Effect of N2, Ne and Ar seeding on Alcator C-Mod H-mode confinement

Reinke, M.L.; Hughes, J.W.; Loarte, A.; Brunner, D.; Hutchinson, I. H.; LaBombard, B.; Payne, J.; Terry, J. L.

doi:10.1016/j.jnucmat.2010.10.055

Cited by 81 publications

(61 citation statements)

References 14 publications

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“…The intense radiation and plasma-neutral interaction results in a ''detached'' divertor with cold dense divertor plasma, T e 6 2 eV, where plasma pressure and heat flux at the target are greatly reduced from that at the entrance to the divertor region. The DIII-D detached divertor plasmas have similar characteristics to those in metal wall tokamaks where injected low-Z impurities, typically nitrogen or neon, provide the additional radiation [7,8]. In general the H-mode pedestal pressure dependence on density is found to be consistent with the EPED model [9] of pedestal stability and transport, indicating no inherent interaction with the detached divertor plasma causing further degradation of the H-mode pedestal.…”

Section: Introductionmentioning

confidence: 87%

Compatibility of detached divertor operation with robust edge pedestal performance

Leonard

Makowski

McLean

et al. 2015

Journal of Nuclear Materials

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a b s t r a c tThe compatibility of detached radiative divertor operation with a robust H-mode pedestal is examined in DIII-D. A density scan produced low temperature plasmas at the divertor target, T e 6 2 eV, with high radiation leading to a factor of P4 drop in peak divertor heat flux. The cold radiative plasma was confined to the divertor and did not extend across the separatrix in X-point region. A robust H-mode pedestal was maintained with a small degradation in pedestal pressure at the highest densities. The response of the pedestal pressure to increasing density is reproduced by the EPED pedestal model. However, agreement of the EPED model with experiment at high density requires an assumption of reduced diamagnetic stabilization of edge Peeling-Ballooning modes.

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Section: Introductionmentioning

confidence: 87%

Compatibility of detached divertor operation with robust edge pedestal performance

Leonard

Makowski

McLean

et al. 2015

Journal of Nuclear Materials

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“…[10] Since those experiments, additional ICRF power has been made available, allowing their extension to even higher performance. Using the puffing method described in Section 2, and in more detail in [5], EDA discharges were seeded with varying amounts of Ar, Ne and N 2 . The resulting values of loss power and core radiated power are shown in Figure 4.…”

Section: Eda H-modes With Impurity Seedingmentioning

confidence: 99%

“…Yet common scaling laws for H-mode confinement, including H 98 , do not account for P rad and instead simply use the input power, corrected for the time-evolving stored energy. This paper describes research conducted on the Alcator C-Mod tokamak [ 4 ] with the goal of characterizing the role of both input and radiated power on normalized confinement, building on initial results discussed in [ 5 ]. We examine two key questions.…”

Section: Introductionmentioning

confidence: 99%

Power requirements for superior H-mode confinement on Alcator C-Mod: experiments in support of ITER

et al. 2011

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Power requirements for maintaining sufficiently high confinement (i.e., normalized energy confinement time H 98 ≥1) in H-mode and its relation to H-mode threshold power scaling, P th , are of critical importance to ITER. In order to better characterize these power requirements, recent experiments on the Alcator C-Mod tokamak have investigated H-mode properties, including the edge pedestal and global confinement, over a range of input powers near and above P th . In addition, we have examined the compatibility of impurity seeding with high performance operation, and the influence of plasma radiation and its spatial distribution on performance. Experiments were performed at 5.4T at ITER relevant densities, utilizing bulk metal plasma facing surfaces and ion cyclotron range of frequencies waves for auxiliary heating. Input power was scanned both in stationary enhanced D α (EDA) H-modes with no large ELMs and in ELMy H-modes in order to relate the resulting pedestal and confinement to the amount of power flowing into the scrape-off layer, P net , and also to the divertor targets. In both EDA and ELMy H-mode, energy confinement is generally good, with H 98 near unity. As P net is reduced to levels approaching that in L-mode, pedestal temperature diminishes significantly and normalized confinement time drops. By seeding with low-Z impurities such as Ne and N 2 , high total radiated power fractions are possible, along with substantial reductions in divertor heat flux (>4x), all while maintaining H 98 ~ 1. When the power radiated from the confined vs. unconfined plasma is examined, pedestal and confinement properties are clearly seen to be an increasing function of P net , helping to unify the results with those from unseeded H-modes. This provides increased confidence that the power flow across the separatrix is the correct physics basis for ITER extrapolation. The experiments show that P net /P th of one or greater is likely to lead to H 98~1 operation, and also that such a condition can be made compatible with a low-Z radiative impurity solution for reducing divertor heat loads to levels acceptable for ITER.

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“…Critical gradients in the edge profiles are known to exist 35,36 , and they may be playing a role in these cases. The prompt power being supplied to the SOL by the LHRF can influence the neutral fueling both in H-mode and in L-mode, and this too is known to affect confinement 35,18 .…”

Section: Discussion and Summarymentioning

confidence: 99%

“…The experiments were carried out with freshly boronized molybdenum-tile PFC surfaces 17 in order to obtain long-duration steady-state Hmodes with relatively low concentrations of high-Z Mo impurity. These plasmas were also seeded with low-Z recycling Ne impurity in order to reduce divertor heat flux 12,18 . (There was little change in the recycling neon flux during the LHRF.)…”

Section: The Lower Hybrid Rf System and Eda H-mode On Alcator C-modmentioning

confidence: 99%

Improved confinement in high-density H-modes via modification of the plasma boundary with lower hybrid wavesa)

et al. 2015

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Injecting Lower Hybrid Radio Frequency (LHRF) waves into Alcator C-Mod's high-density H-mode plasmas has led to enhanced global energy confinement by increasing pedestal temperature and pressure gradients, decreasing the separatrix density, modifying the pedestal radial electric field and rotation, and decreasing edge turbulence. These experiments indicate that edge LHRF can be used as an actuator to increase energy confinement via modification of boundary quantities. H 98 -factor increases of up to ~35% (e.g. H 98 from 0.75 to 1.0) are seen when moderate amounts of LH power (P LH /P tot~0 .15) are applied to H-modes of densities n e~3 x10 20 m -3 , corresponding to values ~0.5 of the Greenwald density. However, the magnitude of the improvement is reduced if the confinement quality of the target Hmode plasma is already good (i.e. H 98 target~1 ). Ray-tracing modeling and accessibility calculations for the LH waves indicate that they do not penetrate to the core. The LHRF power appears to be deposited in plasma boundary region, with a large fraction of the injected power increment appearing promptly on the outer divertor target. There is no evidence that the LH waves are driving current in these plasmas. The LHRF-actuated improvements are well correlated with suppressed pedestal density fluctuations in the 100-300 kHz range. There is also a correlation between the improved confinement and a drop in separatrix density, a correlation that is consistent with previous H-mode results with no LHRF.

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Effect of N2, Ne and Ar seeding on Alcator C-Mod H-mode confinement

Cited by 81 publications

References 14 publications

Compatibility of detached divertor operation with robust edge pedestal performance

Compatibility of detached divertor operation with robust edge pedestal performance

Power requirements for superior H-mode confinement on Alcator C-Mod: experiments in support of ITER

Improved confinement in high-density H-modes via modification of the plasma boundary with lower hybrid wavesa)

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