Dynamic Behavior of the Thermomagnetic Gas Torque

Abstract

The recently discovered thermomagnetic gas torque shows twofold behavior in the presence of a large modulating magnetic field collinear with the steady magnetic field. At steady fields of about one-half $H_0$ (the field for maximum torque), the torque $N$ is decreased by an amount $\Delta N$ if the modulation frequency $\nu$ is low compared to a collision frequency. As $\nu$ is increased, the torque returns to the value it had in the absence of modulation. From plots of $\Delta N$ versus $\nu$, we have determined correlation frequencies ${\nu }_c$ for the collision processes. We find that, for high-enough pressure $P$, ${\nu }_c$ varies linearly with $P$ for NO, ${\mathrm{O}}_2$, ${\mathrm{N}}_2$, and HD gases. However, it appears that although elastic collisions are the controlling process in the gas torque for NO, ${\mathrm{O}}_2$, and ${\mathrm{N}}_2$, inelastic collisions determine the gas-torque behavior for HD (at least for the low-field torque peak). At steady fields greater than $2H_0$, a precession-related phenomenon shows up as dispersionlike curves in plots of $\Delta N$ versus $\nu$. For NO, such curves are observable at the molecular magnetic Larmor frequency and twice the Larmor frequency. From double-frequency curves, we obtain $g_J=-49.3\mathrm{}{\mu }_N$, in good agreement with Ramsey's value of -48.2. Although the dispersion curves cannot be resolved, precession-related phenomena are observable for ${\mathrm{O}}_2$, and HD. We can estimate that the $g_J$ values effective in the gas-torque phenomena are ∼ -10 and +1 ${\mu }_N$ for ${\mathrm{O}}_2$ and HD. The value for ${\mathrm{O}}_2$ agrees quite well with the average expected for $M_s=0\mathrm{}$ molecules in fast rotational states ($K\ge 9$). The value for HD is in order-of-magnitude agreement with measurements of Ramsey.