Density limit experiments on FTU

Abstract

One of the main problems in tokamak fusion devices concerns the capability to operate at a high plasma density, which is observed to be limited by the appearance of catastrophic events causing loss of plasma confinement. The commonly used empirical scaling law for the density limit is the Greenwald limit, predicting that the maximum achievable line-averaged density along a central chord depends only on the average plasma current density. However, the Greenwald density limit has been exceeded in tokamak experiments in the case of peaked density profiles, indicating that the edge density is the real parameter responsible for the density limit. Recently, it has been shown on the Frascati Tokamak Upgrade (FTU) that the Greenwald density limit is exceeded in gas-fuelled discharges with a high value of the edge safety factor. In order to understand this behaviour, dedicated density limit experiments were performed on FTU, in which the high density domain was explored in a wide range of values of plasma current (Ip = 500–900 kA) and toroidal magnetic field (BT = 4–8 T). These experiments confirm the edge nature of the density limit, as a Greenwald-like scaling holds for the maximum achievable line-averaged density along a peripheral chord passing at r/a ≃ 4/5. On the other hand, the maximum achievable line-averaged density along a central chord does not depend on the average plasma current density and essentially depends on the toroidal magnetic field only. This behaviour is explained in terms of density profile peaking in the high density domain, with a peaking factor at the disruption depending on the edge safety factor. The possibility that the MARFE (multifaced asymmetric radiation from the edge) phenomenon is the cause of the peaking has been considered, with the MARFE believed to form a channel for the penetration of the neutral particles into deeper layers of the plasma. Finally, the magnetohydrodynamic (MHD) analysis has shown that also the central line-averaged density at the onset of the MHD activity depends only on the toroidal magnetic field.

Scrape-off layer transport and filament characteristics in high-density tokamak regimes

“…edge density, obtained from interferometric edge chord, normalized to Greenwald density value n e e /n G ) for AUG (panel (a) and (b)) and TCV ((e) and (f)), respectively. The choice of normalization (edge density normalized to Greenwald density) is done in order to consider possible effects due to the different density peaking obtained at different edge safety factor [29]. As already observed the integrated ion flux in TCV increases almost linearly with the density up to the threshold A c c e p t e d M a n u s c r i p t followed by a smooth roll-over [30], the latter assumed in the following as a proxy for plasma detachment.…”

A unified model of density limit in fusion plasmas

“…Now, we benchmark scalings ( 14), (15) with experimental results. As far as ohmic tokamak is concerned, recent FTU experiments [30,31], realized in clean machine conditions with negligible content of metals (Z eff <1.5, D as main ion), established a Greenwald-like scaling for the edge density limit: (n edge ) DL ≈0.35×n G . The similarity with equation ( 14) stimulates a more quantitative comparison.…”

“…Section 2 is the core of the work, being devoted to the analytical derivation of the DL scaling laws, and their comparison with some experimental results. To this purpose, ohmic experiments performed on FTU [22,23], and NBI-heated, L-mode experiments realized on TEXTOR-94 [24] are considered for the tokamak. RFX-mod experiments [3,11] are taken for the RFP; finally, results from LHD [5] are considered for the stellarator.…”

Review of SIRIO interferometer experimental results

“…It is ring of strong radiation, toroidally symmetric and poloidally localized in the high-field side (HFS) of a tokamak chamber, appearing above a density threshold related to the plasma current [18][19][20]. In FTU the MARFE instability appears when the average electron density is of the order of 𝑛 ∼ 0.5𝑛 𝐺 [21], with 𝑛 𝐺 = 𝐼 𝑝 (𝑀 𝐴)/𝜋𝑎 2 , at the same time the plasma edge temperature drops, the 𝐷 𝛼 signal increases and the visible light camera shows a toroidal ring of strong emission from this zone (figure 5 at left). Some MARFEs move poloidally, other ones MARFEs evolve into detached plasmas after moving poloidally.…”