Exploring new directions for advanced tokamaks reveals promising potential

Economical fusion energy from tokamak power plants will likely depend on full-metal first walls (plasma facing) and fully noninductive current driving that ensures steady-state operation. Recent experiments at Germany’s ASDEX — or Axially Symmetric Divertor Experiment — Upgrade (AUG) facility recently demonstrated some promising Advanced Tokamak (AT) scenarios that bring such design features closer to practical reality. As described in Physics of Plasmas, AUG’s conversion to a full-tungsten interior makes it an ideal venue for the investigation of AT scenarios, which allow for long-pulse operation, for future demonstration plants.

Achieving AT conditions in ASDEX-Upgrade requires precise tailoring of the so-called q-profile, or “safety factor,” a key parameter of the magnetic fields. This involves a controlled ramp up of the plasma current, external current or intrinsic bootstrap current while maintaining low collisionality with an increasing electron temperature. Since the need for low density precludes gas puffing, a fueling technique for controlling plasma density, alternative strategies such as RF heating are used to keep the bulk plasma tungsten-free.

To improve determination of the q-profile, the team combined several diagnostic methods as well as newly added parameters from Faraday rotation measurements to make accurate reconstruction of equilibrium plasma profiles. Several forms of motional Stark effect measurements are also now included in the reconstructions.

The authors describe their investigations of two AT scenarios specifically. The first, while unsuitable for an actual power plant, achieves almost fully noninductive conditions at a relatively high value of q₉₅ and is valuable as a theoretical benchmark for models of noninductive plasma current.

A second, more reactor-relevant candidate scenario, at q₉₅ ≈ 5.3, for application in ITER/DEMO, shows excellent confinement although the noninductive current was comparable to other steady-state scenarios and further increase in noninductive current was limited by stability, at ß_N = 2.7. Nevertheless, these results demonstrate that noninductive operation is achievable in future full-metal tokamaks. Further studies will concentrate on overcoming the stability limit, improving diagnostics and examining other steady-state AT scenarios.

Source: “Advanced tokamak investigations in full-tungsten ASDEX upgrade,” by A. Bock, H. Doerk, R. Fischer, D. Rittich, J. Stober, A. Burckhart, E. Fable, B. Geiger, A. Mlynek, M. Reich, H. Zohm, and the ASDEX Upgrade Team, Physics of Plasmas (2018). The article can be accessed at https://doi.org/10.1063/1.5024320 .