Dynamical evolution of pedestal parameters in ELMy H-mode in the National Spherical Torus Experiment

Abstract

Abstract. Characterizations of the pedestal parameter dynamics throughout the edge localized modes(ELM) cycles are performed on the National Spherical Torus Experiment (NSTX, [M. Ono et al., Nucl. Fusion 40, 557 (2000)]). A clear buildup of the pedestal height is observed between ELMs for three different plasma currents, which tends to saturate prior to the onset of ELM at low and medium plasma current. Similarly, the pedestal width increases with no clear evidence of saturation during an ELM cycle. The maximum pedestal gradient increases as a function of plasma current, reaches a nominal value after the ELM crash, and remains constant until the end of the ELM cycle. The pedestal height just prior to the onset of ELM is shown to increase quadratically with plasma current. The pedestal width ∆ is proportional to the square-root of the poloidal β at the top of the pedestal. Coherent density fluctuations strongly increasing at the plasma edge are observed to be maximum after the ELM crash and to decay during the rest of the ELM cycle. Finally, the pedestal parameters evolution during the ELM cycle as well as the scaling with Ip of the pedestal pressure prior to the onset ELM are found to be qualitatively consistent with the peeling ballooning theory.

Pedestal study across a deuterium fuelling scan for highδELMy H-mode plasmas on JET with the carbon wall

“…Other studies which support the pedestal width scaling with ( pol,ped ) 0.5 include [18,24,25], which present results from JT-60U as well as spherical Tokamaks such as MAST and NSTX. It is important to note the constant c 1 for spherical machines is higher than medium aspect ratio tokamaks, for example for NSTX c 1 =0.17 [25]. More generally, for high triangularity discharges such as those in the new fuelling database, EPED predicts a pedestal pressure that first increases, and then decreases with density [12,61].…”

“…the width and height, and pedestal dynamics are quantified by least squares mtanh fits to ELM synchronised HRTS profiles. Fitting an mtanh function to radial kinetic profiles is a common technique used on many machines such as JET [19,20], AUG [23,26], DIII-D [22,27], Alcator C-Mod [45,46], MAST [24,28,29,47], NSTX [25] and JT-60 [48]. The mtanh fits to the JET HRTS profiles presented in this section are used in Section 5 when evaluating the Peeling Ballooning stability and comparing experimental results to EPED1 predictions for the pedestal pressure.…”

“…Another important parameter is the normalised poloidal pedestal pressure ( pol,ped ) as studied in detail by [12,15,18,[20][21][22][23][24][25][26]. The average of the electron temperature and density pedestal width in normalized poloidal flux is shown to scale with ( pol,ped ) 0.5 .…”

Correlations between quasi-coherent fluctuations and the pedestal evolution during the inter-edge localized modes phase on DIII-Da)

“…Multiple machines observe that edge pressure gradient establishes fairly rapidly in the recovery phase after an ELM. 11,14,15 In addition, AUG determined that the density gradient recovers faster than the temperature gradient for various fueling rates. 16 Once the gradient recovers, the pedestal pressure has been observed to expand at nearly constant gradient until the next ELM.…”

Exploration of the Equilibrium Operating Space For NSTX-Upgrade

“…However, the applicability of this model to the ST is not yet established. There is evidence that peeling-ballooning physics plays an important role in determining the edge stability [83][84][85][86], and some evidence that kinetic ballooning modes can be the dominant instability in the pedestal region [87]. However, the detailed experiment/theory comparisons of pedestal structure have not been completed as at conventional aspect ratio.…”