D.L. Rudakov, M.G. Shats, R.W. Boswell

Electric or Langmuir probes are among the most basic plasma diagnostics. They are generally simple in construction and use, relatively inexpensive and robust enough to withstand considerable heat fluxes. They have excellent spatial resolution, limited only by the probe size and by the accuracy of the positioning mechanism. Langmuir probes can provide measurements of such basic plasma parameters as electron density n_e , electron temperature T_e , and plasma potential V_p . Good temporal resolution make electric probes a useful tool in plasma fluctuation studies. Although probes generally perturb their local surroundings to some extent and interpretation of the data is not always straightforward, the above-mentioned advantages usually justify the use of probe techniques wherever possible.

At the current stage of operation of H-1, moderate parameters of the plasma and comparatively short pulse duration allow extensive use of electric probes throughout the plasma cross-section. The probes used on H-1 include single Langmuir probes, a quadruple probe, a double-quadruple "fork" probe, a Mach probe and a 24-channel probe array. Single and quadruple probes are used to measure local electron density, electron temperature and plasma potential. The quadruple probe and the "fork" probe are used in fluctuation studies to simultaneously measure fluctuations of the above-mentioned parameters, which allows for an experimental estimate of the fluctuation-induced particle flux. The Mach probe is used to measure poloidal and toroidal ion flow velocities. The 24-channel probe array is used for fast time-resolved acquisition of the radial profiles and poloidal maps of the electron density and floating potential. It is also employed to measure poloidal wave numbers of the low frequency coherent fluctuations observed in H-1.

Most of the Langmuir probes used on H-1, with the exception of the 24-pin probe array, have similar construction. Figure 1 presents the construction of the quadruple and Mach probes as an example.

Figure 2 shows the construction of the 24-pin probe array. Individual probes (see the insert) have a construction similar to those mentioned above. The distance between the pins is 10 mm.

The double-quadruple "fork" probe consists of two identical quadruple probes (similar to that shown in Figure 1) installed on a single support tube with their axes parallel to the support tube axis. The distance between the axes of the two probes is 15 mm.

Few plasma diagnostics are capable of directly measuring the ion temperature. One successful approach employs the retarding field energy analysers (RFEA). The RFEA is a versatile diagnostic capable of measuring not only the ion temperature but the whole ion or electron parallel energy distribution, as well as the plasma potential.

Operation of the RFEA is based on selectively rejecting or retarding either the plasma electrons or the ions. RFEA typically consists of a collector electrode with two or more separately biased fine grids placed in front of it. The plasma particles enter the analyser through a small input slit or hole. In the ion mode of operation the electrons are rejected by a negatively biased grid, and the ions are then retarded by a positively biased grid. Varying the retarding grid voltage one can obtain an integral distribution of the ion parallel energy above the level defined by the grid voltage. In the electron mode of operation the grid voltages are reversed.

The RFEA used on H-1 was developed in the Space Plasma Group of the Plasma Research Laboratory, ANU. The design of the RFEA is schematised in Fig. 3. The analyser biasing circuit in the ion mode of operation is shown in Fig. 4.