Electric field spectra beyond the strong turbulence regime of relativistic beam-plasma interactions

Using a Stark-effect diagnostic, we have explored a new regime of relativistic beam-plasma turbulence, 〈${\mathrm{E}}^{2}$〉>${\mathrm{nT}}_{\mathrm{e}}$. Measurements of E and of the plasma frequency used nonperturbing optical techniques during the electron-beam pulse. Fields up to 150 kV/cm display an exponential spectrum, exp(-${\mathrm{E}}^{2}$/${\mathrm{E}}_{0}^{2}$) with ${\mathrm{E}}_{0}$=85 kV/cm. This spectrum does not resemble that given by strong-turbulence theory, and involves stronger fields than are dealt with in current theory. '