A method of using CO2 laser interferometer is described for the density measurement of intermediate region (the product of denstity n and plasma diameter d is around 1015 cm-2). Michelson type interferometer is adopted and one-tenth of fringe corresponds n x d≅1.05 x 1015 cm-2. Ge-Cu photoconductor cooled by liquid helium is used as a dector with fast time reponse. These are applied for the density measurement of plasma of BSG device8)9) and it is confirmed that this method is reliable and easy technique for the purpose.
