The Microwave Concentrator Stack: A Source for Free‐Standing, Linearly Extended Plasmas

Abstract

A new type of plasma source, the Microwave Concentrator Stack, presented in this paper is a device of a geometry that can be described as an elliptical cylinder with additional metallic structures for shaping the electric field in its inside. The basic idea is to concentrate the electric field strength of microwaves on one of its focal lines by coupling microwave power into it on the other focal line. Even though the dimensions of the Concentrator Stack are too small compared to the wavelength of the microwaves to assume wave-optics behavior, a pronounced maximum of the electric field intensity is created on the focal line.By placing an evacuated glass tube around the focal line, free-standing plasmas can be generated by the strong electric field. In the low pressure range (10 to 200 Pa), the plasma displays very good one-dimensional homogeneity along the focal line of the Concentrator Stack. At higher pressures the plasma contracts to several ball-shaped discharges, whose positions correspond to the aforementioned metallic structures inside the Concentrator Stack.The electric field distribution was calculated and optimized for various versions of the Concentrator Stack using the finite element software Comsol Multiphysics TM and also measured with PET absorber sheets. Measurements and calculations are in excellent agreement.