P. R. Smy

Ion Current from a Collision‐Dominated Flowing Plasma to a Cylindrical Electrode Surrounded by a Thin Sheath

Experimental results for the ion current to a cylindrical electrode in a flowing continuum plasma are compared with the currents calculated from a convection‐dependent thin‐sheath theory obtained by extending the theory of Lam to cover convection rather than diffusion‐generated currents. The relation derived with this theory Ii≈5.3e01/4e3/4μi1/4rp1/4vf3/4ne3/4V1/2l,  (mks) where e0 is the permittivity of free space, e is the electronic charge, μi is the ion mobility, rp is the probe radius, vf is the plasma/probe velocity, ne is the ionization density, V is the probe bias, and l is the probe length, is found, on the average, to predict the experimental currents to within ±30% over ranges of 0.1–5 mm in probe radius, 5×102−4×103 cm/sec in probe/plasma velocity, 5×1016−2×1018/m3 ionization density, and 10–100 V in probe bias. Further support for a convection‐dependent thin‐sheath model is provided by two experimental observations: (1) The current is observed to vary as V1/2 as predicted by the convection‐de...

Electrostatic‐Probe Studies in a Flame Plasma

Simple theoretical considerations have yielded an approximate formula for the ion current per unit length Ii to a cylindrical probe (radius rp, bias voltage V) immersed in a collision‐dominated moving plasma (subsonic velocity=vf, electron density=ne, ion mobility=μi). Ii=2(πμie0)1/3(neevfV)2/3[log(Ii/2neevfrp)]2/3 (mks) where e is the electronic charge. This relation has been checked with a propane/air flame and a moving probe; it is found to be correct within ±50% over several decades of probe voltage, probe‐flame velocity and flame ionization. A more favorable choice of μi in the above expression improves the agreement between theory and experiment to ±30%. Measurements of sheath thickness obtained from the mutual interference of two adjacent probes support the basic tenets of the theoretical model. It is concluded that, in many instances of probe measurements in flames and other collision‐dominated plasmas, the velocity‐dependent approach adopted in this paper will be necessary since very large errors...

The variation of ionization with air/fuel ratio for a spark-ignition engine

A negatively biased flush‐mounted planar probe operated both in a pulsed and dc mode has been used to measure the ionization density in the plasma generated by combustion in a single‐cylinder spark‐ignition engine. The ionization was measured as a function of the air/fuel ratio and was found to exhibit a broad peak (Q?2) whose maximum occurs slightly to the rich side of stoichiometric. The peak ionization was calculated from two slightly different models for ion collection to be ?1019/m3. Values of this magnitude appear to be more reasonable than the very high value of 4×1021/m3 deduced by Rado using a similar experimental setup. Finally, it is proposed that this type of measurement may be useful to monitor engine performance.

Efficiency and erosion characteristics of plasma jet igniters

The rate of erosion of both ceramic and metal in a typical plasma jet igniter has been monitored as a function of various experimental parameters. Even under the most favorable conditions it appears that working lifetimes may be limited to the order of ten hours-a figure in line with previous measurements of arc erosion. Direct measurements of the thermal energy delivered by the igniter to the adjacent gas reveal that overall efficiencies approximately 10% are attained. In order to minimise erosion while maintaining a given delivery of thermal energy it is found essential to work with low initial voltages on the energy storage capacitor.

An experimental investigation of the effect of microwave radiation on a propane-air flame

Abstract The electron temperature, ionization density and flame speed were measured for a propane-air flame which was immersed in a microwave field at frequency 2.5 GHz and maximum field strength of 1.9 × 104 V/m. Although the electron temperature in the flame front was increased by 55% and that in the downstream burnt gases by over 100% there was no increase in either the ionization density or flame speed to within the accuracy of the measurements (a few percent, depending on the technique in question). The electron temperature was measured for a near stoichiometric flame at field strengths approximately one-third of the maximum value, while the flame speeds were measured at maximum field strengths and for flames as lean as φ = 0.69.

High‐pressure Langmuir probe in a weak flowing plasma or a plasma sheath

A theory is developed for the current to a negatively biased probe in a moving weakly ionized plasma or a stationary ion sheath. It is found that at low ionization densities the current is no longer space‐charge limited but is, in fact, limited by the electric field generated by the probe. For a cylindrical probe the current per unit length I=2πμn0eV[log(I/n0evfrp)−1/2] −1, where μ is the mobility of the ion, e is the electronic charge, V is the probe potential (negative), I is the probe current, vf is the plasma flow velocity, n0 is the plasma ionization density, and rp is the probe radius. For a spherical probe the current I=4πμn0erpV. Measurements with a cylindrical probe at densities down to 1012/m3 show good agreement with theory.

Radio‐Frequency Floating Double Probe as a Plasma Diagnostic

A floating double probe operating at radio frequencies in the MHz range has been developed to the point where measurements of sheath capacitance and resistance yield values for plasma ionization density and electron temperature with an accuracy of ≈8% and ≈16%, respectively. The experimental technique used supersedes previous methods in that it enables sheath capacities as low as 10−2 pF to be measured. The analysis of the results has been extended to cover the important nonplanar (cylindrical) sheath case. As a result of these two developments, the technique is now applicable to small diameter probes which are essential for measurements in most gas discharge plasmas. The performance of the probes has been checked over a range of plasma densities and working frequencies and has been found to give results which are in good agreement with microwave cavity and dc probe measurements. This type of measurement has various advantages over the usual dc probe methods in terms of complete isolation, but it is expec...

Experimental investigation of the low‐frequency capacitive response of a plasma sheath

Several theoretical models are examined for the low‐frequency capacitive response of the ion sheath surrounding both planar and cylindrical Langmuir probes in a collisionless plasma. It is found that a recent theoretical approach by Shkarofsky, based on the total probe surface charge derived from Gausss law, predicts values for both the low‐frequency, capacitance and the transient charge flow which are as much as an order of magnitude smaller than that predicted based on the electron current flow caused by the motion of the sheath edge, e.g., Kamke and Rose. This discrepancy is attributed to the neglect by Shkarofsky of the transient ion conduction current through the sheath. Additionally, a recent theory by Rosa for the complex impedance of a planar sheath is seen, in the limit of low frequency, to be in agreement with that predicted following Kamke and Rose. Experimental measurements have been made, for both pulsed and rf‐driven probes, of the transient charge flow to the probe and the effective low‐fr...

Chemical activity and transport processes in the vicinity of a plasma jet igniter

Abstract Because of the well-known enhancement of combustion observed with plasma jet ignition, various measurements have been carried out upon the subsonic puff of highly turbulent gas ejected from a pulsed plasma jet igniter. “Spot” measurements of temperature show that the puff propagates with a sharp temperature front whose thickness is ∼ 10 −1 mm. Measurements of the flow velocity within the puff reveal that it is approximately equal to the front velocity of the puff and is thus capable of propagating both heat and particles from the igniter to the front of the puff. Finally measurements of the nitric oxide generated at various distances from the igniter when the plug is filled with N 2 or O 2 and is discharged into a vessel containing the complementary gas show that atomic nitrogen or oxygen is generated in amounts greatly in excess of equilibrium dissociation in a uniformly heated plasma. The production of NO extends several centimeters away from the igniter, where, in view of the low measured gas temperature, it can only be ascribed to the reaction between atomic nitrogen and molecular oxygen (or vice versa). Since such atoms are only created in the plasma within and close to the igniter, we conclude that they survive for appreciable times and distances and that radicals as well as heat are propagated by the internal flow processes.

Plasma Diagnostics Using CO2 Laser Absorption and Interferometry

Absorption of CO2 laser radiation is experimentally shown to be a useful plasma‐diagnostic technique, spatially, temporally, and quantitatively. Electron densities determined from absorption agree well with shock‐produced plasma values. Comparison of densities obtained via absorption, interferometry, and Langmuir probes in a plasma jet also show good agreement in magnitude and spatial dependence.