A 'gridded energy analyzer' (GEA) will be installed for the measurements of the
distribution functions of ions and electrons.
Several grids are biased with different voltages in order to repel one species
of particles and collect the other species (see fig.1 below).
The version installed on TORPEX has 4 grids, plus the electrode (collector). The voltage of one of the grids (V2 on grid2) can be swept to allow only particles with energies higher than V2 to reach the collector.
In addition to a better characterization of the plasma parameters, it will be possible to investigate the link between macroscopical parameters, as density and temperatures, and microscopical properties.
The GEA can be moved along the major radius and turned around the axis of the movable shaft, allowing a complete rotation of 360¡ (Fig. 2). The spatial resolution is given by the dimensions of the probe, about 1.5cm. The angular resolution depends on the mechanical system used to move the probe on a shot-by-shot basis, and can be estimated to be a few degrees.
The discriminating grid can be swept with a frequency of 1kHz ; a complete I-V characteristic can be acquired in a half-millisecond. This period corresponds to the maximum time resolution we can have in the ideal case, when it is not necessary to average the results over many periods.
The sampling frequency of 250kHz and the peak-to-peak amplitude Vpp=100V of the sweeping voltage give an energy resolution of about 1eV, which is comparable to the ion temperature. The energy resolution can be increased by decreasing the amplitude of Vpp.
The main parameters influencing the choice of the dimensions of the probe are the Debye length, the ion thermal velocity and the absolute value of the magnetic field. The potential applied to a grid will influence the particles dynamics as long as the grid spacing dgrid is smaller than the Debye length; this imposes an upper limit to the grid spacing. In addition, as the transmission factor of the grid is a decreasing function of dgrid, when dgrid is too small only a few number of particles can be collected, and the low signal-to-noise ratio (SNR) makes hard the data analysis. It could be possible to increase the dimensions of the probe to get a stronger signal, but this would introduce larger perturbations in the plasma.
The parameters collected in Tab I are a good compromise among all the
physical and geometrical constraints. With such a parameters the maximum ion current is estimated
to be about 359 mA for Te = 20 eV and
ne = 1017 m-3.
| External diameter of the probe | 15 mm |
| Effective diameter of hte grids dg | 6 mm |
| Grid spacing m | 0.125 mm (200 mesh) |
| Minimum distance between grids Smin | 0.7 mm |
| Trasmission of the entrance grid TG0 | 40% |
| Trasmission of the other grids TG1,G2,G3 | 70% |
| Overall trasmission factor Ttot | 13% |