Measurements of HF‐Plasma Oscilations by means of a Laser‐Heated Emissive Probe

Abstract

Indirectly heated emissive probes have advantages compared to conventional emissive wire probes. We have developed a laser-heated emissive probe consisting of a 1 mm diameter and 2 mm long pin of LaB6, heated by a focussed laser beam of 808 nm wavelength with a power up to 50 W. This probe is smaller and simpler than electrically heated emissive wire probes. Materials of low work function, high temperature stability and longer lifetime such as LaB6 can be used. There is no deformation in a magnetic field and no voltage drop along the probe wire. In this contribution we show the good time resolution of such a probe. Whereas emissive wire probes need two cables and an electric power supply or battery, for our probe one connection to an oscilloscope with high input impedance suffices. Therefore the probe system has a much lower stray capacitance than conventional emissive wire probes.

New diagnostic tools for transport measurements in the scrape-off layer (SOL) of medium-size tokamaks

“…In a realistic experiment the procedure to verify that the emission current is sufficient for shifting V fl,em as close as possible to Φ pl is to measure V fl,em as function of the EEP heating power P heat . It is very helpful that for increasing heating power the transition from the cold floating potential V fl towards Φ pl (being more positive than V fl by a few time T e -equation (1)) is usually rather abrupt and that above a certain value of P heat the value of V fl,em almost saturates (see for instance [10,30,35,36]). Also figure 1 illustrates the rather abrupt transition of the floating potential of an unheated, or only slightly heated EEP from that of a CLP (horizontal blue line) to that of an EEP, being close to the plasma potential Φ pl (horizontal red line).…”

Emissive probes

“…The probe itself can be made of a variety of materials, including platinum, tungsten, graphite and LaB 6 , but the greatest benefit of heating with a laser is that there is no potential distribution across the probe. Because of this, laser-heated probes have attracted attention from the fusion community in recent years [51][52][53][54][55]. Particularly noteworthy is the movable laser-heated emissive probe designed by Schrittwieser et al as shown in figure 17 [52].…”

Simulation of a Planar Emissive Probe in a Mid‐Sized Tokamak Plasma

“…Most common are current-heated wire loops [10,11] but also laser heated probes have recently gained momentum [12,13]. The emitted electron current reduces the difference between the floating and the plasma potential to (4):…”