Overview on plasma operation with a full tungsten wall in ASDEX Upgrade

Abstract

Operation with all tungsten plasma facing components has become routine in ASDEX Upgrade. The conditioning of the device is strongly simplified and short glow discharges are used only on a daily basis. The long term fuel retention was reduced by more than a factor of 5 as demonstrated in gas balance as well as in post mortem analyses. Injecting nitrogen Preprint submitted to Elsevier 23 November 2012for radiative cooling, discharges with additional heating power up to 23 MW have been achieved, providing good confinement (H98 y2 = 1), divertor power loads around 5 MW m −2 and divertor temperatures below 10 eV. ELM mitigation by pellet ELM pacemaking or magnetic perturbation coils reduces the deposited energy during ELMs, but also keeps the W density at the pedestal low. As a recipe to keep the central W concentration sufficiently low, central (wave) heating is well established and low density H-Modes could be re-established with the newly available ECRH power of up to 4 MW. The ICRH induced W sources could be strongly reduced by applying boron coatings to the poloidal guard limiters.

First scenario development with the JET new ITER-like wall

“…It has to be noted that the deuterium gas puffing rate is still an order of magnitude below the recycling neutral flux but its impact on the confinement appears stronger [1]. In addition, the lower observed Z eff could play a role in the edge transport barrier and pedestal stability, which would be consistent with the observation made by the first nitrogen seeding experiments with the JET-ILW [21] and AUG results [3], where the H~1 could be recovered when injecting nitrogen gas which can radiate at the plasma edge.…”

“…Hybrid H-modes above β N = 3 at I p = 2MA (q 95~4 , β N~3 ) with and H 98y2 factor exceeding 1.2 for 2s. Although operational issues such as the limited steady state heat load or the changes in plasma impurity composition were foreseen, the landscape of scenario operational domain has changed significantly and are consistent with the observation on other metallic devices such as ASDEX Upgrade [3] or C-MOD [4]. The most important results is that the access to H~1 confinement is in general much more restricted than with the carbon wall for the baseline H-mode scenario in both low (δ = 0.2) and high shape (δ = 0.4) mainly because of the need to control core radiation which was achieved by significant gas puffing rate.…”

“…By scanning down the gas bleed with the JET-ILW, a minimum ELM frequency of typically 10Hz was found necessary to prevent excessive core plasma radiation peaking. The role of the ELM on core radiation control has also been observed in ASDEX Upgrade and the property they have to flush out tungsten evidenced [3].…”

“…The resonance layer was located close to the magnetic axis and H minority (~5%) scheme used throughout this experiment. Electron heating (electron cyclotron heating and ion cyclotron heating) has been used in another metallic device, ASDEX Upgrade, to control the core impurity concentration and therefore the radiation level in the discharge [3]. Although an elevation of the core temperature and enhanced sawtooth activity are clearly observed in the ICRH phase in JET, there is no clear evidence yet that core ICRH heating had an impact on the high Z impurity level indicating that higher level of ICRH power is probably required..…”

Automated in situ line of sight calibration of ASDEX Upgrade bolometers

“…Tomographic reconstruction algorithms can be applied on the measurement in order to retrieve the radiation distribution of a poloidal cross section. Especially for ITER, bolometers will be a key technology to control the quantity of the radiation in the divertor limiting the heat load on the divertor blanket modules [1] [2]. Therefore, the geometrical properties, the alignment of the LOS and their characteristics, are key parameters for the accuracy Email address: florian.penzel@ipp.mpg.de (F. Penzel) of the diagnostic.…”