Theory of the fast particle generation in front of LH grills
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V. Petrzilka , V. Fuchs , J. Gunn , A. Ekedahl , N. Fedorczak , M. Goniche , J. Hillairet ,
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P. Pavlo
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Association EURATOM-IPP.CR, Za Slovankou 3, 182 21 Praha 8, Czech Republic
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CEA, IRFM, 13108 Saint Paul-lez-Durance, France
During tokamak operation with lower hybrid (LH) power a few percent of the launched
power is absorbed by the scrape-off layer (SOL) plasma in magnetic flux tubes in front of
the LH grill [1]. At strike points of these flux tubes, intense plasma-wall interaction is seen
in visible and infra-red (IR) wavelengths, and local wall damage can occur. The parallel
power flux within these ‘‘hot spots” is estimated to be up to about 10 MW/m2 by IR
imagery [2]. According to theory [3], Landau damping transfers the power carried by the
high refractive indices n// of the wave to thermal SOL electrons with energies of a few tens
of eV and accelerates them up to a few keV. The high n// spectral components are expected
to be absorbed immediately in front of the LH grill within a few mm [4]. However, recent
experimental results from retarding field analyzer measurements on Tore Supra [5] as well
as JET IR camera measurements [6] have shown the existence of fast electrons as far as a
few centimeters from the grill mouth. This finding cannot be explained by the standard
theory [3], and also contradicts earlier PIC simulation results [4], which predict interaction
zones of the order of up to about 5 mm. In addition to a summary of experiments and
standard theory [1-4], the present contribution also aims at understanding recent
experimental results [5,6] on the basis of new theoretical ideas [7,8]. We present a theory
explaining the fast electron generation in a wide layer of several centimeters in front of the
LH grill by taking into account LH wave propagation features closely connected with the
blob character of SOL turbulence [9]. An alternative theoretical explanation considers
plasma density modulations due to ponderomotive force effects in front of the LH grill
[8,10]. Then, the LH wave can scatter on these local SOL density perturbations, and
higher n// necessary for the fast electron generation can be produced at positions indicated
by recent measurements. Last but not least, we discuss the charge separation effects in
front of LH grills [11] and possible resulting ion acceleration [12].
Support by the Czech Republic Science Foundation Projects GACR 202/07/0044 and
205/10/2055 is acknowledged.
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[4] K. Rantamäki et al., Nucl. Fusion 40 (2000) 1477.
[5] J.P. Gunn et al., J. Nucl. Mater. 390–391 (2009) 904.
[6] P. Jacquet et al., 18th Conf. on RF Power in Plasmas, Gent (2009) paper A 55.
[7] V. Fuchs et al., 18th Conf. on RF Power in Plasmas, Gent (2009), paper A52.
[8] V. Petrzilka et al., 36th EPS Conf. on Plasma Physics 2009, Sofia, Bulgaria, P4.207.
[9] O.E. Garcia et al., J. Nucl. Mater. 363-365 (2007) 575.
[10] R. Klima, Sov. Phys. JETP 23 (1966) 534.
[11] F. Zacek et al., Plasma Phys. Contr. Fusion 47 (2005) L17.
[12] V. Petrzilka et al., 18th IAEA Conference, Sorrento 2000, paper CN-77/EXP4/07.