  A new type of laser photodetachment (LPD) technique has been developed for
the measurement of electron sheath thickness around an electrostatic probe and
for the measurement of the length of collection region of photodetached
electrons (PDE). When a thickness of the sheath formed around an electrostatic
probe is thicker than about 0.1 mm, modification of the temporal evolution of
the LPD signal is observed. By making use of the modification, we evaluated
sheath thickness around a cylindrical probe in the existence of the magnetic
field of 15 mT. It was found that the thickness of electron sheath along the
magnetic field was comparable to the calculated plane-parallel Child-Langmuir
sheath thickness when the probe-bias voltage was high. Furthermore, by
inserting a small screening object in the laser beam channel, we can measure
the sheath thickness from the modification of LPD signal. From the variation of
the signal intensity as scanning the screening object perpendicular to the
laser beam channel, we can observe the collection region of photodetached
electrons, because this procedure changes the relative displacement between
shadow and the probe electrode.
