Confinement of Washer-Gun Plasma and Tailoring of Location and Scale Length of Electron Density Gradient to Meet the Requisites of Microwave Plasma Interaction

Experimental investigations on interaction of High Power Microwave (3 MW, 3 GHz) with an over-dense (Plasma-frequency > Microwave-frequency) plasma has been initiated in SYMPLE (SYstem for Microwave PLasma Experiments) 1 , aiming at understanding electromagnetic wave absorption in plasma. The microwave power is so chosen to meet the condition: wave energy-density $\frac{1}{2}{{\text{E}}_{\mu }}^2$ plasma particle pressure n e kT e (E µ : wave-field, n e : plasma electron density, T e : electron temperature). The microwave coupling-scheme ensures having E µ -vector parallel to plasma density gradient □n e . The plasma system consists of a washer-gun based plasma (n e ~ (1-10)x10 17 /m 3 ) ejected to a cylindrical chamber (30 cm diameter, 1 m long) 2 . As requirement of having unmagnetized plasma prevents use of electromagnets for confinement, a permanent magnet based 3 - 5 scheme is adopted, which enables the confining-field to be locally restricted to the inner chamber-periphery, retaining desired radial extent of central plasma column unmagnetised. As wave-absorption has critical dependence on scale-length (Ln) of □n e , there is a requirement on controlling L n , which is also achieved by this confinement scheme. This scheme also enables access on location of □n e . The present work discusses design and simulation of a 6-pole cusped magnetic field geometry enabling confinement of plasma while retaining the central plasma column of ~ 12 cm radial extent unmagnetised, experimental verification of the field profile and results on tailoring of n e, L n and □n e location.