TJ-II: Influence of magnetic configuration on filament dynamics

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Experimental campaign

2021

Proposal title

TJ-II: Influence of magnetic configuration on filament dynamics

Name and affiliation of proponent

Daniel Carralero, Ulises Losada, Gustavo Grenfell, Boudewijn van Milligen et al.,

Details of contact person at LNF

Daniel Carralero

Description of the activity

SOL transport in tokamaks is generally thought to be dominated by the macroscopic convective cells usually known as “filaments” or “blobs”. These filaments propagate ballistically in the radial direction due to the ExB velocity associated to the dipole generated around a pressure oscillation in the presence of a magnetic field featuring curvature (IC instability) [1]. Depending on several characteristics of the SOL, filaments may be in a number of propagation regimes [2], which affect their size, frequency, speed, amplitude and eventually the magnitude of the transport associated to them.


In tokamaks, a substantial amount of work has been done to validate experimentally these simple filament models and to measure the transport associated to them [3][4]. However, substantially less studies in stellarators are found in the literature, leaving the open question of how does the complex geometry of a non-axially symmetric SOL influence these structures. Recently, a first filament characterization has been carried out in the novel optimized stellarator Wendelstein 7-X, in which filaments feature sizes comparable to those found in tokamaks, but substantially lower speeds and transport [5]. This result has been explained invoking the reduced curvature drive associated to the large aspect ratio of W7-X with respect to a tokamak. However, this effect remains largely intertwined with the complex geometry of the stellarator, involving islands, and long connection lengths which cause filaments to alternate many regions of good and bad curvature along the parallel direction.


In this context, the stellarator TJ-II provides a very appropriated experimental setup to test this hypothesis. While it shares with W-7X a fully 3D SOL, it differs from it in two relevant aspects: in the first place, due to its heliac configuration and smaller size, there are regions in it with substantial curvature, closer to that found in a tokamak. Since it is equipped with a dual system of multi-probes arrays placed at two different toroidal and poloidal locations, it is posible to measure filaments in regions with different curvatures (probe B lies in a neutral to favorable curvature probe D is in an unfavorable negative curvature zone). In the second place, it does not typically feature a rational close to the edge for a typical operation setting, although one can be intruduced by selecting the right magnetic configuration. By this means, it would be posible to disentangle the effects of curvature and SOL islands on filaments.


Therefore, the goal of this research proposal is to :

1. Characterize filaments and evaluate filament propagation models, already tested in tokamaks.

2. Describe and understand the effect of topologic features (e.g. islands, different curvature and connection lengths, etc.) to filament characteristics.

International or National funding project or entity

If applicable, enter funding here or write N/A

Description of required resources

In a previous characterization of the boundary of TJ-II [6][7][8], sufficient data for a preliminary evaluation of filaments was gathered [9]. Form this first analysis a number of additional requirements were established for a complete characterization of filaments, including:

a) Radially and poloidally separated $ I_{sat} $ measurements to carry out correlation-based analysis of filament shape and velocity.

b) Identical probeheads with similar layouts in both manipulators to provide equivalent filament measurements in regions with different curvatures.

c) The best possible characterization of $ n_e $ and $ T_e $ profiles at the edge and SOL regions.

d) Repeat experiments with a magnetic configuration featuring long connection lengths for several cm. after the LCFS.


From this, an overview of the experimental program would be:

a) In a first day of operation, ECRH and NBI plasmas in standard configuration would be probed replicating previous discharges by Kobayashi et al. In this ocasion, both probes would be used to characterize filaments across the SOL in their respective positions and to measure $ n_e $, $ \phi_f $ and $ T_e $ profiles to the innermost posible position. Time permitting, polarization experiments could be attempted as well. Simultaneously, the GPI system would be used in a slow setting to provide a broad evaluation of $ n_e $ and $ T_e $ in the edge region.

b) On top of these main diagnostics, profile characterization will be carried out using TS, He beam, ECE, interferometry and profile reflectometry. DR will be used to provide measurements of $ E_r $ at the edge, thus complementing probe measurements from floating potential.

c) In a second day of operation, experiments would be repeated once again, using a magnetic configuration featuring enhanced connection length after the separatrix. One viable way to achieve this is to reduce the volume of the magnetically confined region, thus increasing the distance between the LCFS and the limiting elements of the vacuum vessel.

d) In a third day of operation, experiments carried out on the first day would be repeated using a magnetic configuration featuring a low rational in the vicinity of the LCFS, thus creating a set of islands in the edge. These measurements would as well be carried both with normal and reduced volume.


Required resources:

  • Number of plasma discharges or days of operation: 3
  • Essential diagnostic systems: Dual Langmuir probe system in TJ-II
  • Type of plasmas (heating configuration): ECRH & NBI
  • Specific requirements on wall conditioning if any: Sufficient density control for good reproducibility in NBI plasmas.
  • External users: need a local computer account for data access: yes
  • Any external equipment to be integrated? Provide description and integration needs:

Preferred dates and degree of flexibility

Preferred dates: (format dd-mm-yyyy)

References

  1. S. I. Krasheninnikov, D. A. D'Ippolito and J. R. Myra, J. Plasma Phys. 74 (2008) 679717
  2. P. Manz, D. Carralero, G. Birkenmeier et al., Physics of Plasmas 20 (2013) 102307
  3. D’Ippolito D.A., Myra J.R. and Zweben S.J., Phys.Plasmas 18 (2011) 060501,4
  4. D. Carralero, P. Manz, L. Aho-Mantila, et al. Phys. Rev. Lett. 115 (2015) 215002
  5. C. Killer, B. Shanahan, O. Grulke et al., Plasma Phys. Control. Fusion 62 (2020) 085003
  6. E. de la Cal and The TJ-II Team 2016 Nucl. Fusion 56 106031
  7. B.Ph. van Milligen, J.H. Nicolau, B. Liu et al., Nucl. Fusion 58 026030 (2018)
  8. T. Kobayashi, U. Losada, B. Liu et al., Nuclear Fusion 59, 044006 (2019)
  9. G. Grenfell et al., to be submitted

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