Plasma sheath potential

Figure 2. Schematic diagram of a solid surface exposed to a plasma. Typical values for important parameters are indicated. The symbols AK, and vi represent the sheath thickness, sheath potential, ion concentration, and ion velocity, respectively.

The most essential plasma device characteristics that are needed in order to obtain impurity release rates are the fluxes of photons and of charged and neutral particles to the wall. It will be necessary to have detailed information on the energy spectra and fluxes to walls, limiters and beam dumps of thermal electrons and ions, photons, a-particles, runaway electrons, charge exchange neutrals, neutral beam and impurity neutrals and ions. The effects of sheath potentials, secondary electron emission and unipolar arcing need to be included in these calculations. 

The principal method of introducing metal impurities into early pinch discharges was considered to be arcing. The externally applied voltages, although low, still permit the occurrence of unipolar arcing between the plasma and the wall when driven by the sheath potential. Local electron emission from a cathode spot is balanced by a uniform flow back to the surface of energetic electrons in the tail of the Maxwellian distribution. 

The sheath potential is minimized, when the electrode area ratio equals 1. The design of Fig. 3 fulfills this requirement. The lowest possible pressure, for which a stable plasma can be established, is 30 Pa. 

Secondly, the plasma potential will vary with time, depending on the cycle of the applied potential.22 If the ion can transit the sheath before the applied electric field reverses, it can experience the maximum sheath potential. As the frequency is raised, the ion cannot cross the sheath before the field reverses, so it experiences the average sheath potential which is approximately one-third the maximum. Therefore, ion bombardment will be less intense at the highest frequencies. 

Additionally, the incident impurity ions in a fusion device will be multiply charged, e.g., a charge state of 4 can be assumed for Be, C, O, and even higher values for W ions. This will result in increased acceleration of the ions in the sheath potential such that the most probable energies for multiply charged ions in a divertor plasma with Te = 10 eV will be around 200 eV, i.e., well above the threshold energy. 

The high energy efficiency (high plasma density and low sheath potentials)... 

Typical spatiotemporal profiles of potential are shown in Fig. 8 [26]. The left electrode is driven by a sinusoidal radio frequency voltage Frf = Fosin(a)t). The case shown is for an argon discharge with Fq = 100 V and u>/2n — 13.56 MHz. The potential distribution is such that the electrodes are bombarded by positive ions during the whole period of the RF cycle, while most electrons are trapped in the plasma. Only electrons with kinetic energy greater than the sheath potential can reach the walls. Electrons leak to the walls during a short time in the cycle when... 

The sheath potential (voltage) is the difference between the plasma potential and the electrode (or wall) potential. The potential of the sheath over the grounded electrode (right electrode in Fig. 8) is equal to the plasma potential. The potential of the sheath over the powered electrode (left electrode in Fig. 8) is the difference between the plasma potential and the electrode potential. The time-average potential of the powered electrode (with respect to ground) is called the DC self-bias or DC bias. Actually, the DC-bias is the difference of the time-average potential of the sheath... 

Application to capacitively-coupled reactors Figure 24a shows the electron temperature distribution in an argon discharge sustained in a one-dimensional parallel plate reactor of the kind shown in Fig. 7. The temperature peaks near the plasma-sheath interface, where the product of the current and electric field (Eq. 31) is highest, and steep gradients develop in that region. Electrons which diffuse towards the electrode during the sheath potential minimum (around r = 0.25 at left electrode, see also Fig. 

Ions in a bulk plasma can be accelerated with the electric potential of a plasma sheath near a solid surface so that they reach the siuface, quickly neutralize, and finally attach to the surface. It is experimentally difficult to discriminate this mechanism from free radical mechanism 5, the deposit of reactive species near the surface. 

Numerically, the Child law sheath can be of the order of 100 Debye lengths in conditions of typical low-pressure discharges applied for surface treatment. More details regarding sheaths, including collisional sheaths, sheaths in electronegative gases, radioftequency plasma sheaths, and pulsed potential sheaths can be found, in particular, in the book of Lieberman and Lichtenberg (1994). 

Rather than use either microwave (MW, 2.45 GHz) or radio frequency (RF, 13.56 MHz) power to sustain our plasma, we often combine the two power sources to generate a so-called "mixed" (or dual-) frequency plasma, as shown in a schematic view of the plasma tqrparatus. Fig. 3 [15]. While MW excitation generates a high concoitration of active i redes in the gas phase, as pointed out above, the role of the RF power is to create a negative DC self-bias voltage Vg on the powered, electrically isolated substrate holder electrode. This causes ions to be accelerated 1 the potential drop (Vp-Vg) across the RF-induced plasma sheath, to their maximum kinetic energy... 

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