Pedestal structure and energy confinement studies on TCV

Abstract

High external gas injection rates are foreseen for future devices to reduce divertor heat loads and this can influence pedestal stability. Fusion yield has been estimated to vary as strongly as T 2 e,ped so an understanding of the underlying pedestal physics in the presence of additional fuelling and seeding is required. To address this, a database scanning plasma triangularity, fuelling and nitrogen seeding rates in neutral beam (NBH) heated ELM-y H-mode plasmas was constructed on TCV. Low nitrogen seeding was observed to increase pedestal top pressure but all other gas injection rates led to a decrease. Lower triangularity discharges were found to be less sensitive to variations in gas injection rates. No clear trend was measured between plasma top Pe and stored energy which is attributed to the non-stiffness of core plasma pressure profiles. Peeling ballooning stability analysis put these discharges close to the ideal MHD stability boundary. A constant for D in the relation pedestal width w = D β P ed θ , was not found. Experimentally inferred values of D were used in EPED1 simulations and gave good agreement for pedestal width. Pedestal height agreed well for high triangularity but was overestimated for low triangularity. IPED simulations showed that relative shifts in pedestal position were contributing significantly to the pedestal height and were able to reproduce the measured profiles more accurately.

Role of the pedestal position on the pedestal performance in AUG, JET-ILW and TCV and implications for ITER

“…When the pedestal is PB limited, EPED can correctly predict the pedestal height in a wide range of experimental conditions and in several devices [23]. Recent results from TCV [11] have…”

“…This subsection briefly describes the method for the PB stability analysis in JET and the implementations of the EPED-like models used for the JET, AUG and TCV plasmas. For the description of the PB stability analysis techniques in AUG and TCV, we refer the reader to the relevant literature [11,17] and references therein (no AUG and TCV PB stability diagrams are shown in this work).…”

“…According to the model, the pedestal pressure increases till the PB modes become unstable and trigger the ELM. Indeed, most of the machines, including AUG [17], TCV [11],…”

“…The most common version of the EPED model (EPED1 [22]) assumes D=0.076 (as determined from an experimental fit of DIII-D low ν* plasmas). In literature, the experimental value of D has been observed to vary from D=0.084 in Alcator C-mod [24] to D=0.11 in AUG [25] and in a wide range D=0.06-0.13 in JET-ILW [26] and TCV [11]. When the pedestal is PB limited, EPED can correctly predict the pedestal height in a wide range of experimental conditions and in several devices [23].…”

Investigation of turbulent transport regimes in the tokamak edge by using two-fluid simulations

“…Regarding the H-mode, we note that, for typical values of density and temperature at the top of the pedestal for neutral beam heated discharges of a medium size tokamak such as TCV, ranges from 0.05 ( and ) to 0.4 ( and ), depending on the external gas injection rate (Sheikh et al. 2018), thus providing a justification for the use of a fluid model. On the other hand, in the case of JET tokamak, typical values of density and temperature at the top of the pedestal (Beurskens et al.…”