AIP). - 0735404445 - 9780735404441 ; , s. 261-264
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Konferensbidrag (refereegranskat)abstract
- Good coupling of LH power at plasma-launcher distance of 15 cm has been obtained at JET. Near-gas injection is used to increase the density in front of the grill. The role of LH power in the density increase at constant gas level is demonstrated. For the first time at JET the temperature of the hot spots caused by parasitic absorption of LH power has been measured.
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| 5. |
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| 6. |
- Corre, Y., et al.
(författare)
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Hybrid H-mode scenario with nitrogen seeding and type III ELMs in JET
- 2008
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 0741-3335 .- 1361-6587. ; 50:11, s. 115012-
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Tidskriftsartikel (refereegranskat)abstract
- The performance of the 'hybrid' H-mode regime (long pulse operation with high neutron fluency) has been extensively investigated in JET during the 2005-2007 experimental campaign up to normalized pressure beta(N) = 3, toroidal magnetic field B-t = 1.7T, with type I ELMs plasma edge conditions. The optimized external current drive sources, self-generated non-inductive bootstrap current and plasma core stability properties provide a good prospect of achieving a high fusion gain at reduced plasma current for long durations in ITER. One of the remaining issues is the erosion of the divertor target plates associated with the type I ELM regime. A possible solution could be to operate with a plasma edge in the type III ELM regime (reduced transient and stationary heat loads) obtained with impurity seeding. An integrated hybrid type III ELM regime with a normalized pressure beta(N) = 2.6 (PNBI similar to 20-22 MW) and a thermal confinement factor of H-98* 98(y, 2) similar to 0.83 has been recently successfully developed on JET with nitrogen seeding. This scenario shows good plasma edge condition (compatible with the future ITER-like wall on JET) and moderate MHD activity. In this paper, we report on the experimental development of the scenario (with plasma current I-p = 1.7MA and magnetic field B-t = 1.7T) and the trade-off between heat load reduction at the target plates and global confinement due to nitrogen seeding and type III ELM working conditions.
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| 7. |
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| 8. |
- de Vries, P. C., et al.
(författare)
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Effect of toroidal field ripple on plasma rotation in JET
- 2008
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 48:3
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Tidskriftsartikel (refereegranskat)abstract
- Dedicated experiments on TF ripple effects on the performance of tokamak plasmas have been carried out at JET. The TF ripple was found to have a profound effect on the plasma rotation. The central Mach number, M, defined as the ratio of the rotation velocity and the thermal velocity, was found to drop as a function of TF ripple amplitude (3) from an average value of M = 0.40-0.55 for operations at the standard JET ripple of 6 = 0.08% to M = 0.25-0.40 for 6 = 0.5% and M = 0.1-0.3 for delta = 1%. TF ripple effects should be considered when estimating the plasma rotation in ITER. With standard co-current injection of neutral beam injection (NBI), plasmas were found to rotate in the co-current direction. However, for higher TF ripple amplitudes (delta similar to 1%) an area of counter rotation developed at the edge of the plasma, while the core kept its co-rotation. The edge counter rotation was found to depend, besides on the TF ripple amplitude, on the edge temperature. The observed reduction of toroidal plasma rotation with increasing TF ripple could partly be explained by TF ripple induced losses of energetic ions, injected by NBI. However, the calculated torque due to these losses was insufficient to explain the observed counter rotation and its scaling with edge parameters. It is suggested that additional TF ripple induced losses of thermal ions contribute to this effect.
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| 9. |
- de Vries, P. C., et al.
(författare)
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Effect of toroidal field ripple on the formation of internal transport barriers
- 2008
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 0741-3335 .- 1361-6587. ; 50:6
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Tidskriftsartikel (refereegranskat)abstract
- The effect of a toroidal field (TF) ripple on the formation and performance of internal transport barriers (ITBs) has been studied in JET. It was found that the TF ripple had a profound effect on the toroidal plasma rotation. An increased TF ripple up to delta = 1% led to a lower rotation and reduced the rotational shear in the region where the ITBs were formed. ITB triggering events were observed in all cases and it is thought that the rotational shear may be less important for this process than, for example, the q-profile. However, the increase in the pressure gradient following the ITB trigger was reduced in discharges with a larger TF ripple and consequently a lower rotational shear. This suggests that toroidal rotation and its shear play a role in the growth of the ITB once it has been triggered.
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| 10. |
- Eriksson, L. -G, et al.
(författare)
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Toroidal rotation in RF heated JET plasmas
- 2007
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Ingår i: RADIO FREQUENCY POWER IN PLASMAS. - : AIP. - 9780735404441 ; , s. 59-62
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Konferensbidrag (refereegranskat)abstract
- Experiments have been carried out on JET aimed at studying rotation in RF heated plasmas with low external momentum input. Both plasmas with Ion Cyclotron Resonance Frequency (ICRF) heating and Lower Hybrid Current Drive (LHCD) have been investigated. The rotation profiles are measured by Charge Exchange recombination spectroscopy, using short diagnostic Neutral Beam Injection (NBI) pulses. Moreover, the temporal evolution of the central rotation could in some cases be deduced from MHD activity. While most of the measurements were focussed on ICRF heating, the profiles measured in plasmas with LHCD are interesting since they are the first reported from JET in such plasmas. In particular, they allowed for studies of rotation in RF heated plasmas with q>1. The experimental results are presented together with an analysis of the torque from ICRF heated fast ions.
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