Density limits in TEXTOR-94 auxiliary heated discharges

Rapp, J.; Vries, P. C. de; Schüller, F. C.; Tokaŕ, M. Z.; Biel, W.; Jaspers, R.; Koslowski, H. R.; Krämer‐Flecken, A.; Kreter, A.; Lehnen, M.; Pospieszczyk, A.; Reiser, D.; Samm, U.; Sergienko, G.

doi:10.1088/0029-5515/39/6/305

Cited by 60 publications

(85 citation statements)

References 22 publications

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“…The reduction of the plasma contact with the wall at the HFS was balanced by a stronger impurity release from limiters at the low field side. 11 Recently, similar results have been obtained on JET. 14 By increasing the plasma wall clearance, the outflow of charged particles and energy to the inner wall is reduced and, consequently, the influx of recycling neutrals of the working gas and eroded impurities into the plasma is decreased.…”

Section: Introductionsupporting

confidence: 72%

“…In limiter tokamaks, this ''wall'' MARFE normally leads to the density limit. [8][9][10][11] Recently, the wall MARFE was also observed in the divertor configuration in Joint European Torus ͑JET͒, where it occurs after the X-point MARFE and ultimately determines the maximum density in L-mode deuterium plasmas. 12 As demonstrated earlier, 7 unstable perturbations leading to the wall MARFE have their largest amplitude on the HFS.…”

Section: Introductionmentioning

confidence: 99%

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The role of plasma–wall interactions in thermal instabilities at the tokamak edge

Tokar

Kelly

2003

Physics of Plasmas

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Plasma-wall interaction leads to the release of impurities and neutrals of the working gas, which contribute significantly to the energy losses from the plasma edge, and therefore, crucially affects the development of thermal instabilities in fusion devices. An analytical model for impurity radiation is proposed, which takes into account the erosion mechanisms of wall material and the motion of impurity particles across magnetic surfaces. The temperature dependence of radiation losses is found to be very different from that predicted by the coronal approximation often used in considering thermal instabilities. The consequences for the development of poloidally symmetric detachment and multi-faceted asymmetric radiation from the edge ͑MARFE͒ are analyzed. It is demonstrated that the MARFE threshold principally depends on the mechanism by which working gas neutrals are released from the wall and on the neutral's properties, e.g., their ionization rate.

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Section: Introductionsupporting

confidence: 72%

Section: Introductionmentioning

confidence: 99%

The role of plasma–wall interactions in thermal instabilities at the tokamak edge

Tokar

Kelly

2003

Physics of Plasmas

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“…It is found that the improved particle confinement phase exists for about 90 ms and the particle confinement time τ P increased about 1.6 times. The results are similar to the experiments in TFTR [13], and are consistent with the theoretical explanation [33] given for the TFTR results. The MARFE cools the plasma edge, and the electron density profile is observed to become more narrow and peaked.…”

Section: Discussionsupporting

confidence: 91%

“…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]. Therefore, it suggests experimentally the importance of impurity radiation and localized power balance in the MARFE formation.…”

Section: Introductionmentioning

confidence: 69%

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

Asif¹

2006

Braz. J. Phys.

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A new set of actively cooled toroidal double-ring graphite limiters have been developed on the HT-7 tokamk for long pulse operation. In this paper, a Multifaceted asymmetric radiation from the edge (MARFE) phenomena and improved particle confinement induced by a MARFE, characterized by H α line emissions drops and the lineaveraged density increase have been studied in the HT-7 ohmic discharges (where the plasma current I p =145 kA, loop voltage V loop =2-3 V, toroidal field B T =1.7 T, and Z e f f =2-8). It is found that the improved particle confinement phase exists for about 90 ms and the particle confinement time τ P increased about 1.6 times.

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“…values close to the operational limit of TEXTOR-94 (b n ഠ 2) for the typical RI-mode conditions [10]. The key factor to obtain these results is appropriate tailoring of the deuterium gas fueling in addition to (i) careful wall conditioning (in this case fresh siliconization) and a proper positioning of the discharge with respect to the limiters, leading to conditions which avoid multifaceted radiation from the edge (MARFE) formation at high density by reducing the recycling at the inner wall [11], and (ii) edge radiation cooling (in this case due to sputtering of Si), these two last conditions being necessary for high confinement radiative mantle discharges in TEXTOR. Similar results are found in discharges with additional neon seeding to produce the radiative belt under boronized or siliconized wall conditions.…”

mentioning

confidence: 99%

Quasistationary High Confinement Discharges with trans-Greenwald Density on TEXTOR-94

Mank

Ongena

et al. 2000

Phys. Rev. Lett.

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Confinement quality as good as ELM-free H-mode at densities substantially above the Greenwald density limit (n e,0 ͞n GW 1.4) has been obtained in discharges with a radiative boundary under quasistationary conditions for 20 times the energy confinement time. This is achieved by optimizing the gas-fueling rate of RI-mode discharges which tailors their favorable energy confinement and leads to discharges with beta values just below the operational limit b n 2 of TEXTOR-94, thereby effectively avoiding confinement back transitions or disruptions. In addition, this high-density regime is favorable for helium removal and results in figures of merit t ‫ء‬ p,He ͞t E ഠ 10 15, relevant for a future fusion power reactor. A subject of intense investigations in present day fusion research is the development of an operational scenario, providing at the same time (i) confinement with at least H-mode quality and (ii) high densities around or above the empirical Greenwald density value n GW I P ͞pa 2 [1] [with I P the plasma current (MA), the minor plasma radius (m), and n GW in units 10 20 m 23 ]. Success in this direction has been obtained in the past years on limiter [2,3] and divertor tokamaks by pellet injection or adjusting deuterium fueling and pumping [4,5]. Recently, high energy confinement at a Greenwald numbern e,0 ͞n GW 1.4 (n e,0 being the central averaged chord density) was obtained at DIII-D by optimizing divertor pumping in gas puffed H-mode discharges with edge localized modes (ELM) [6,7]. This is in contrast to most H-mode discharges which undergo a confinement degradation when approaching this value [8]. In this paper we show that it is possible in addition to substantially overcome the Greenwald limit under quasistationary conditions with confinement of ELM-free H-mode quality, high b, q 3.4, and a regime favorable for heat and particle exhaust. This is obtained by optimizing the gas-fueling rate in radiatively improved (RI) mode plasmas on the limiter tokamak TEXTOR-94 in order to maintain a sufficiently low neutral pressure in the scrape off layer (SOL).The RI-mode on TEXTOR-94 is obtained with edge radiation cooling due to neon seeding under boronized wall conditions or sputtering of silicon under siliconized wall conditions. A careful adjustment of the edge and heating conditions is necessary to reach RI-mode confinement, proportional to the electron density, leading to high confinement at high density. The confinement improvement observed in the RI-mode has been interpreted as resulting from the suppression of ion thermal gradient (ITG) modes caused by the presence of impurities in the edge and by the resulting core density peaking [9]. TEXTOR-94 is a medium size tokamak (R 1.75 m, a 0.46 m) equipped with the toroidal belt limiter ALT-II which consists of eight pumped blades. Some blades are equipped with gas inlet apertures located at the surface of the limiter tiles. The gas flow through the ALT fueling apertures is smoother than it is for the standard TEXTOR gas feeders equipped with a fast feedback ...

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Density limits in TEXTOR-94 auxiliary heated discharges

Cited by 60 publications

References 22 publications

The role of plasma–wall interactions in thermal instabilities at the tokamak edge

The role of plasma–wall interactions in thermal instabilities at the tokamak edge

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

Quasistationary High Confinement Discharges with trans-Greenwald Density on TEXTOR-94

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