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Comparison of beryllium and graphite first-walls in JET

Description: JET has operated with beryllium as a first-wall material in 1989 and 1990. An initial period with beryllium evaporation onto the original graphite surfaces was followed by operation with beryllium belt-limiter tiles. Beryllium Faraday shields for the ICRH antennae and lower X-point target tiles were installed for experiments in 1990. The use of beryllium has increased the density limit, significantly reduced deconditioning following disruptions, allowed heavy gas fueling for impurity control, reduced the impurity influx from the ICRH antennae so that ICRH-only H modes were possible for the first time and permitted hot-ion plasmas on the outer limiters. This paper describes the primary effects of beryllium which led to these improvements in performance. 11 refs., 3 figs.
Date: January 1, 1990
Creator: Thomas, P.R.
Partner: UNT Libraries Government Documents Department

Edge Stability and Transport Control with Resonant Magnetic Perturbations in Collisionless Tokamak Plasmas

Description: A critical issue for fusion plasma research is the erosion of the first wall of the experimental device due to impulsive heating from repetitive edge magneto-hydrodynamic (MHD) instabilities known as 'edge-localized modes' (ELMs). Here, we show that the addition of small resonant magnetic field perturbations completely eliminates ELMs while maintaining a steady-state high-confinement (H-mode) plasma. These perturbations induce a chaotic behavior in the magnetic field lines, which reduces the edge pressure gradient below the ELM instability threshold. The pressure gradient reduction results from a reduction in particle content of the plasma, rather than an increase in the electron thermal transport. This is inconsistent with the predictions of stochastic electron heat transport theory. These results provide a first experimental test of stochastic transport theory in a highly rotating, hot, collisionless plasma and demonstrate a promising solution to the critical issue of controlling edge instabilities in fusion plasma devices.
Date: June 13, 2006
Creator: Evans, T E; Moyer, R A; Burrell, K H; Fenstermacher, M E; Joseph, I; Leonard, A W et al.
Partner: UNT Libraries Government Documents Department

ELM Suppression in Low Edge Collisionality H-Mode Discharges Using n=3 Magnetic Perturbations

Description: Using resonant magnetic perturbations with toroidal mode number n = 3, we have produced H-mode discharges without edge localized modes (ELMs) which run with constant density and radiated power for periods up to about 2550 ms (17 energy confinement times). These ELM suppression results are achieved at pedestal collisionalities close to those desired for next step burning plasma experiments such as ITER and provide a means of eliminating the rapid erosion of divertor components in such machines which could be caused by giant ELMs. The ELM suppression is due to an enhancement in the edge particle transport which reduces the edge pressure gradient and pedestal current density below the threshold for peeling-ballooning modes. These n = 3 magnetic perturbations provide a means of active control of edge plasma transport.
Date: July 11, 2005
Creator: Burrell, K H; Evans, T E; Doyle, E J; Fenstermacher, M E; Groebner, R J; Leonard, A W et al.
Partner: UNT Libraries Government Documents Department

Development of ITER 15 MA ELMy H-mode Inductive Scenario

Description: The poloidal field (PF) coil system on ITER, which provides both feedforward and feedback control of plasma position, shape, and current, is a critical element for achieving mission performance. Analysis of PF capabilities has focused on the 15 MA Q = 10 scenario with a 300-500 s flattop burn phase. The operating space available for the 15 MA ELMy H-mode plasma discharges in ITER and upgrades to the PF coils or associated systems to establish confidence that ITER mission objectives can be reached have been identified. Time dependent self-consistent free-boundary calculations were performed to examine the impact of plasma variability, discharge programming, and plasma disturbances. Based on these calculations a new reference scenario was developed based upon a large bore initial plasma, early divertor transition, low level heating in L-mode, and a late H-mode onset. Equilibrium analyses for this scenario indicate that the original PF coil limitations do not allow low li (<0.8) operation or lower flux states, and the flattop burn durations were predicted to be less than the desired 400 s. This finding motivates the expansion of the operating space, considering several upgrade options to the PF coils. Analysis was also carried out to examine the feedback current reserve required in the CS and PF coils during a series of disturbances and a feasibility assessment of the 17 MA scenario was undertaken. Results of the studies show that the new scenario and modified PF system will allow a wide range of 15 MA 300-500 s operation and more limited but finite 17 MA operation.
Date: October 16, 2008
Creator: Kessel, C. E.; Campbell, D.; Gribov, Y.; Saibene, G.; Ambrosino, G.; Casper, T. et al.
Partner: UNT Libraries Government Documents Department

Suppression of Large Edge Localized Modes with a Stochastic Magnetic Boundary in High Confinement DIII-D Plasmas

Description: Large sub-millisecond heat pulses due to Type-I ELMs have been reproducibly eliminated in DIII-D for periods approaching 7 energy confinement times with small dc currents driven in a simple magnetic perturbation coil. The current required to eliminate all but a few isolated Type-I ELM impulses during a perturbation coil pulse lasting several seconds is less than 0.4% of plasma current. Based on vacuum magnetic field line modeling, perturbation fields from the coil resonate strongly with plasma flux surfaces across most of the pedestal region (0.9 {le} {psi}{sub N} {le} 1.0) when q95 = 3.7{plus_minus}0.2 creating small remnant magnetic islands surrounded by weakly stochastic field lines. Under the best ELM suppression conditions, the stored energy, {beta}N and H-mode quality factor are unaffected by the perturbation field along with the electron pressure profile, radial electric field and poloidal rotation across the pedestal. Consequently, the H-mode transport barrier and global energy confinement time is also unaltered in these cases. Although some isolated ELM-like events typically occur during the perturbation coil pulse, long periods free of large Type-I ELMs ({Delta}t > 4-6 {tau}{sub E}) have been reproduced numerous times, on multiple experimental run days. Several Type-I ELM suppression and modification behaviors have been identified and studied over a range of discharge conditions including those matching the ITER scenario 2 flux surface shape and aspect ratio scaled down by a factor of 3.5 to fit in the DIII-D vacuum vessel. Since large Type-I ELM impulses represent a severe constraint on the survivability of the divertor target plates in future fusion devices such as ITER, a proven method of eliminating these impulses is critical for the development of tokamak reactors. Results presented in this paper suggest that non-axisymmetric edge magnetic perturbations could be a promising option for controlling ELMs in future tokamaks such as ITER.
Date: October 18, 2004
Creator: Evans, T E; Moyer, R A; Watkins, J G; Osborne, T H; Thomas, P R; Becoulet, M et al.
Partner: UNT Libraries Government Documents Department