Structure, stability and ELM dynamics of the H-mode pedestal in DIII-D

Abstract

ABSTRACT. Experiments are described that have increased understanding of the transport and stability physics that set the H-mode edge pedestal width and height, determine the onset of Type-I edge localized modes (ELMs), and produce the nonlinear dynamics of the ELM perturbation in the pedestal and scrape-off layer (SOL). Models now exist for the n e pedestal profile and the p e height at the onset of Type-I ELMs, and progress has been made toward models of the T e pedestal width and nonlinear ELM evolution. Similarity experiments between DIII-D and JET suggested that neutral penetration physics plays an important role in the relationship between the width and height of the n e pedestal. Plasma physics appears to dominate in setting the T e pedestal width. Measured pedestal conditions including edge current at ELM onset agree with intermediate-n peeling-ballooning (P-B)

H-mode pedestal scaling in DIII-D, ASDEX Upgrade, and JET

“…The dataset used here is well described in [30],reanalysed with the linear fit method [33], and contains a power scan at q 95~ 4.8 and δ=0.23. It is important to note that this experiment describes a power scan from P NET =6.5-13.5MW and is not a dimensionless experiment as the collisionality varies as 0.9>ν e *>0.05 and is correlated to 0.3<β p,ped <0.7 through β∼nxT vs ν~n e /T e 2 (at relatively fixed n e =6x10 19 m -3 in the scan). …”

“…Experiments in which all the dimensionless quantities were matched at the top of the pedestal on two different tokamaks indicate that the electron temperature pedestal width scales with machine size minor radius as ΔT e /a ≈ 3% [17,18,19,20]. However there can be differences in the behaviour of the density pedestal width, which plays an equally important role in setting the pedestal pressure height and gradient.…”

“…For all three devices, data are accumulated from the last 20%-30% of the ELM cycle for many ELMs, e.g. [19,32]. A composite profile in normalized-flux space is then built from the many ELM cycles by mapping the profile at a specific time-point onto the flux surface geometry of the MHD equilibrium.…”

Analysis of temperature and density pedestal gradients in AUG, DIII-D and JET

“…For a given plasma discharge a stability boundary can be illustrated in a j-α diagram. For most published cases of type-I ELMy H-modes the experimental point lies near the peeling-ballooning stability boundary [9,[25][26][27][28]. The position of the boundary in a j-α diagram varies with plasma parameters as does the location of the operational point on this boundary.…”

Edge transport barriers in magnetic fusion plasmas

“…The peeling-ballooning mode model can be used for predictions of the maximum pressure gradient expected in next-step fusion devices such as ITER. This approach, together with the use of dimensionless parameters, has been used with inter-machine pedestal measurements to determine scaling of the transport barrier width [124]. A more theory-based approach using turbulent transport models applied across the pedestal region may provide a better understanding of the ETB structure [125] and will be used for future comparisons with multi-machine data.…”