Electron impact ionization of cold atoms for the electron truncated Maxwellian distribution was studied. The floating potential V_f of a plate immersed in plasma is determined by the condition of equal fluxes of electrons and ions, e V_f / T_e = 0.5 ln ( 2 π Z_i m_e / m_i ) for cold plasma ions, where Z_i , m_e and m_i denote the atomic number and the masses of plasma electron and ion, respectively, and T_e is the electron temperature. The lighter electrons make the plate potential negative and heavier ions form the deeper floating potential. As the high energy electrons, which overcome the negative wall potential, are absorbed to the plate, the velocity distribution of repelled electrons is truncated. The floating potential V_f corresponds to the truncation energy of repelled electrons. By using this truncated velocity distribution function the rate coefficients of electron impact ionization are calculated according to semi-empirical formulae, e.g. the Lotz formula [1]. The effect of truncation is quite small for the higher electron temperature, e.g. for T_e = 10 eV the ratios are 0.97, 0.93, and 0.99 for hydrogen, helium, and argon plasma, respectively. Here Z_i = 1, 2, 3, which are the most probable charge states for T_e = 10 eV. The lower electron temperature affects appreciably the rate coefficients, e.g. for T_e = 1 eV and Z_i = 1 the ratios reduce to 0.72, 0.68, and 0.76, respectively. In a detached divertor plasma, where the electron temperature is as low as few eV, the effect of truncation might be important for the electron impact ionization.

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