741 research outputs found
Main chamber wall plasma loads in JET-ITER-like wall at high radiated fraction
Future tokamak reactors of conventional design will require high levels of exhaust power dissipation (more than 90% of the input power) if power densities at the divertor targets are to remain compatible with active cooling. Impurity seeded H-mode discharges in JET-ITER-like Wall (ILW) have reached a max- imum radiative fraction ( F rad ) of ∼75%. Divertor Langmuir probe (LP) measurements in these discharges indicate, however, that less than ∼3% of the thermal plasma power reaches the targets, suggesting a missing channel for power loss. This paper presents experimental evidence from limiter LP for enhanced cross-field particle fluxes on the main chamber walls at high F rad . In H-mode nitrogen-seeded discharges with F rad increasing from ∼30% to up to ∼75%, the main chamber wall particle fluence rises by a factor ∼3 while the divertor plasma fluence drops by one order of magnitude. Contribution of main chamber wall particle losses to detachment, as suggested by EDGE2D-EIRENE modeling, is not sufficient to explain the magnitude of the observed divertor fluence reduction. An intermediate detached case obtained at F rad ∼60% with neon seeding is also presented. Heat loads were measured using the main chamber wall thermocouples. Comparison between thermocouple and bolometry measurements shows that the frac- tion of the input power transported to the main chamber wall remains below ∼5%, whatever the divertor detachment state is. Main chamber sputtering of beryllium by deuterium is reduced in detached condi- tions only on the low field side. If the fraction of power exhaust dissipated to the main chamber wall by cross-field transport in future reactors is similar to the JET-ILW levels, wall plasma power loading should not be an issue. However, other contributions such as charge exchange may be a problem.EURATOM 63305
An FPGA-based bolometer for the MAST-U Super-X divertor
A new resistive bolometer system has been developed for MAST-Upgrade. It will measure radiated
power in the new Super-X divertor, with millisecond time resolution, along 16 vertical and 16
horizontal lines of sight. The system uses a Xilinx Zynq-7000 series Field-Programmable Gate Array
(FPGA) in the D-TACQ ACQ2106 carrier to perform real time data acquisition and signal processing.
The FPGA enables AC-synchronous detection using high performance digital filtering to achieve a high signal-to-noise ratio and will be able to output processed data in real time with millisecond
latency. The system has been installed on 8 previously unused channels of the JET vertical bolometer system. Initial results suggest good agreement with data from existing vertical channels but with higher bandwidth and signal-to-noise ratio.Engineering and Physical Sciences Research Council EP/L01663X/1EURATOM 63305
First Observation of a Stable Highly Dissipative Divertor Plasma Regime on the Wendelstein 7-X Stellarator
The evolution of the bound particle reservoir in Wendelstein 7-X and its influence on plasma control
Improved Confinement in JET High {beta} Plasmas with an ITER-Like Wall
The replacement of the JET carbon wall (C-wall) by a Be/W ITER-like wall
(ILW) has affected the plasma energy confinement. To investigate this,
experiments have been performed with both the C-wall and ILW to vary the
heating power over a wide range for plasmas with different shapes.Comment: 50 pages, 32 figures. This is an author-created, un-copyedited
version of an article submitted for publication in Nuclear Fusion. IoP
Publishing Ltd is not responsible for any errors or omissions in this version
of the manuscript or any version derived from i
Comparison of H-mode plasmas in JET-ILW and JET-C with and without nitrogen seeding
In high confinement mode, highly shaped plasmas with edge localized modes in JET, and for heating power of 15-17 MW, the edge fluid code EDGE2D-EIRENE predicts transition to detachment assisted by nitrogen at the low field side (LFS) target when more than 50% of the power crossing the separatrix between ELMs is radiated in the divertor chamber, i.e. ∼4 MW. This is observed both in the ITER-like wall (JET-ILW) and in the carbon wall (JET-C) configurations and is consistent with experimental observations within their uncertainty. In these conditions, peak heat fluxes below 1 MW m-2 are measured at the LFS target and predicted for both wall configurations. When the JET-C configuration is replaced with the JET-ILW, a factor of two reduction in the divertor radiated power and 25-50% increase in the peak and total power deposited to the LFS divertor plate is predicted by EDGE2D-EIRENE for unseeded plasmas similar to experimental observations. At the detachment threshold, EDGE2D-EIRENE shows that nitrogen radiates more than 80% of the total divertor radiation in JET-ILW with beryllium contributing less than a few %. With JET-C, nitrogen radiates more than 70% with carbon providing less than 20% of the total radiation. Therefore, the lower intrinsic divertor radiation with JET-ILW is compensated by stronger nitrogen radiation contribution in simulations leading to detachment at similar total divertor radiation fractions. 20-100% higher deuterium molecular fraction in the divertor recycling fluxes is predicted with light JET-C materials when compared to heavy tungsten. EDGE2D-EIRENE simulations indicate that the stronger molecular contribution can reduce the divertor peak power deposition in high recycling conditions by 10-20% due to enhanced power dissipation by molecular interaction
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