Perpendicular Drift Velocity Research Articles

Electron drift velocities in a moderate and in a strong crossed magnetic field†

From energy balance considerations in the presence and absence of a crossed magnetic field and without assuming an a priori constant electron collision frequency, an equivalent reduced electric Held concept is evolved which reduces to the more familiar equivalent pressure one when the electric field is kept constant. Using the concept and assuming a Maxwellian electron energy distribution, calculation of the electron transverse and perpendicular drift velocity and their ratio tan θ is extended to a moderate and strong magnetic field for hydrogen gas. The results are in good agreement with experimental data in the electron energy range where ionization by collision occurs. It is shown numerically that average electron collision frequency values, derived from the various transport coefficients, inherently vary widely. If the average electron collision frequency is derived from a given transport coefficient, then its value still depends on the magnitude of the reduced magnetic field even when the average ele...

Calculated magnetic electron drift velocities in H2, N2, O2, air and C2H6for 10

The influence of the ratio of magnetic field to gas pressure on the perpendicular and transverse drift velocity of an electron swarm is examined quantitatively for H2, N2, O2, air and C2H6 for mean energies from 0?1 to 10 ev. Whereas the perpendicular drift velocity reaches a peak value when the electron collision frequency is approximately equal to the cyclotron frequency, the transverse velocity starts to decline when this equality is reached. In contrast, the ratio of the perpendicular to transverse velocity, which is shown to be an important quantity governing gaseous discharge movement, continues to increase with B/p; the increase is almost linear throughout except for gases exhibiting the Townsend-Ramsauer effect. Expressions are derived for the low and high magnetic field case and values in the intermediate region, which lies in the range 10-3 B/p 10-1 Wb m-2 torr-1, are obtained numerically. The mobility values for the different gases at a fixed value of mean energy and B/p differ by up to a factor of 10, and this should allow discharge propagation velocities to be identified with particular movement mechanisms unless obscured by feedback processes. At low magnetic fields the perpendicular drift velocity is derived as a function only of B/p and E/p. If the equivalent pressure concept were used in the derivation of Amp?re's law, the magnitude of the force on a conductor in a crossed magnetic field would be higher than hitherto accepted. The relevance and application of the calculated magnetic transport data to practical discharges are mentioned briefly.

The influence of a crossed magnetic field on a gaseous Townsend discharge

The early treatment by Townsend of the influence of a crossed magnetic field on the movement of an electron swarm in a uniform electric field is extended to high electric fields when ionization occurs and secondary processes can no longer be neglected. It is shown that the discharge moves bodily in the ev×B direction with a velocity, when secondary electron emission by positive ion bombardment of the cathode takes place, given by v = vT+ (tan θ− + tan θ+)(τ+/τs + τ+) in which νT+ is the transverse ion drift velocity, θ− and θ+ are the electron and ion deflection angles and τ+ and τs are the ion transit and site times. The site time is the delay which occurs between the impact of a positive ion on to a surface and the subsequent emission of an electron; the transit time is the time it takes for the ion to cross the gap. When secondary electron emission by photon bombardment takes place, the propagation velocity of the gaseous discharge is given by v = vperp(v*/vT− + v*) in which ν* is the photon velocity and νperp- and νT- are the electron perpendicular and transverse drift velocity respectively. Because of the importance of θ-, the dependence of the perpendicular and transverse electron drift velocity on collision cross section, mean energy, E/p and B/p is examined in detail over a wide range and quantitative results obtained for hydrogen. Whereas νperp− reaches a maximum with increase in B/p when v0 approximately equals eB/mp, tan θ− continues to increase linearly with B/p from low to high magnetic fields. Good agreement is obtained with some but not all of the experimental data reported in the literature. The need for similar data on positive ions is pointed out.