Abstract
In the initial phase of a plasma discharge with EC-assisted breakdown, the wave-particle interaction is nonlinear and wave trapping provides the mechanism for transition from a very low energy state to a much larger energy state. A Hamiltonian adiabatic approach provides the condition for the energy variation to occur in a rigorous way, together with quantitative estimate as a function of the wave frequency, harmonic number, polarisation and EC power and beam width, for the first, and second cyclotron harmonic. The interaction is highly localized in space close to the EC resonance. The spatial profile of the energy gain are reported together with the estimates of the width of the radial region where the interaction takes place in the case of a tokamak configuration. The analysis provides a physics based description of the process as well as indications for an optimized experimental setup.
Highlights
In ITER it is foreseen to startup the plasma injecting Electron Cyclotron (EC) waves to reduce the volt-second consumption of the main transformer during breakdown
In the initial phase of a plasma discharge with EC-assisted breakdown, the wave-particle interaction is nonlinear and wave trapping provides the mechanism for transition from a very low energy state to a much larger energy state
Various physical mechanisms occur during r.f. assisted plasma startup, like wave-particle interaction, particle collisions with both neutrals and ionised plasma, particle confinement and plasma build-up, and the overall process is quite complex
Summary
In ITER it is foreseen to startup the plasma injecting Electron Cyclotron (EC) waves to reduce the volt-second consumption of the main transformer during breakdown. During the initial phase of the EC-assisted breakdown, electrons are assumed to behave as free particles at room temperature moving along the static magnetic field, under the action of a localised microwave beam. As it was first pointed out long ago in [3], such cold electrons can acquire a large energy via nonlinear wave trapping for beam power values like those injected in fusion machines (100 kW - 1 MW in single rf source), whenever the e.m. amplitude is. In ITER typical parameters are τcycl 6 × 10−12 s, τtrap ≈ 10−8 s, τ f light ≈ 1 − 5 × 10−6 s, τcoll ≈ 5 × 10−4 − 5 × 10−3 s, Equation (1) identifies a nonlinear adabiatic interaction regime
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