New results on a laser-heated emissive probe

Emissive probes are used to determine the plasma potential directly since under ideal conditions their floating potential is equal to the plasma potential, when the emission current compensates the probe electron saturation current. This makes them useful also for non-Maxwellian plasmas, such as plasmas with drifting electrons or an electron beam. Usually emissive probes consist of a loop of refractory metal wire heated by an electric current until sufficient electron emission. However, such wire loop probes have several disadvantages which we can avoid by an indirectly heated probe. We have developed laser-heated emissive probes where an infrared laser beam is focused on a pin probe [1]. The beam is delivered from an infrared diode laser with 808 nm wavelength and a maximum output power of 50 W. Such a probe can have the simple form of a pin and allows the use of alternative materials such as LaB6 or graphite. With a cylindrical probe of 1,5 mm diameter and a length of about 2 mm of graphite or LaB6 we have achieved higher probe temperatures without evaporation or melting and thus higher emissivity and longer lifetime than with wire probes. In contrast to an electrically heated emissive wire probe, there is no deformation of the probe in a magnetic field, no potential drop along the probe wire and a faster time response. In the VINETA helicon discharge, with probe tips of LaB6 we have achieved several amperes of emission current [2]. With a combination of a cold and a laser-heated emissive probe close to each other, in a Maxwellian plasma, also the electron temperature Te can be determined from the difference of the floating potential Vfl of the cold probe and the plasma potential pl, measured as floating potential of the emissive probe. We obtain Te from the relation: Te = (pl – Vfl)/, with  being ln(Ies/Iis); in a magnetized argon plasma   4,2. Here we present new results on measurements of plasma potential and electron temperature fluctuations in the edge region of the VINETA plasma, in particular with two equally sized probe pins of LaB6 or graphite.