First result on a laser-heated emissive probe

The floating potential of a probe, emitting a sufficiently high electron current, yields a fairly accurate approximation of the plasma potential. This is an advantage in comparison to the conventional Langmuir probe where, after determination of the electron temperature Te, the plasma potential can only be derived indirectly from the formula Vpl = Vfl + aTe, where a is a function of the ratio of the electron to the ion saturation currents (a is around 2.4 in a magnetised hydrogen plasma). In addition, an emissive probe also works if there are electron drifts or beams in the plasma. Emissive probes are usually realised by small directly heated loops of W-wire. Drawbacks of this design are the limited lifetime, the low electron emissivity of W and the voltage drop across the wire. We have developed a new type of emissive probe, which is heated by an infrared high-power diode laser with a maximum output power of 50 W. The probe consists of a small cylinder of LaB6. The probe was inserted into the edge region of the VINETA helicon discharge plasma. Basic features of emissive probes were verified.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004, Nice (France

Oscillatory behavior of hollow grid cathode discharges

Multiple complex space-charge structures in unmagnetized low-temperature plasmas arise from ionization phenomena near additional negatively or positively biased electrodes or due to local constraints. Because of their usually spherical form, such structures are called fireballs. If they appear inside hollow grids, they are called inverted fireballs or plasma bubbles. The temporal evolution of such structures is often accompanied by strong plasma instabilities. The dynamics of complex space-charge structures have been investigated by using single spherical grid cathode with an orifice. Langmuir probe and optical emission spectroscopy were used to diagnose the structures. Measurements delivered the axial profiles of the plasma potential, electron temperature and density, and the densities of excited atoms and ions, that confirmed the formation of a fireball in the region near the orifice (also evidenced by visual observation). Inside the grid, a plasma bubble has developed, with a high ion density inside due to the hollow cathode effect. Information on the nonlinear dynamics of the complex space charge structures was obtained from the analysis of the oscillations of the discharge current

Concentric double hollow grid cathode discharges. Spectral investigations and phenomenological approach

A discharge plasma is created by simultaneously biasing two concentric spherical grids with axisymmetric orifices. In this geometry, space charge structures in the form of multiple quasi-spherical luminous plasma bodies appear simultaneously inside and around the cathodes. The plasma formations are highly interdependent supplying each other with the particle flow and current closure necessary for the maintenance of the discharge. To diagnose these structures, space-resolved cold Langmuir probe measurements and optical emission spectroscopy investigations were performed in the axial direction allowing for the mapping of the axial profiles of plasma potential, electron temperature and density, ion density and optical emission. The existence of an accelerating double layer in the vicinity of the holes has been confirmed here, and in previous research (Teodorescu-Soare C T et al 2016 Phys. Scr. 91 034002; Schrittwieser R W et al 2017 Phys. Scr. 92 044001; Teodorescu-Soare C T et al 2019 Int. J. Mass Spectrom. 436 83). Besides the assessment of the relationship between discharge conditions and plasma parameters in the novel cathode system, the importance of a multiple concentric cathode discharge configuration is revealed for deposition applications

Plasma potential probes for hot plasmas: A review and some news

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

Sheaths and Double Layers with Instabilities

The properties of sheaths and associated potential structures and instabilities cover a broad field which even a review cannot cover everything. Thus, the focus will be on about a dozen examples, describe their observations and focus on the basic physical explanations for the effects, while further details are found in the references. Due to familiarity the review focuses mainly on the authors work but compared and referenced related work. The topics start with a high frequency oscillations near the electron plasma frequency. Low frequency instabilities also occur at the ion plasma frequency.The injection of ions into an electron-rich sheath widens the sheath and forms a double layer. Likewise, the injection of electrons into an ion rich sheath widens and establishes a double layer which occurs in free plasma injection into vacuum. The sheath widens and forms a double layer by ionization in an electron rich sheath. When particle fluxes in "fireballs" gets out of balance the double layer performs relaxation instabilities which has been studied extensively. Fireballs inside spherical electrodes create a new instability due to the transit time of trapped electrons. On cylindrical and spherical electrodes the electron rich sheath rotates in magnetized plasmas. Electrons rotate due to $\mathbf E \times \mathbf B_0$ which excites electron drift waves with azimuthal eigenmodes. Conversely a permanent magnetic dipole has been used as a negative electrode. The impact of energetic ions produces secondary electron emission, forming a ring of plasma around the magnetic equator. Such "magnetrons" are subject to various instabilities. Finally, the current to a positively biased electrode in a uniformly magnetized plasma is unstable to relaxation oscillations, which shows an example of global effects. The sheath at the electrode raises the potential in the flux tube of the electrode thereby creating a radial sheath which moves unmagnetized ions radially. The ion motion creates a density perturbation which affects the electrode current. If the electrode draws large currents the current disruptions create large inductive voltages on the electrode, which again produce double layers. This phenomenon has been seen in reconnection currents. Many examples of sheath properties will be explained. Although the focus is on the physics some examples of applications will be suggested such as neutral gas heating and accelerating, sputtering of plasma magnetrons and rf oscillators