Scaling of density peaking in H-mode plasmas based on a combined database of AUG and JET observations

Abstract

For the first time, scalings for density peaking in tokamaks are obtained from a database consisting of observations from two devices, ASDEX Upgrade and JET. The investigation relies on an inversion method for the interferometer signals which grants consistent reconstructions despite differences in interferometer geometries. By combining observations from these devices, correlations between physics parameters investigated for their role in determining density peaking are reduced. Multiple regression analyses show that in the combined database collisionality is the most relevant parameter. The particle source provided by neutral beam injection provides a contribution to the peaking, which, although not negligible, is not large enough to explain the whole observed variation of density peaking. The device size, introduced as an alias for possible systematic differences between the devices not captured by the regression parameters, is found to play only a small role in regressions which include collisionality. Device size becomes relevant in scalings which exclude collisionality and include the ratio of the density to the Greenwald density limit. This indicates that density peaking is more likely to be a function of collisionality rather than of the fraction of the density limit. All the scalings which include collisionality in the regression variables predict a peaked density profile for the ITER standard scenario.

20 years of research on the Alcator C-Mod tokamak

“…For the typical OH and L-mode plasmas, these two quantities increased monotonically with minor radius, reaching values on the order of 0. Turning to the H-mode, results from AUG and JET 40 showed moderate peaking at low collisionality, but could not distinguish between dependence on collisionality and n/n G .…”

“…38 The density in the H-modes, normalized to the density limit, is typically 0.5-0.7, a bit below that seen in lower field, neutral beam heated devices. One reason for this is that the very strong gas puffing required for higher densities interferes with ICRF antenna operation, 39 though the lack of beam fueling may also be a factor 40 along with limitations of fueling and transport through a high-opacity edge and pedestal. 41 (The new field-aligned (FA) antenna described in Sec.…”

Density profile peaking in JET H-mode plasmas: experiments versus linear gyrokinetic predictions

“…It has already been shown in earlier experiments that density peaking in H-mode plasma scales with the logarithm of the effective collisionality [1][2][3][4][5][6]. The same tendency is also clearly seen in the recent JET plasmas studied in this paper (see figure 2) with an additional feature: around ν eff ∼ 0.5 there appears to be an inflexion point where the collisionality dependence disappears, which is well visible on the n 0.2 / n e measurement and a little less pronounced on R/L n .…”

“…This definition differs from the one used in previous publications [3,4], * = RS NBI /n e χ, where χ is the heat diffusivity. The definition adopted here makes the comparison with simulations straightforward, as explained later.…”

Simulations of density profiles, pellet fuelling and density control in ITER

“…This has led to the development of physics based transport models in reasonable agreement with experimental results [2,3,4] and to a more convincing interpretation of data from present day machines [5,6]. On the other hand the problems of calculating the particle fluxes across the separatrix and the particle source due to neutral influx and pellet injection have not yet been completely solved.…”