Anomalous penetration of high-frequency electromagnetic waves into a plasma

Measurements of anomalously large effective penetration depths of high-frequency (ω≈ωce) electromagnetic waves in a plasma are reported. The penetration depth increases from 7 to 20 times the cold plasma value as the average incident power level is increased from 3 to 10 W/cm2. In this regime the electromagnetic wave energy is found to be concentrated in an annular region in the plasma.