The nonlinear response of a plasma subject to an intense, spatially localized rf field has been investigated experimentally. In a uniform quiescent magnetoplasma of densityne≳109cm−3, temperaturekTe≃5kTi≃1 eV, and magnetic fieldB0∼100 G, a resonance cone (&ohgr;/&ohgr;c≃0.1; where &ohgr;cis the electron cyclotron frequency) is established which converges away from a circular exciter to reach a maximum amplitude at the remote cone apex. When an rf burst of intensity &egr;0Erf2/nekTe≲O(1) is applied, a strong density depression (&dgr;ne/ne≃40%) is formed in the focal region on a time scale (∼1 &mgr;sec) short compared with a collision time. Fast ion bursts [(1/2)mvi2≃35 eV≳100kTi], large amplitude ion acoustic waves, but negligible electron temperature increases are observed. Density and resonance cone field interact nonlinearly and in time develop a state of turbulence. Ion heating (&Dgr;Ti/Ti≃100) dominates over electron heating (&Dgr;Te/Te∼2). The direct measurements of the particle distributions with probes and energy analyzers are supplemented by test wave techniques. Small amplitude ion acoustic, electron plasma, and Bernstein waves are propagated through the region of high rf intensity and the perturbed plasma properties are deduced from the test wave behavior. The observed nonlinear collisionless interactions between particles and intense rf field are qualitatively explained by a model involving the ponderomotive force on electrons and acceleration of ions by space charge fields.