Role of ion magnetization in formation of radial density profile in magnetically expanding plasma produced by helicon antenna

Abstract

Experimentally, the density profile in the magnetic nozzle of a helicon antenna based plasma device is seen to be modified from being centrally peaked to that of hollow nature as the external magnetic field is increased. It occurs above a characteristic field value when the ions become magnetized in the expansion chamber. The density profile in the source chamber behind the nozzle, however, remains peaked on-axis irrespective of the magnetic field. The electron temperature there is observed to be hollow and this nature is carried to the expansion chamber along the field line. In the electron energy distribution near the off axis peak location, a high energy tail exists. Rotation of these tail electrons in the azimuthal direction due to the gradient-B drift in the expansion chamber leads to an additional off-axis ionization and forms the hollow density profile. It seems that if the ions are not magnetized, then the off-axially produced additional plasma is not confined and the density profile retains the on-axis peak nature. The present experiment successfully demonstrates how the knowledge of the ion magnetization together with tail electrons significantly contributes to the design of an efficient helicon plasma based thruster.

Effect of magnetic field on double layer in argon helicon plasma

“…Since the electron temperature was correlated with the enthalpy which could be transferred into potential energy [38], the electrons gathered at outward region contributed to the broad distribution of plasma potential, as shown in Figure 5. Besides, the electrons with higher energy at outward region could enhance the ionization due to the grad‐B drift effect [37]. This contributed to the radially broad distribution of the plasma density, as shown in Figure 6.…”

“…As electrons derived from the source could be magnetized by magnetic field, they were confined around the magnetic field lines and were transported through strong gradient of diverging magnetic field (∇B) into the diffusion chamber. Such transportation was proposed to be grad-B drift [37], in which the electrons rotated in azimuthal direction with little loss of energy. Hence the electron energy was higher at the periphery below the discharge tube and the electron temperature was peaked off-axis.…”

Effect of inhomogeneous magnetic field on plasma generation in a low magnetic field helicon discharge

“…The experiments are performed in the linear helicon plasma experimental device ( Fig. 1), which has been described before 20,21 . The vacuum system consists of a source and expansion chamber.…”

“…Two separate single rf compensated Langmuir probes 21 , one straight, inserted from the top radial port in the source chamber at z = 31 cm and other L shaped, inserted off-axially from the end flange of the expansion chamber at z = 50 cm, are used to measure the plasma density, n 0 . These locations also correspond to before (z bef ore = 31 cm) and after (z af ter = 50 cm) both in the magnetic and geometric expansion Fig.…”

Data Processing Techniques for Ion and Electron Energy Distribution Functions

“…The mean squared error, the correlation coefficient and the residuals of the different methods were compared, coupled with a visual assessment of the EEPF. Alternatively, analog differentiation has been performed by using appropriate electronics circuitry to obtain the IEDF and the EEPF 3,17,[28][29][30] . The IEDF has also been obtained using a Gaussian deconvolution method 2,16,19 .…”