Investigation of the role of electron cyclotron resonance heating and magnetic configuration on the suprathermal ion population in the stellarator TJ-II using a luminescent probe

Abstract

Numerous observation exist of a population of high energetic ions with energies well above the corresponding thermal values in plasmas generated by electron cyclotron resonance (ECR) heating in TJ-II stellarator and in other magnetically confined plasmas devices. In this work we study the impact of ECR heating different conditions (positions and powers) on fast ions escaping from plasmas in the TJ-II stellarator. For this study, an ion luminescent probe operated in counting mode is used to measure the energy distribution of suprathermal ions, in the range from 1 to 30 keV. It is observed that some suprathermal ions characteristics (such as temperature, particle and energy fluxes) are related directly with the gyrotron power and focus position of the heating beam in the plasma. Moreover, it is found that suprathermal ion characteristics vary during a magnetic configuration scan (performed along a single discharge). By investigating the suprathermal ions escaping from plasmas generated using two gyrotrons, one with fixed power and the other modulated (on/off) at low frequency (10 Hz), the de-confinement time of the suprathermal ions can be measured, which is of the order of a few milliseconds (<4 ms). A model that uses a zero-dimensional power balance is used to understand the de-confinement times in terms of the interaction of suprathermal ions and plasma components. This model also can be used to interpret experimental results of energy loss due to suprathermal ions. Finally, observations of increases (peaks) in the population of escaping suprathermal ions, which are well localized at discrete energies, is documented, these peaks being observed in the energy distributions along a discharge.

Possibility of strong anomalous absorption of microwaves in electron cyclotron resonance heating experiments in fusion devices

“…Its concept, supported by the results of theoretical analysis, implies localized heating energy deposition and the absence of any nonlinear phenomena including parametric decay instabilities (PDIs) in the pump wave propagation and damping [1,2]. However, during the last decade both anomalous scattering of microwaves [3,4] and ion acceleration [5,6] were observed in ECRH experiments at different toroidal devices. These phenomena have not been explained by the linear wave theory, nor have been they interpreted within the conventional paradigm of PDIs in inhomogeneous plasma with monotonic density and magnetic field profiles.…”

Anomalous absorption in ECRH experiments due to the parametric low-threshold excitation of localized UH waves

“…The nonlinear three-wave interactions lead to the variation of amplitudes . The averaging procedure of an arbitrary function is determined as in equation (11). For the parameters under consideration ( )( )…”

“…where the averaging procedure over the spot in which the decay occurs is defined as in equation (11). If the PDI saturation results in a stationary regime, the saturation levels of daughter waves can be estimated assuming the nonlinear pumping in the primary decay is balanced by the energy losses due to the secondary decay is compensated by the effect of the secondary wave energy loss from the spot of a pump beam.…”

“…The parametric decay instabilities (PDIs), which can accompany the ECRH experiments, are believed to be deeply suppressed by the huge energy loss of daughter waves from the decay region, according to the predictions of theory developed in 80 th [1]. However, during the last decade numerous ECRH experiments performed at TEXTOR, TCV, TJ-II, ASDEX-UG, LHD, T-10 and FTU [2][3][4][5][6][7][8][9][10][11][12][13] have demonstrated excitation of the anomalous phenomena. The clearest evidence of the nonlinear phenomena excitation was obtained at TEXTOR [2,3] where the strong backscattering signals down-shifted in frequency and amplitude modulated at the frequency of magnetic island were observed.…”

Low-Threshold Induced-Side-Scattering Instability at the Tokamak Edge Transport Barrier Predicted in O-Mode Electron Cyclotron Resonance Heating Experiments

“…Such a system is also discussed for DEMO. However, over the last decade a number of anomalous phenomena -anomalous microwave backscattering [8,9], ion acceleration [10,11], an evident broadening of the ECRH power deposition profile [12,13] and radiation at sub-harmonics of the gyrotron frequency [14] -were discovered in ECRH experiments in different toroidal devices. The explanation of these observations turned out to be impossible within the framework of the traditional theoretical model [7].…”