R. M. Clements

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,u2009u2009(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...

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.

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.

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.

An experimental investigation of the effect of a plasma jet on a freely expanding methane-air flame

Abstract Pulsed plasma jet igniters have been shown to be effective in enhancing the flame speed of combustible mixtures of gases. The mechanism by which this enhancement is achieved is somewhat unclear, although the consensus of recent investigations is that the possible mechanism involves the inherent generation by the jet of uv radiation, radicals turbulance, or some combination of the above. This paper describes a series of measurements of flame speed in which ignition and velocity enhancement processes are separate and independent. With such a configuration it is possible to demonstrate that an enhancement in flame speed is achieved in the vicinity of the jet and that this enhancement is not necessarily related to some jet-related perturbation of the ignition process. For the strengths of mixtures investigated (approximately stoichiometric to φ = 0.67) turbulence generated by the plasma jet igniter is shown to dominate both ignition and flame propagation.

Langmuir probe measurement of electron temperature in high-pressure plasmas

The effect upon measurements of electron temperature in high‐pressure plasmas of the cooling of the plasma electrons by the inherently cold layer of gas surrounding the probe has been investigated. Theoretical considerations indicate that while cooling effects can be substantial, accurate electron temperature measurements can be obtained if the probe is sufficiently small or is moved through the plasma at sufficient velocity. These conclusions are at variance with the calculations of some other workers but are in good agreement with experimental results reported in this paper.

Low Temperature Starting of Diesel Engines Using Timed Spark Discharge

A timed high energy spark discharge system was used as an aid for low temperature starting of two single cylinder, and one multi-cylinder diesel engines. The tests were conducted by cold soaking the engines in a low temperature chamber in temperatures down to -55 deg C. An Arctic (AA) diesel fuel was used at these low temperatures while B(No. 2) fuel was used at warmer temperatures. When used in I.D.I. engines the timed spark discharge system produced more rapid starts and faster warm ups at lower temperatures and consumed equal or less electrical energy when compared to the factory fitted electric glow plugs. An in cylinder glow plug, which was fitted to a D.I. engine, but is not a factory option, produced more rapid starts than the timed spark discharge, but the timed spark system produced smoother running and consumed only one-half the electrical energy.

Resonance behavior of the ion‐sheath capacitance near the plasma ion frequency

A slight minimum in the measured rf capacitance of the ion sheath surrounding negatively biased planar and cylindrical Langmuir probes in a collisionless plasma has been observed. The position of the minimum, which is near the plasma ion frequency ωi, and which closely agrees with that predicted on the basis of simple physical arguments, is studied both as a function of plasma density and probe potential. The experimental results, for the planar probe, are compared with a recent theory by Rosa. The plasma was formed in the positive column of a helium dc discharge at a pressure of 0.3–0.5 Torr, with plasma electron densities and temperatures ranging between 1015 and 1016 m−3 and between 5×105 and 7×105°K, respectively. Measurements of the sheath capacitance were made using a tunnel‐diode oscillator circuit whose frequency could be adjusted approximately an order of magnitude above or below ωi.