The ferromagnetic targets for the Polarized Electron Target are
produced at the base of Cu foils with thickness 30 um and dimensions 30x70
mm. On the foils middle along the bigger dimension the Fe alloys are covered
with the thickness 10 um and the width 10 mm. The iron purity is very high
(99.9 %) and was tested with using the laser mass-spectrometer. The iron
foils are produced in high vacuum device by a plasma-spark method. The
foil thickness was determined by careful weighting of every Cu support
foils and resulting Cu-Fe bimetallic foils. For some samples Cu alloy was
removed by etching from Fe-foil surface on whole foil long. The bimetallic
foils were carefully annealed in vacuum at the 800 deg C temperature during
nearly 10 hours. After magnetization testing the copper alloy was removed
by etching from iron surface near beam point area. The copper-free surface
has the ellipse form with axises dimensions 6 mm and 20 mm across and along
foil piece accordingly. Again, when a foil is inclined at 20 deg the copper-free
zone is seen from beam direction as circle with 6 mm diameter. The bimetallic
design of ferromagnetic foil has some advantages as compared with usual
foils. Those foils more easy in operation, its allow to use pick-up coils
with smaller dimensions. The using additional support material with good
thermoconductivity (as Cu) and liquid nitrogen cooling provides the decreasing
of thermal heating and therefore the demagnetization effect and the operation
with the beam current up to 30 uA for the beam dimension 0.5 mm diameter.
This spot dimension is produced with using defocusing operation mode of
the CEBAF accelerator with the beam spot intrinsically 0.1x0.1 mmxmm and
also prevent extra heating Fe-foil. Additionally calculations were made
to determine the target thermal conditions and demagnetization effect.
As result, the target beam spot temperature would not exceed the room temperature
more than 30 deg C and the demagnetization effects are negligible for above
mentioned conditions. For foil polarization the method of an easy magnetization
in foil plane is used. Typically, on-beam position the foils will be inclined
at the 20 deg angles relatively beam direction. As result the longitudinal
component of the target polarization will be equal 0.94 of the total polarization
value. The iron foils are polarized in the magnetic field of a set of Helmholtz
Coils, mounted on input and output chamber tubes with field direction in
parallel to a beam axis. Every of coils has 2000 turns of copper wire 1.2
mm diameter and is divided on two sections with the same ohmical resistances
in 8 Ohm. All four sections are connected in parallel with result 2 Ohm
ohmical resistance. With using 10 A total supply current the Helmholtz
magnet provides the magnetization force up to 300 Oersted at the chamber
center. For these conditions the iron foils are almost saturated and the
of magnetic field variation along the 7 cm foil length can be negligible.
For magnetization measurements two pick-up coils 3 mm long, 1.1 cm width
are used. The pick-up coils were placed on the foil ends at the 8 mm distance
from foil edges. Every coil has 100 turns of a copper wire 0.04 mm diameter.
