Page 2 - A mass resolved, high resolution neutral particle analyzer for C-2U
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radius) of the 4 concentric divertor electrodes on C-2U, just outside the location of a plasma gun (Fig. 1 (b)) that is used in conjunction with the divertor electrodes to control the radial electric field in the plasma [1]. The plasma gun is biased to Vbias, as much as 1 kV below machine ground, while each of the divertor electrodes are electrically floating. The outer case of each analyzer as well as its measurement electronics are held at Ve, the potential of the electrode to which they are mounted. Although Ve is floating, in practice it is measured to float close to Vbias. Finally, we note that the measurements presented in this paper were taken during the last two months of operation of C-2U, when the electrode biasing on the north side of the machine (opposite the ELA) was operated in an experimental configuration. This configuration produced relatively short-lived shots that lasted for about 2 ms.
Ion Current Density
Ion current density was measured by the GEA, which consists of four mesh electrodes in front of a current collector (Fig. 2). The GEA can also measure the ion energy distribution, but those results are not presented in this paper. To measure ion current density, the first 3 electrodes (#1-3) are held at Ve. Electrode #1 attenuates the plasma to prevent space charge limitation inside the device, electrodes #2 and #3 do not serve a useful function for this measurement, and electrode #4 is biased to -250 V with respect to Ve in order to repel primary electrons from the plasma and reflect secondary electrons back into the collector.
FIGURE 2: (Color online). The energy analyzer consists of a series of mesh electrodes in front of a current collector.
The GEA’s sensitivity to ion current is determined by the combined transparency of its front attenuation plate (TA) in addition to each of the following nickel meshes that make up the electrodes (TM). The transparency of an individual mesh is set geometrically by the ratio of its open area to closed area, and it has an angular dependence due to the finite thickness of the mesh. Optical measurements of TA and TM are presented in Fig. 3 (a) and (b).
                                                                                                                                                                                                                                                                                         FIGURE 3: (Color online).Optical transparency measurements of an individual (a) attenuation plate and (b) nickel mesh that are used in the GEA electrodes. The angle of incidence is measured with respect to the axis of the GEA. (c) Ion current measured by GEA #1 and GEA #2. The smoothed signal is plotted with the un-smoothed signal shaded in the background.
The mesh and attenuation grid were designed so that their transparency is roughly constant over the range of angles at which ions will approach the analyzer. Each GEA is mounted so that its axis is parallel to the local magnetic field (within 2◦ for typical operation). Ions that travel from the SOL to the GEA location are collimated by the magnetic

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