Christian M. Franck

Mode Transitions in Helicon Discharges

High resolution density measurements in a conventional helicon source show sudden jumps during a rf power ramp. The downwards power ramp enters all three known discharge modes, capacitive, inductive, and helicon wave sustained mode. For an upwards power ramp the discharge jumps directly from the capacitive into the helicon mode. This observation stands in contrast to models that are based on the plasma density as the critical parameter for the transition to the helicon mode.

Transition from unbounded to bounded plasma whistler wave dispersion

Whistler wave dispersion measurements are done in a linear magnetized helicon plasma experiment. The waves are excited by an induction loop and detected by movable magnetic probes for a frequency range of 100–800 MHz, corresponding to 0.05–0.9 ωce. The dispersion of whistler waves is measured for various plasma densities and magnetic field strengths. A key issue is to study the transition from an unbounded to bounded plasma wave dispersion. A comparison with theoretically derived dispersion relations is made. For small wavelengths, the dispersion can be described with whistler wave theory for unbounded plasmas whereas for larger wavelengths, the bounded geometry must be taken into consideration. The experimental results agree with theoretical dispersion relations derived for the bounded and the unbounded situation.

Magnetic fluctuation probe design and capacitive pickup rejection

In this article the capacitive pickup of magnetic fluctuation probes for plasma applications is studied. The nine most commonly used probe designs are compared with respect to their capacitive pickup rejection, magnetic sensitivity, and minimum plasma disturbance. For absolute calibration, well defined electric and magnetic field fluctuations are produced separately in a Faraday cup and in a Helmholtz magnetic field coil configuration, respectively. A sample measurement in a radio frequency helicon plasma demonstrates that the optimum probe design is well suited for measuring magnetic fluctuations in a plasma environment.

Measurements of spatial structures of different discharge modes in a helicon source

This paper reports on two-dimensional measurements of plasma parameters and magnetic eigenmode profiles in capacitive, inductive and helicon wave sustained discharge modes of a helicon source with high spatial resolution. It is demonstrated that plasma densities ranging over four orders of magnitude can be achieved. The plasma profiles of the capacitive and inductive discharges are completely consistent with the accepted discharge models. The magnetic eigenmode structure in the helicon mode shows great differences in the mode number and axial wavelength compared to the capacitively coupled discharge. In particular, multiple reflections of the obliquely propagating helicon wave fronts at the plasma boundaries are observed in the helicon wave sustained plasma. Moreover, the consistent connection of plasma parameters with discharge parameters via the helicon wave dispersion is demonstrated with varying magnetic field and for various discharge power levels.

Dynamics of periodic ion holes in a forced beam–plasma experiment

Experimental observation of the propagation of periodic ion phase space vortices is reported. Density fluctuation measurements in a double plasma device show an apparently spontaneous acceleration of these periodic structures from ion thermal to ion acoustic velocity. A nonlinear kinetic description explains this as a transition from a new type of nonlinear electrostatic mode (periodic ion holes) to an ion acoustic mode which is caused by trapped particle scattering.

Transition from unbounded to bounded whistler wave dispersion: Reconsidered

The whistler wave dispersion relation in the transition region between unbounded and bounded plasma geometry is investigated experimentally and numerically. Measurements are done in a linear magnetized helicon plasma covering the large frequency range from 100–800 MHz, corresponding to 0.06–0.5fce. Small wavelength wave propagation (λ≪d: plasma diameter) is well explained by unbounded plasma whistler wave dispersion. In contrast to previously reported measurements [Franck et al., Phys. Plasmas 9, 3254 (2002)], the experimental findings are compared to numerical results obtained from the differential equations of a plasma-filled waveguide. Long wavelength wave measurements show that there is only qualitative agreement even with dispersion theory of whistler wave propagation in bounded plasmas. This is attributed to the perpendicular wave mode structure that influences the parallel wavelengths. Measurements of the perpendicular wave mode structure shows that it is basically given by the diameter of the plas...

Excitation and Propagation of Alfvén Waves in a Helicon Discharge

An experimental study of shear Alfven waves in a linearly magnetized plasma is presented. Shear Alfven waves are electromagnetic waves propagating parallel to the background magnetic field without compression of the plasma at a frequency well below the ion cyclotron frequency and a wavelength inversely proportional to the square root of the plasma density. A basic condition on laboratory investigations is that the Alfven wavelength must be significantly smaller than the device dimension. This makes Alfven waves difficult to investigate in laboratory experiments and most studies are performed in space, where typical Alfven wavelengths of several kilometers are observed. The results of these studies are often ambiguous due to difficulties concerning the measurements of plasma parameters and the magnetic field geometry. The primary motivation for the present paper is the investigation of Alfven wave propagation in a well defined laboratory situation. The experiments are conducted in the linear VINETA device....

Investigation of the Influence of Different Boundary Conditions on Helicon Discharges

In this paper we present investigations on the influence of boundary conditions on helicon discharges. This is done using two approaches: Firstly, the influence of the plasma size is studied by excitation of whistler waves with different wavelengths (frequency range 100 – 1000 MHz). For wavelengths much smaller than the plasma dimensions (4.5 m in length and up to 40 cm in diameter), the unbounded whistler dispersion relation turns out to be appropriate. For increasing wavelengths, the measured dispersion deviates more and more from the predictions of unbounded plasma theory and the helicon wave dispersion relation must be used. Secondly, the boundary condition between the antenna and the plasma is investigated. Standard right‐helical antennae are used to operate an rf helicon plasma at a typical frequency of 13.56 MHz. In the linear magnetised plasma experiment VINETA, two different setups for the RF plasma source are available: a standard helicon setup with a glass cylinder attached to one end of the ch...

Ion and Electron Whistler Wave Dispersion Experiments

Although whistler waves are studied for almost a century, they are still subject of intense research. Laboratory experiments are of particular value for the interpretation of satellite data, which is often ambiguous or at least difficult to understand. The linear plasma experiment VINETA, which is designed to form a large (4.5 m column length, up to 40 cm diameter) and dense (ne ⩽ 1019 m−3) plasma with great flexibility in the magnetic field configurations (max. field B0 = 100mT). In the present contribution we discuss investigations on electron whistler waves, right‐hand polarised electromagnetic waves in the frequency range ωci ≪ ω ⩽ ωce, and ion whistler waves, left‐hand polarised electromagnetic waves below the ion cyclotron frequency ω < ωci. Firstly, the dispersion behaviour of electron whistler waves is studied at wavelength up to the plasma dimensions. For smaller wavelengths it turns out that the dispersion relation derived for unbounded whistler waves describes the experimental observations well...