Cold Kilo-Electron Ball as Probe for Charge-Proportional Cyclotron Frequency Shift in Penning Trap Cavity

We report the realization of a cold (4 K), dense spherical droplet of one-component plasma made up of about 1000 electrons. This kilo- $e$ droplet experiences a 1000-fold enlarged shift, ${\delta }_{\mathrm{ball}}^{*}$, of the center-of-mass cyclotron frequency due to interaction with ${\mathrm{TE}}_{1mn}$ and ${\mathrm{TM}}_{1mn}$ ( $n$ odd) modes near 164 GHz in our $\approx 1\mathrm{cm}$ diam Penning trap cavity. For the first time we were able to search for this shift and obtain ${\delta }_{\mathrm{ball}}^{*}\approx -5\mathrm{}\mathrm{kHz}$ for 1000 electrons, by comparing data for balls of $\approx 100$ and $\approx 1000$ electrons. A drastic decrease of $|{\delta }_{\mathrm{ball}}^{*}|$, as in 4 h a $1500e$ drop flattened, spread in radius from 140 to 390 μm, and partly decoupled from the cavity modes responsible for the shift, yielded supporting evidence. The kilo- $e$ ball's small initial radius and slow spreading may make it useful in other applications.