Schematically Polarized Electron Target design is shown on Figure
1. The target chamber (1) is produced from the stainless steel tube 6"
outer diameter.There are input and output tubes with Helmholtz Coils
(2) and flanges for connection to beamline, left and right flanges for
the mounting of a target structure and of a moving mechanism and top side
viewing port. The left chamber flange has a port to connect with linear moving mechanism.
Two bellows blocks are used for connection to beamline vacuum system.
These bellows and support structure positioning devices provide the
mechanical unbinding between the target chamber and beamline vacuum system
and get the possibility to place the target axises relatively the horizontal
plane and beam axis with tolerances better than 0.2 mm in position and 2 mrad in angular inclination.
The target chamber is installed on lifting mechanismt using two support structures.
The plane of lifting mechanism is inclined about on 20 deg.that allows to move target
chamber in up-forward and down-backward directions in range +/-3cm.
It provides three fixed positions: 1)Center, 2) Up, 3) Down
The moving mechanism
with two concentric cylinders, mounted on top and bottom bellows block
flanges, and with a lead screw and a gear is moved with a step-motor drive.
It provides the target elevation under the additional atmosphere pressure
near 60 kG with 60 sec time for full travel range. Two tubes for cryogenic
cooling of the target structure and vacuum electrical feed-throught port
for the let passing of the pick-up coils and thrmoresistors signals are
mounted on the top chamber flange. The cryogenic cooling of the target
structure is provided with using liquid nitrogen, supplied inside vacuum
chamber through stainless steel tubes to the copper dewar (4) with near
0.1 litre capacity. The cooling from the dewar is transfered to copper
drum of the target structure with using of the flexible copper cable 10
cm long and 7 mm diameter. The cooling support structure is made from long
and thin stainless steel tubes that provide good thermal isolation from
room temperature with thermal flux less than 5 Watts. The target support
structure (5) is made as two drums placed at the vacuum chamber axis one
inside another. The external stainless steel drum is connected with using
the thin wall stainless steel tube to a top chamber flange. The drum bottom
is connected to the same kind tube that is centered with using two rollers
placed on the corrector device. The corrector device, mounted on the bottom
chamber flange, is able to correct the target structure position across
the vertical axis at the 2.0 mm range. At the top and the bottom of the
stainless steel drum there are bronze bearings, that fix the internal drum
axises. The internal drum and targets frames are produced from copper.
The drum design and its wall thickness provide the high thermal conductivity.
The cylindrical wall has the holes and windows, that are used for free
passing of the electron beam with escaping the structure material. The
beam hole has 25 mm diameter. The target frames are connected to top and
to bottom lids of the copper drum. The each frame is produced from two
copper plates with thicknesses 2 mm. The target frame hole, that is covered
with foil-target, has the elliptical form with 20 mm and 60 mm as ellipse
axises. The target frame hole and drum wall window have the minimum dimension
of the 20 mm diameter in a projection onto the beam perpendicular plane
with a targets on beam axis and inclination angles 20 deg and 160 deg.
Between these angular positions the passing target windows are wider up
to 60 mm with foil in perpendicular to beam plane position. The low shaft
of the copper drum is connected to a thin stainless steel tube. The target
rotary motion is provided with using of the movable connection this tube
and the shaft of the rotary motion vacuum feed-through mounted on the outside
of the bottom chamber flange. A step-motor with incremental encoder is
used as rotary mechanism driver (6). In the zero position of rotary motion
system the beam hole or one of two targets, inclined at 20 deg relatively
to a vertical plane and beam direction, can be placed at the beam axis.
The rotary motion mechanism provides the rotation of the drum with targets
at the 140 deg relatively to zero position what means the Top and Bottom
Targets inclination ranges from +20 deg to +160 deg and from -20 deg to
-160 deg, accordingly. The target rotation provides the additional possibilities
for a false asymmetry measurements, allows to use each of two Fe-foils
for the longitudinal or transversal beam polarization measurements, etc.
The copper drum and targets cooling is provided with using above-mentioned
flexible copper cable. The cable is soldered to the upper shaft of the
copper drum and is connected to dewar copper wall. The connection thermal
conductivity provides the temperature drop less than 10 K relatively liquid
nitrogen temperature for dissipated beam power up to 1 Watt. This value
corresponds to using of the 30 uA beam current, the foil thickness 10 um
and the inclination angle 20 deg. Through the hole in the upper copper
drum shaft the signal wires from pick-up coils and thermoresistors are
passing. Then these wires pass inside the upper support tube and through
a hole are directed to the electric feed-through. The lifting and rotary
motion systems use the Step-Motors in microstep mode. For remote operation
a VME Crate with Controller Model VME44 and drives Model MD10 and MD10A
of Oregon Micro System Corp are used. The lifting system has the Limit
Top, the Home (Zero) and the Limit Bottom position microswitches for positioning
target structure on beam axis. The additional switches outside the allowed
travel range: the Kill Top and the Kill Bottom will provide turn off the
Step-Motor Drive and prevent mechanical system from any destruction in
the emergency situation. There are the mechanical restrictions for the
vertical travel range. The rotary motion Step-Motor with an incremental
encoder allows to control the rotary angle with the 0.18 deg accuracy.
The system has the Limit Right, the Home (Zero) and the Limit Left position
microswitches. Also the Kill Right and the Kill Left switches and the mechanical
restrictions prevent the target structure from multiturn operation and
destruction in emergency situation. All hardware blocks with VME-crate
and Controller are placed at Hall A near Möller Polarimeter and operate
in remote control with using operator screen from Machine Control Center.
