For the first time, high amplitude (&Dgr;n/n≈ 40&percent;), high Q (up to 100,000) BGK modes have been controllably excited in a plasma. The modes are created by sweeping an excitation voltage downwards in frequency, thereby dragging a phase space “bucket” of low density into the bulk of the plasma velocity distribution. The modes have no linear limit, and differ markedly from plasma waves and Trivelpiece‐Gould modes. © 2003 American Institute of Physics