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Title: Relativistic Fermi-Ulam map: Application to WEGA stellarator lower hybrid power operation

Analytical and numerical support is here provided in support of the explanation [Laqua et al., Plasma Phys. Controlled Fusion 56, 075022 (2014)] for the observation of ∼MeV electrons during Lower Hybrid (LH) operation in EC pre-heated plasma at the WEGA stellarator [Otte et al., Nukleonika, 57, 171 (2012)]. In the quoted experiments, LH power from the WEGA TE{sub 11} circular waveguide, 9 cm diameter, un-phased, 2.45 GHz antenna, is radiated into a B ≅ 0.5 T, n{sup ¯}{sub e} ≅ 5 × 10{sup 17} 1/m{sup 3} plasma at T{sub e} ≅ 10 eV bulk temperature with an EC-generated 50 keV population of electrons. In response, the fast electrons travel around flux or drift surfaces essentially without collisions, repeatedly interacting with the rf field close to the antenna mouth, and gaining energy in the process. Our WEGA antenna calculations indicate a predominantly standing electric field pattern at the antenna mouth. From a simple approximation of the corresponding Hamiltonian equations of motion, we derive here a relativistic generalization of the simplified area-preserving Fermi-Ulam (F-U) map [M. A. Lieberman and A. J. Lichtenberg, Phys. Rev. A 5, 1852 (1972), Lichtenberg et al., Physica D 1, 291 (1980)], allowing phase-space global stochasticity analysis. At typical WEGA plasma and antenna conditions, and with correlated phases between electron–antenna electricmore » field interaction events, the F-U map and supporting numerical simulations predict an absolute energy barrier in the range of 300 keV. In contrast, with random phases intervening between interaction events, the electron energy can reach ∼MeV values, compatible with the measurements on WEGA [Laqua et al., Plasma Phys. Controlled Fusion 56, 075022 (2014)].« less
Authors:
; ; ; ; ;  [1] ;  [2]
  1. Institute of Plasma Physics AS CR, v.v.i., Prague (Czech Republic)
  2. Max-Planck-Institute for Plasma Physics, Wendelsteinstr. 1, 17491 Greifswald (Germany)
Publication Date:
OSTI Identifier:
22299956
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 21; Journal Issue: 6; Other Information: (c) 2014 EURATOM; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; ANTENNAS; COMPUTERIZED SIMULATION; ELECTRIC FIELDS; EQUATIONS OF MOTION; EV RANGE 01-10; LOWER HYBRID CURRENT DRIVE; LOWER HYBRID HEATING; PHASE SPACE; PLASMA; RELATIVISTIC RANGE; WEGA STELLARATOR