211 research outputs found
Multi-machine analysis of termination scenarios with comparison to simulations of controlled shutdown of ITER discharges
To improve our understanding of the dynamics and control of ITER terminations, a study has
been carried out on data from existing tokamaks. The aim of this joint analysis is to compare
the assumptions for ITER terminations with the present experience basis. The study examined
the parameter ranges in which present day devices operated during their terminations, as
well as the dynamics of these parameters. The analysis of a database, built using a selected
set of experimental termination cases, showed that, the H-mode density decays slower than
the plasma current ramp-down. The consequential increase in fGW limits the duration of the
H-mode phase or result in disruptions. The lower temperatures after the drop out of H-mode
will allow the plasma internal inductance to increase. But vertical stability control remains
manageable in ITER at high internal inductance when accompanied by a strong elongation
reduction. This will result in ITER terminations remaining longer at low q (q95 ~ 3) than
most present-day devices during the current ramp-down. A fast power ramp-down leads
to a larger change in βp at the H–L transition, but the experimental data showed that these are manageable for the ITER radial position control. The analysis of JET data shows that
radiation and impurity levels significantly alter the H–L transition dynamics. Self-consistent
calculations of the impurity content and resulting radiation should be taken into account when
modelling ITER termination scenarios. The results from this analysis can be used to better
prescribe the inputs for the detailed modelling and preparation of ITER termination scenariosDoE Awards DE-FC02-99ER54512DoE Awards DE-AC02- 76CH03073DoE Awards DE-FC02-04ER54698EURATOM 63305
On the extrapolation to ITER of discharges in present tokamaks
An expression for the extrapolated fusion gain G = Pfusion /5 Pheat (Pfusion
being the total fusion power and Pheat the total heating power) of ITER in
terms of the confinement improvement factor (H) and the normalised beta (betaN)
is derived in this paper. It is shown that an increase in normalised beta can
be expected to have a negative or neutral influence on G depending on the
chosen confinement scaling law. Figures of merit like H betaN / q95^2 should be
used with care, since large values of this quantity do not guarantee high
values of G, and might not be attainable with the heating power installed on
ITER.Comment: 6 Pages, 3 figures, Submitted to Nuclear Fusion on the 29th of
November 200
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Current driven due to localized electron power deposition in DIII-D
Due to spatial localization of electron cyclotron wave injection in DIII-D, electrons heated in an off-axis region must toroidally transit the tokamak 25--50 times before re-entering the heating region. This distance is of the order of the mean free path. The effect of such RF localization is simulated with a time-dependent Fokker-Planck code which is 2D-in-velocity, 1D-in-space-along-B, and periodic in space. An effective parallel electric field arises to maintain continuity of the driven current. Somewhat surprisingly, the localized current drive efficiency remains equal to that for a uniform medium
Experimental investigation and validation of neutral beam current drive for ITER through ITPA Joint Experiments
Joint experiments investigating the off-axis neutral beam current drive (NBCD) capability to be utilized for advanced operation scenario development in ITER were conducted in four tokamaks (ASDEX Upgrade (AUG), DIII-D, JT-60U and MAST) through the international tokamak physics activity (ITPA). The following results were obtained in the joint experiments, where the toroidal field, B t, covered 0.4-3.7 T, the plasma current, Ip, 0.5-1.2 MA, and the beam energy, Eb, 65-350 keV. A current profile broadened by off-axis NBCD was observed in MAST. In DIII-D and JT-60U, the NB driven current profile has been evaluated using motional Stark effect diagnostics and good agreement between the measured and calculated NB driven current profile was observed. In AUG (at low δ ∼ 0.2) and DIII-D, introduction of a fast-ion diffusion coefficient of Db ∼ 0.3-0.5 m2 s-1 in the calculation gave better agreement at high heating power (5 MW and 7.2 MW, respectively), suggesting anomalous transport of fast ions by turbulence. It was found through these ITPA joint experiments that NBCD related physics quantities reasonably agree with calculations (with Db = 0-0.5 m2 s-1) in all devices when there is no magnetohydrodynamic (MHD) activity except ELMs. Proximity of measured off-axis beam driven current to the corresponding calculation with Db = 0 has been discussed for ITER in terms of a theoretically predicted scaling of fast-ion diffusion that depends on Eb/Te for electrostatic turbulence or βt for electromagnetic turbulence. © 2011 IAEA, Vienna
On the mechanisms governing gas penetration into a tokamak plasma during a massive gas injection
A new 1D radial fluid code, IMAGINE, is used to simulate the penetration of gas into a tokamak plasma during a massive gas injection (MGI). The main result is that the gas is in general strongly braked as it reaches the plasma, due to mechanisms related to charge exchange and (to a smaller extent) recombination. As a result, only a fraction of the gas penetrates into the plasma. Also, a shock wave is created in the gas which propagates away from the plasma, braking and compressing the incoming gas. Simulation results are quantitatively consistent, at least in terms of orders of magnitude, with experimental data for a D 2 MGI into a JET Ohmic plasma. Simulations of MGI into the background plasma surrounding a runaway electron beam show that if the background electron density is too high, the gas may not penetrate, suggesting a possible explanation for the recent results of Reux et al in JET (2015 Nucl. Fusion 55 093013)
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