Electron cyclotron heating during ECRH-assisted pre-ionization in a tokamak

Abstract

In this research, we investigate ECRH for low-energy electrons analytically and numerically, which is applicable to ECRH-assisted plasma start-up in a tokamak. From the previous experimental studies, it is wellknown that the first harmonic ECRH is more effective than the second harmonic ECRH. In this work, we developed an analytic model of ECRH in the start-up process and present comparisons of the efficiency between the first harmonic and the second harmonic. It is found that electrons gain a large energy from the waves at first harmonic resonance up to several hundred eV. However, electrons gain a large energy from the waves only up to ∼10 eV and energy-gain starts decreasing afterward at second harmonic resonance. When seed electrons are heated from the room temperature to far away above the ionization energy, seed electrons can bring about an avalanche of electrons. Thus, pre-ionization with the second harmonic can be delayed since electrons need more time to be heated up to the breakdown temperature due to the slow heating speed compared to the first harmonic ECRH.

Comparison between Non-Inductive Plasma Current Start-Up Using ECRH with and without Fundamental Resonance on QUEST

“…The electron energy is sourced from the electrical field of the high-frequency wave when electrons pass through the ECR layer is calculated using a model given in [9,10]. The phase difference between the particle gyromotion and the wave electric field as Ψ = nψ − ωt and the dimensionless variable Γ = γ − 1 are defined, where, n, ψ, ω, γ are the harmonic number, the gyro-phase of electrons, the wave angular frequency, and relativistic factor of electron, respectively.…”

“…plausible explanation has been developed [9,10]. Nevertheless, plasma current start-up without 1st ECRH has not been reported yet.…”

Development of plasma sources for Dipole Research EXperiment (DREX)

“…It is necessary to search for a way to lower the breakdown conditions. ECR pre-ionization has been used to assist current startup in tokomaks [11,23,24]. In these experiments, the reduction of loop voltage with rf was typically up to 50% of the initial voltage without rf and the delay in initiation of the plasma current was substantially reduced.…”

“…In addition, we plan to try to employ ECR as a pre-ionization assisted startup for the discharge. An ECR pre-ionization assisted startup scheme has been demonstrated in a number of tokamaks to generate reproducible and performance-enhancing start-up [22][23][24]. In particular, it shows a substantial reduction in the requirement of loop voltage for the ohmic current and thus lowers the transformer flux consumption in the initial phase of startup.…”

Triple-probe Diagnostic Measurements in Plasma of GLAST Spherical Tokamak

“…The toroidal electrical field requirement for breakdown can & A. Qayyum abdul.qaui@yahoo.com be reduced significantly by adopting various pre-ionization schemes during startup phase [15][16][17]. ECRH-assisted current startup is a scheme for generating toroidal plasma currents in tokomaks [18][19][20].…”