Physics research on the TCV tokamak facility: from conventional to alternative scenarios and beyond

Abstract

The research program of the TCV tokamak ranges from conventional to advanced-tokamak scenarios and alternative divertor configurations, to exploratory plasmas driven by theoretical insight, exploiting the device’s unique shaping capabilities. Disruption avoidance by real-time locked mode prevention or unlocking with electron-cyclotron resonance heating (ECRH) was thoroughly documented, using magnetic and radiation triggers. Runaway generation with high-Z noble-gas injection and runaway dissipation by subsequent Ne or Ar injection were studied for model validation. The new 1 MW neutral beam injector has expanded the parameter range, now encompassing ELMy H-modes in an ITER-like shape and nearly non-inductive H-mode discharges sustained by electron cyclotron and neutral beam current drive. In the H-mode, the pedestal pressure increases modestly with nitrogen seeding while fueling moves the density pedestal outwards, but the plasma stored energy is largely uncorrelated to either seeding or fueling. High fueling at high triangularity is key to accessing the attractive small edge-localized mode (type-II) regime. Turbulence is reduced in the core at negative triangularity, consistent with increased confinement and in accord with global gyrokinetic simulations. The geodesic acoustic mode, possibly coupled with avalanche events, has been linked with particle flow to the wall in diverted plasmas. Detachment, scrape-off layer transport, and turbulence were studied in L- and H-modes in both standard and alternative configurations (snowflake, super-X, and beyond). The detachment process is caused by power ‘starvation’ reducing the ionization source, with volume recombination playing only a minor role. Partial detachment in the H-mode is obtained with impurity seeding and has shown little dependence on flux expansion in standard single-null geometry. In the attached L-mode phase, increasing the outer connection length reduces the in–out heat-flow asymmetry. A doublet plasma, featuring an internal X-point, was achieved successfully, and a transport barrier was observed in the mantle just outside the internal separatrix. In the near future variable-configuration baffles and possibly divertor pumping will be introduced to investigate the effect of divertor closure on exhaust and performance, and 3.5 MW ECRH and 1 MW neutral beam injection heating will be added.

Real-time feedback control of the impurity emission front in tokamak divertor plasmas

“…The authors would like to thank the entire TCV team (see author list of 22 ) for making these experiments possible and are especially grateful to Dr. Labit for his expertize on and modifications to the small-ELM plasma scenario. Furthermore, the entire EUROfusion MST1 team (see author list of 41 ) is greatly acknowledged, in particular input from Dr. Bernert.…”

“…In this paper we demonstrate a strategy to control impurity emission front locations, as a proxy for divertor detachment, on the Tokamak Configuration Variable (TCV) 22 . (i) We apply real-time analysis of multi-spectral video images obtained from the MANTIS diagnostic 23 to reconstruct the poloidal location of a chosen spectral line’s emission front.…”

Kinetic modelling of runaway electron generation in argon-induced disruptions in ASDEX Upgrade

“…For this reason, much effort is directed towards the development of schemes to avoid, limit or mitigate the formation of such a beam. One proposed measure is using massive material injection (MMI) in the form of gas (massive gas injection – MGI) or frozen pellets (shattered pellet injection) to avoid or dissipate the runaway electrons (Hollmann et al 2015 a ; Lehnen et al 2015), and its efficiency has been demonstrated in medium-sized tokamaks (Hollmann et al 2015 b ; Reux et al 2015; Pautasso et al 2016; Esposito et al 2017; Paz-Soldan et al 2017; Carnevale et al 2018; Coda et al 2019; Mlynar et al 2019; Pautasso et al 2020). However, the plasma currents, temperatures and densities of future devices such as ITER will be significantly larger than what can be achieved in current experiments, and simulations are necessary to foresee the effectiveness of massive material injections for disruption mitigation under such conditions.…”

Bifurcation phenomena in magnetically confined toroidal plasmas

“…Turbulence spreading is also important for the turbulence in the scrape-off-layer (SOL), where the turbulence spreading from the pedestal region is expected to be more dominant than the locally driven turbulence [243]. There are many experiments reported regarding the broadening of the radial profile of heat flux and the increase of power decay length, that are required for the reduction of the heat load on the divertor in tokamaks [267][268][269][270]. Since enhancement of turbulence in SOL is one of the candidates for broadening of the radial profile of heat flux, deeper understanding of turbulence in SOL is an emerging issue in the future.…”