G.S. Gogna

Interpreting the behavior of a quarter-wave transmission line resonator in a magnetized plasma

The quarter wave resonator immersed in a strongly magnetized plasma displays two possible resonances occurring either below or above its resonance frequency in vacuum, fo. This fact was demonstrated in our recent articles [G. S. Gogna and S. K. Karkari, Appl. Phys. Lett. 96, 151503 (2010); S. K. Karkari, G. S. Gogna, D. Boilson, M. M. Turner, and A. Simonin, Contrib. Plasma Phys. 50(9), 903 (2010)], where the experiments were carried out over a limited range of magnetic fields at a constant electron density, ne. In this paper, we present the observation of dual resonances occurring over the frequency scan and find that ne calculated by considering the lower resonance frequency is 25%–30% smaller than that calculated using the upper resonance frequency with respect to fo. At a given magnetic field strength, the resonances tend to shift away from fo as the background density is increased. The lower resonance tends to saturate when its value approaches electron cyclotron frequency, fce. Interpretation of the...

Revised formulation of electron density for partially shielded floating hairpin probe

The electron density formulation for the floating hairpin probe considers a uniform plasma dielectric surrounding the hairpin. However, a fraction of the short-circuited end of the hairpin is attached to the dielectric probe-tip holder. The external dielectric shifts the probe’s resonances towards the lower frequency domain. So far, its influence on the accuracy of the measured electron density has been widely ignored. This letter presents a revised formulation for the electron density when a substantial part of the hairpin is shielded by the dielectric. The formulation was experimentally verified and highlights the practical significance of the partially shielded hairpin probe.

Introducing hairpin probe for electron density measurement in a KAMABOKO-III negative ion source

Neutral beam injection based on a negative ion source is one of the most promising candidates for heating and current drive in future fusion reactors. The physical and chemical processes involved in the filter field region of an ion sources are especially important to investigate, as this is the region where the negative ions are mainly generated and extracted. The externally applied magnetic/filter field around 0.012T cools the high-energy electrons in order to avoid collisional destruction of the negative ions. The electron density (ne) measurements using conventional diagnostic techniques such as Langmuir Probe (LP) and Optical Emission Spectroscopy becomes complicated in presence of negative ions as well as magnetic field and gives a qualitative result. For accurate det ermination of ne in such complex plasma, the floating resonance hairpin probe1 (HP) is applied in KAMABOKO-III negative ion source, at the MANTIS test bed in CEA Cadarache, France. The technique is based on measuring the plasma permittivity using a U-shaped microwave resonating structure whose relative shift of characteristics frequency in plasma from that in vacuum directly gives the ne without rely on any other plasma parameter. However, in the presence of magnetic field the plasma density becomes anisotropic and thus the HP gives an effective ne measurement within its spatial resolution. The measured ne is compared with the positive ion density measured by planar LP. The ne is studied as a function operating source parameters such as arc power, pressure, applied magnetic field and the plasma grid bias exclusively in the filter field region. The effect of probe orientation to the B-field on the resonant properties is also studied under given plasma conditions. The performance of the HP shows that the technique is more reliable, sensitive and accurate diagnostic tool for such ion source.

Study of resonant properties of hairpin probe for high-density operation

A well-defined sharp drop in the amplitude of the reflected signal characterizes the resonance signal of the hairpin probe (HP) when the frequency is varied over a given range typically of the order of electron plasma frequency. A quantity that defines the ratio of width of the resonance signal, Δf (fupper - flower) to the central frequency, fcentral is the quality/Q factor of the HP. For lower Q it is difficult to resolve the fcentral against the background noise1. The factors governing the Q are the losses of the energy stored in the resonator due to the resistive heating of the probe that depends on the probes temperature and the specific resistivity of the wire material as shown in figure. The coupling between the adjacent loop antenna and the hairpin also determines the shape of the resonance peak. In the plasma, the losses are further enhanced due to resistive heating of the oscillating electrons influenced by the electric field between the probe tips and the presence of external magnetic field. In this work, we present an analytical model based on the transmission line theory of the hairpin resonator circuit that explains the physical factors responsible for the observation of broad resonance peaks.