Measurement of the Coulomb energy loss by fast protons in a plasma target

We report measurements of the energy loss by 1-MeV protons in a plasma target created by electric discharge in the hydrogen gas. Combined with independent measurements of the mean ionization degree and the electron areal density of the plasma column, based on two-wavelength laser interferometry, the energy-loss data let us infer the value of the Coulomb logarithm: ${\mathit{L}}_{\mathit{f}\mathit{e}}$=14.9±2.8 for the stopping power of the free plasma electrons. This value is 3.1±0.6 times higher than that given by the Bethe formula for the neutral hydrogen and, within the experimental errors, agrees with the theoretical predictions. © 1996 The American Physical Society.