Parametric dependence of density limits in the Tokamak Experiment for Technology Oriented Research (TEXTOR): Comparison of thermal instability theory with experiment

Kelly, F. A.; Stacey, Weston M.; Rapp, J.

doi:10.1063/1.1409958

Cited by 5 publications

(4 citation statements)

References 41 publications

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“…In particular, a 20%-30% DL increase is predicted when halving f O . This is compatible with the TEXTOR observations [41]. The result is the same for different choices of f C /f O , as shown by the dotted and dashed black curves.…”

Section: Textor-94 L-mode Experimentssupporting

confidence: 89%

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A unified model of density limit in fusion plasmas

et al. 2017

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A limit for the edge density, ruled by radiation losses from light impurities, is established by a minimal cylindrical magneto-thermal equilibrium model. For ohmic tokamak and reversed field pinch the limit scales linearly with the plasma current, as the empirical Greenwald limit. The auxiliary heating adds a further dependence, scaling with the 0.4 power, in agreement with Lmode tokamak experiments. For a pure externally heated configuration the limit takes on a Sudolike form, depending mainly on the input power, and is compatible with recent Stellarator scalings.A discharge-terminating density limit (DL) is found in all the magnetic confinement fusion devices [1]. One of the main interpretative branches invokes impurity radiation losses, which scale with the square of the density. The consequent cooling of the plasma can become critical at high density, giving rise to a variety of instabilities, both thermal [2][3][4][5][6][7][8], and MHD [9][10][11][12][13]. Given the rich phenomenology, DL seems elusive of an explanation based on a single mechanism. This letter presents a complementary approach to the problem, analysing, in cylindrical geometry, the feasibility of a magneto-thermal equilibrium with realistic temperature profile, rather than addressing specific instabilities. Such a study provides a unified interpretation of the phenomenon, given that a DL ruled by light impurities radiation (experiments show that any significant contamination by heavy impurities is just detrimental towards the achievement of high densities [1]), quantitatively consistent with experimental scalings, emerges naturally for all the magnetic configurations. In particular, we found a Greenwald-like scaling [1] for tokamak and reversed field pinch (RFP), and a Sudo-like scaling [14,15] for a pure externally heated configuration, taken as approximation of the stellarator. We are aware that this analysis cannot 2 exhaust the topic, since some instability mechanism is necessary to describe the dynamical route to the plasma termination. Consequently, we speak of an 'equilibrium DL' and not of the DL in the ultimate sense. This work has been inspired by analyses of the ohmic tokamak presented in[16] (section 7.8) and in [17] (section 8). The differences rely in a more general approach, besides a more accurate treatment of the profile dependent terms.We introduce a minimal cylindrical equilibrium model (each quantity depending on the radial coordinate r only), analytically treated with a formalism able to unify the configurations with an applied electric field, namely the tokamak, both ohmic and with additional heating, and the RFP, considered as purely ohmic. Basically, we will take integral relations from the heat transport equation, in some way similar to those carried out in [18] apart for the simpler geometry, and combine them with Ohm's law at r=0 (on-axis). Ohm's law is replaced by a suitable scaling for the energy confinement time in the case of pure auxiliary heating. [ ]Here, T is the electron temperature and K an effec...

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Section: Textor-94 L-mode Experimentssupporting

confidence: 89%

“…Another interesting feature exhibited by TEXTOR experiments, in both ohmic and auxiliary heated plasmas, is the DL dependence on the wall condition. In fact, the DL has been progressively increased by the following steps (see figure 1 of [41]):…”

Section: Textor-94 L-mode Experimentsmentioning

confidence: 99%

A unified model of density limit in fusion plasmas

et al. 2017

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“…After coating graphite on the wall in 2012, the operation region is shown in figures 1(b)-(d) for 2012, 2013, 2014 and 2015, respectively. Carbonization leads to a significant reduction of metallic impurities in the plasma, with an increase in the density limit from 0.42 n G to 0.67n G in 2012; to 0.7n G in 2013, and to 0.72n G in 2014 and 2015 over the entire current range as shown in figures 1(b)-(d), which is similar to the observation made on the TEXTOR tokamak when comparing different wall mat erials [25,26].…”

Section: Operation Regionsupporting

confidence: 83%

The operation region and MHD modes on the J-TEXT tokamak

Huang

Shi

et al. 2016

Plasma Phys. Control. Fusion

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The operation region and the parameter region of magnetohydrodynamic (MHD) modes are analyzed for J-TEXT Ohmic discharges. The operation region is described by the Hugill diagram, which combines low-q and high density limits. It is found that the operation region has expanded over the years on J-TEXT. In detail, the high density limit has increased from less than 0.5n G to 0.7n G and the low-q limit has lowered from 2.8 to 2.2; this is due to the reduced impurity content that results from coating graphite on the wall. Furthermore, the operation region has further expanded to 0.85n G and q a ~ 2.0, respectively-a result of suppressing the disruptive precursor MHD by using externally-applied resonant magnetic perturbations (RMPs). Here, n G and q a are the Greenwald density limit and edge safety factor, respectively. Corresponding to the results of the operation region, the parameter regions of MHD modes are presented. It is found that a m/n = 2/1 tearing mode (TM) appears for a wide parameters region with 2.4 < q a < 4 and n e < 3 × 10 19 m −3 -here m and n are the poloidal and toroidal mode numbers. Furthermore, other MHD modes such as m/n = 5/2, 3/1, 4/1 and 7/2, appear only when their rational surfaces are close to the plasma edge or m/n ~ q a , and these MHD modes may transit to a 2/1 TM when changing the plasma parameters. In addition, correlation analysis between the amplitude and frequency of the dominant 2/1 TM for different plasma conditions reveals that there is a threshold between normal discharges and density-limit discharges, which would be a reference to predict density-limit disruptions.

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“…However, the authors of the theory claim that its relative simplicity is beneficial as it compactly explains the connection between radiative collapse and the density limit, and the density limit's lack of or weak dependence on heating power, effective ion charge Z eff , plasma shaping, and safety factor q [36]. There are, of course, alternative theories that could be explored, such as one also derived from radiation balance and related to MARFEs [64] that results in a similar expression for density, although with a term proportional to the square root of the auxiliary heating power [65].…”

Section: A Note On the Connection Between Density And Radiative Limitsmentioning

confidence: 99%

Density limits as disruption forecasters for spherical tokamaks

Berkery

Sabbagh

Ham

et al. 2023

Plasma Phys. Control. Fusion

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Fusion power output from spherical tokamaks would benefit from increased confined plasma density, but there exists a limit on the density before confinement is lost and the plasma current is disrupted. This density limit has long been characterized by a simple, global Greenwald limit proportional to the plasma current and inversely proportional to the cross sectional area of the plasma. It is shown that in the database of discharges from the NSTX and MAST spherical tokamaks, the likelihood of disruption does increase above the Greenwald limit, and especially in the plasma current rampdown phase. The physics of the density limit has been recently theoretically explored through local criteria. Several of these are tested using the disruption event characterization and forecasting (DECAFTM) code for their potential effectiveness as disruption warning signals. For a limited set of NSTX discharges, a local island power balance criteria was found to be less reliable, presently, than the Greenwald limit. An empirical critical edge line density and a boundary turbulent transport limit were both tested for MAST-U, which has an electron density profile measurement with high spatial resolution in the outer part of the plasma. Both were found to have similar dependencies on key plasma parameters. In a limited set of MAST-U discharges that appear to disrupt due to rising density at values under the Greenwald limit, crossing of the boundary turbulent transport limit occurred close to the time of disruption. Finally, these limits were evaluated for their potential use in real-time, and it was found that with the necessary real-time inputs and with refinement through further testing, these limits could be implemented in a real-time disruption forecasting system.

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Parametric dependence of density limits in the Tokamak Experiment for Technology Oriented Research (TEXTOR): Comparison of thermal instability theory with experiment

Cited by 5 publications

References 41 publications

A unified model of density limit in fusion plasmas

A unified model of density limit in fusion plasmas

The operation region and MHD modes on the J-TEXT tokamak

Density limits as disruption forecasters for spherical tokamaks

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