It has been possible to observe the occurrence of diamagnetism in active discharges by measuring shifts in spin absorption resonance frequencies of foreign substances located near the plasma. Narrow spin electronic resonances in DPPH have been used to show that the magnetization of a gaseous discharge in low-pressure mercury vapor increases linearly with applied magnetic field up to about 40 gauss, where maximization occurs. Observation of the phase change in the Larmor precession of protons in a strong homogeneous field showed that the diamagnetism in a modified Penning ionization gauge (P.I.G.) discharge increased linearly with power input and decreased approximately as $\frac{1}{H}$. Irradiation of the plasma with high-power micro-waves with frequency near the free-electron gyrofrequency resulted in an increase of diamagnetism of the discharge. The discharge magnetic moment ranged from -0.001 to -0.32 erg/${\mathrm{cm}}^3$-gauss, depending on experimental conditions. The sensitivity of the nuclear resonance technique is one part in ${10}^6$, and it permits observation of diamagnetism to be deferred until the discharge is over. The theory of plasma diamagnetism is summarized.

• Received 15 December 1960

DOI:https://doi.org/10.1103/PhysRev.122.367