  We have developed a Thomson scattering measurement system for the cascade arc
discharge device designed for the plasma window (PW) application study. The PW
is one of the plasma application techniques that sustain the steep pressure
gradient between high pressure (10-100 kPa) and a vacuum environment due to the
thermal energy of the plasma. Since the plasma thermal energy is the essential
parameter for the pressure separation capability of PW, we installed the
Thomson scattering measurement system to observe the electron density and
temperature within the anode and cathode of the PW for the detailed analysis of
the pressure separation capability. The frequency-doubled Nd:YAG laser (532 nm,
200 mJ, 8 ns) was employed for the probe laser. The scattered light was fed to
the triple grating spectrometer. The notch filter between the first and second
grating eliminated the stray light, realizing a sufficiently high
signal-to-noise ratio. The Thomson scattering measurement system successfully
obtained the electron density and temperature of the cascade arc plasma at 20
mm downstream from the tip of the cathode. The installed system successfully
obtained the Thomson scattering spectrum and showed that the electron density
increased from $2\times10^{19} {\rm m}^{-3}$ to $7\times10^{19} {\rm m}^{-3}$
with the discharge power, while the electron temperature was almost constant at
about 2 eV. The obtained data successfully contributed to the study of the
pressure separation capability of the PW.
