High density operation on Frascati Tokamak Upgrade

Abstract

Plasma behaviour in the high density regime has been investigated on the Frascati Tokamak Upgrade (FTU). The items particularly addressed are density limit, MARFE characteristics and fuelling efficiency. With gas puffing, a maximum line average density of 3.2 x 10" m-3 has been obtained in the ohmic regime at q = 5.1, corresponding to 1.7 times the Greenwald limit; while with pellet injection, a value of 3.7 x 10' ' m-' has been reached at Q = 3.5. The density limit appears to be connected with the impurity content and edge parameters, so the best results are obtained with very clean plasmas and peaked electron density profiles. The MARFE phenomenon always appears beyond a critical density that depends on the total input power and the effective charge: emissivities in the range of 2 to 12 MW/m3 have been measured in this highly radiating region. The fuelling efficiency, starting from a value of about 50% at low density, progressively deteriorates, falling to 10% near the density limit: this behaviour is interpreted as being due to a decrease of the scrape-off layer transparency to incoming neutrals.

Density limits in toroidal plasmas

“…Machines as diverse as Alcator-C, C-Mod, Doublet III, DITE, and TFTR found MARFE thresholds in the range 0.4 -0.55 n G [66]. FTU has found that MARFEs form at a variable fraction of the density limit with dependences on input power, Z EFF and limiter material [39]. Experiments with beryllium first wall materials in JET found a MARFE formation density which scaled as P IN 0.5 [37].…”

“…The Greenwald limit was derived from a relatively small set of data, but predicted the density limits (at least for discharges with flat profiles) for a wide range of devices that were subsequently commisioned including FTU [127,39], JET [57,40], DIII-D [72], TFTR [128], TUMAN-3 [11], TEXTOR [53,48], C-Mod [54], START [129,130], JT60-U [38], TCV [12], ASDEX-Upgrade [41], NSTX [15], MAST [27]. The last devices, large spherical tokamaks with aspect ration R/a < 1.8, may have a density limit 20-40% higher than n G , reasonably good agreement given that the scaling law was derived for machines with a narrow range (3-5) of aspect ratios.…”

“…While this is usually accomplished in discharges with some central particle source from neutral beams, it has also been demonstrated in ohmic and RF-heated plasmas with no core fueling whatsoever [145,146,39,147]. The physics which initiates this transport modification is not well understood, but is generally thought to be due to a drop in ITG growth rate via modification of the ion density profile [148,147].…”

Study of confinement improvement in presence of a MARFE in the HT-7 superconducting tokamak

“…The critical conditions for the occurrence of a MARFE may be correlated to the edge plasma parameters, because it is a localized instability within the edge region. It was evidence that the critical value of n e (0.7a)Z e f f in the MARFE onset always correlated with the total input power which has been observed in the FTU [12] and the HT-7 limiter tokamak [7][8], here n e (0.7a) is the line average density, measured at the outermost interferometer channel at r =20 cm (r =0.7a, the minor radius a = 27∼28 cm). In the TEXTOR-94, dependence of the edge electron density (r = a + 1 cm) on the heating power was also observed [13].…”

“…In the TEX-TOR tokamak the MARFE generally leads to a disruption. In the FTU tokamak the onset of a MARFE always occurs before achieving the density limit [12]. The critical density for its onset is not a constant for the same magnetic field and plasma current, but depends on the vacuum vessel conditioning procedure and on the material used for the limiter surface [17].…”