Plasma potential probes for hot plasmas

Abstract

Plasma probes are well established diagnostic tools. They are not complicated, relatively easy to construct and to handle. The easiest and fastest accessible parameter is their floating potential. However, the floating potential of a cold probe is not very significant. Much more important and relevant is the plasma potential. But in most types of plasmas, consisting mainly of electrons and only positive ions, the floating potential is more negative than the plasma potential by a factor proportional to the electron temperature. Obviously this is due to the much higher mobility of the electrons. We present a review on probes whose floating potential is close to or ideally equal to the plasma potential. Such probes we name Plasma Potential Probes (PPP) and they can either be Electron Emissive Probes (EEP) or so-called Electron Screening Probes (EPS). These probes make it possible to measure the plasma potential directly and thus with high temporal resolution. An EEP compensates the plasma electron current by an electron emission current from the probe into the plasma, thereby rendering the current-voltage characteristic symmetric with respect to the plasma potential and shifting the floating potential towards the plasma potential. Only the simplest case of an EEP floating exactly on the plasma potential is discussed here in which case no sheath is present around the probe. An ESP, principally operable only in strong magnetic fields, screens off most of the plasma electron current from the probe collector, taking advantage of the fact that the gyro radius of electrons is usually much smaller than that of the ions. Also in this case we obtain a symmetric current-voltage characteristic and a shift of the probe's floating potential towards the plasma potential. We have developed strong and robust EEPs and two types of ESPs, called BUnker Probes (BUP), for the use in the Scrape-Off Layer (SOL) of Medium-Size Tokamaks (MST), and other types of strongly magnetized hot plasmas. These probes are presented in detail.

Analysis of thermal and structural responses of a new diagnostic probe to repeated exposure in ASDEX upgrade tokamak

“…The newly constructed probe head (Figure 1) will be the first one used in a tokamak that incorporates RFAs, EEP and CLPs in one place [5]. This is very useful for correct evaluation of the heat fluxes at the SOL mid-plane.…”

“…Therefore, a set of diagnostics consisting of two Cold Langmuir Probes (CLP), one Electron-Emissive Probe (EEP), two Retarding Field Analysers (RFA) and two Magnetic Pickup Coils has been mounted on a single relatively small probe head -called the New Probe Head (NPH). The development of the diagnostics for the NPH is described in [5]. The probe head design, and later the prototype, has to go through a number of tests (numerical and experimental) before it can be used in the real tokamak, where it will have to withstand repeated exposure to the high heat flux in SOL plasma.…”

Diamond-Coated Plasma Probes for Hot and Hazardous Plasmas

“…Therefore, particular research efforts have been made on various methods of obtaining a reliable direct measure of the plasma potential via the probe’s floating potential. To this end, electron-emissive probes (EEP) and electron-screening probes (ESP) have been developed (see, e.g., Ionita et al [ 4 ]).…”

“…In the meantime, a probe head [ 4 , 23 ] (called: the “New Probe Head”—NPH) has been developed for measurements in the EUROfusion MSTs (see below), combining two CLPs, an EEP and two retarding field analyzers (RFA) for ion energy measurements. The NPH also carries two magnetic pickup coils (MPC) to measure magnetic field fluctuations on two radial positions [ 24 , 25 ].…”

Intense boundary emission destroys normal radio-frequency plasma sheath

“…A recent experiment reported a positive potential of highly emissive probe relative to plasma in ICP, though the complete sheath profile was unknown 35 . Yet most others reported negative potential relative to plasma [58][59][60][61] . Future works are encouraged for IRP with transverse waves, which offers possibilities to overcome blockage of signal by plasma layer in spacecraft communications 62 .…”