Comparison of theBfield dependency of plasma parameters of a weakly magnetized inductive and Helicon hydrogen discharge

Abstract

The discharge properties of a weakly magnetized inductively coupled hydrogen discharge (operating pressure 1 Pa) are evaluated by using optical emission spectroscopy. The behaviour of the electron density n e , temperature T e and the density ratio of atomic to molecular hydrogen n H /n H2 with varying magnetic field strength (up to 12 mT) is investigated. The results obtained from the OES measurements performed with a line of sight directed along the central axis of the cylindrical discharge vessel are compared to the case when the ICP antenna is replaced by a Nagoya-type-III Helicon antenna. In the ICP case, the electron temperature and density at the axis of the cylindrical discharge vessel decrease with increasing magnetic field due to the hindered radial electron diffusion. This results in a gradual transition from a homogeneous radial emission profile to a hollow profile with minimal emission in the discharge centre. Concerning the density ratio of atomic to molecular hydrogen, one obtains very high values of up to 0.32 at low B field and a decreasing behaviour with higher magnetic fields. For the Helicon case, the obtained values of n e and T e are virtually unaffected by the external magnetic field. Furthermore, a hollow radial emission profile is observed already at low B field strengths. In the Helicon setup one obtains an increasing trend for n H /n H2 with a maximum of about 0.2 at 12 mT.

Experimental benchmark of the NINJA code for application to the Linac4 H−ion source plasma

“…This LOS is used for recording lateral intensity profiles of the atomic b H emission line at particular z positions. From these measurements, the radial emission profile of b H is determined via Abel inversion as described in [19]. Prerequisites of the numerical approach are a cylindrical symmetry of the emission and a negligible optical thickness of the emission line.…”

Investigation of the RF efficiency of inductively coupled hydrogen plasmas at 1 MHz

“…Via Abel inversion, the lateral intensity profiles are numerically transformed into radial emission profiles (r) [17]. For this setup, the followed approach is described in detail in [7]. Requirements for an application of the Abel inversion are the cylindrical symmetry of the discharge and a negligible optical thickness of the detected emission line.…”

“…Requirements for an application of the Abel inversion are the cylindrical symmetry of the discharge and a negligible optical thickness of the detected emission line. Due to the geometry of the experiment and the low population density of the lower state n = 2 of the H β line, both requirements are fulfilled for this setup [7].…”

“…Due to its flexibility, it allows for various systematic investigations in continuous wave hydrogen and deuterium operation, including e.g. the application of different antenna types and the influence of external magnetic fields on the plasma parameters [7,8,9]. In addition to the 13.56 MHz / 600 W generator already available, the experiment has recently been equipped with a 1 MHz / 1 kW generator, which allows for dedicated investigations at the excitation frequency relevant for ITER.…”

Conceptual design of the beam source for the DEMO Neutral Beam Injectors

“…Instead of aiming to operate in the full helicon mode for which several 100 mT are required, the investigations are focussed to achieve the better coupling due to the so-called low-field peak [19]. It has already been demonstrated that the low-field peak appears below 10 mT in hydrogen and deuterium plasmas at the relevant source pressure, showing that in the peak a higher dissociation degrees and plasma densities are achieved [20]. Further studies concentrate on the dependence on the RF frequency and source diameter to optimize the system.…”