Special Plasma Seminar
Thursday, November 21
4274 Chamberlin Hall
Speaker: Prabal Chattopadhyay, IPR, Gandhinagar, India
"Wave Propagation and Potential Structures in an Expanding Helicon Plasma"
Abstract: Diverging magnetic fields are found naturally in universe including our magnetosphere and in solar coronal funnels. Diverging magnetic fields are used in expanding plasmas to accelerate particles by electric fields produced by localized potential structures, called double layer, formed self consistently inside the plasma. Acceleration of charged particles in low temperature plasmas is of interest to surface function modification as well as to development of electrostatic thrusters. This presentation will discuss the role of diverging magnetic fields in helicon source operation and self consistent potential structure formation in bulk of plasma. A geometrically expanding (small diameter source attached to a bigger diameter expansion chamber) linear helicon device along with various diagnostics is designed and built with a diverging magnetic field. The helicon plasma produced with an m = +1 half helical antenna powered by a 2.5 kW RF power source at 13.56 MHz is characterized. Mode transitions are observed and mode structures are studied at low magnetic fields (<100 G). Though a monotonic increase in density with magnetic field is expected for helicon plasma, multiple density peaks are observed for the first time for field variations at low magnetic fields and are explained on the basis of oblique resonance of helicon waves in a bounded geometry for the first time. Characterization on both sides of the antenna at low magnetic fields revealed the role of left circularly polarized waves in electron cyclotron absorption in bounded plasmas. Changing the magnetic field topology at low magnetic fields, it is found that diverging magnetic fields near antenna can increase the efficiency of the source as high as 80 % from the zero field case. With a magnetic field ~ 100 G near the source and ~ 10 G at the end of the expansion chamber, density peaks on axis are observed nearly two wavelengths away from the antenna where the field is ~ 35 G. Helicon wave phase measurements show that the wave does not propagate in the source region owing to the density cut-off of the wave but starts to propagate in the downstream for lower densities and magnetic fields. Finally, diverging magnetic fields and their gradient near the geometrical expansion location are varied to create strong potential structures along the magnetic field direction. Observation of multiple potential structures in current free plasmas will also be presented.