The Polarized Electron Target design is shown on Picture 1.
The target chamber is produced of a stainless steel tube 6" OD.
There are input and output tubes with Helmholtz Coils and flanges for
connection to the beamline, left and right flanges for
mounting an internal target structure with a linear moving mechanism.
The left (looking down to the beamdump) chamber flange has a port to connect with the linear moving mechanism.
Bellows are used for connection to the beamline vacuum system.
The target chamber is installed on a lifting mechanism.
The target structure is installed inside of target chamber.
It consists of a slide and a target frame with installed five ferromagnetic
foils as the targets (a scheme is here).
The target frame is easily movable along the slide, perpendicularly to the beam.
The targets are installed on the frame with a distance of ~89mm between its centers.
Moving and positioning the targets in the beam is provided by the linear moving mechanism.
The target position on the beam is determined by a switch for every of five targets and also by 2
limit switches. Additionally, a position transducer is used as the encoder for precise position
measurement. There is no obstructed part in the target frame (except the targets themselves -
ferromagnetic foils 10-30um thickness) that could cross the beam.
The Professional Rack and Pinion(PRP)
(picture) from
UHV Design is used as the linear moving mechanism.
It provides 440mm travel range from left limit switch to the right limit switch, and
it is mounted on a CF64 (4-1/2\x{201D} OD) flange, with straddled-through holes. The device
incorporates a precision ground stainless steel rail, combined with a stainless steel rack
to support the load. The rack in turn is driven by a pinion coupled to an MD25 MagicDrive.
This design provides an extremely stiff, smooth, precise and reliable solution for linear travel
The BiSlide assembly from
Velmex,Inc. with a stepper motor is used as
the lifting mechanism. The plane of BiSlide assembly is inclined at about 20 deg.
that allows to move the target chamber in forward-up or backward-down directions in a range of
+/-30mm along of slide plane.
The position is determined by limit switches, installed on the BiSlide assembly and also
by a position transducer , used as the encoder - see the specs.
There are three limit switches located at the
following positions:
- Center (0mm)- the beam is at the target center,
- Top Limit (+30mm) - the beam is below the target center by 30*sin(20)~10.3mm,
- Bottom Limit (-30mm) - the beam is above the target center by 10.3mm
The lifting and linear motion systems use the Step-Motors that operated in
microstepping mode. For remote operation, a VME Crate with Controller
Model VME44
and motor drivers Model MD125
from Oregon Micro Systems, Inc.. are used.
Mechanical stoppers prevent the target structure from moving beyond the safe limits.
All the hardware along with the VME-crate and Controller are placed inside Hall A, close
to the location of the power supplies for the Möller quadrupole magnets.
Target Frame Parameters
- The distance between the center of the targets is: 3.5"
- The distance between the edges of the targets is: 2.3"
- The distance between the edge of a target foil and the edge of the frame is: 2.7"
- The target foil width is : 1.2"
- The target foil length is : 5.5"
- The window for the beam in ether right or left limit position(WxH) is: 2.7"x??
Linear Moving System
The stepper motor and the limit switches are connected to T-Channel of the motor controller.
Basic Parameters:
- number of positions : 7,
- 2 limit positions (no target in the beam):"Left" and "Right" (looking toward the beamdump);
- 5 positions (a target in the beam): "Target-1","Target-2","Target-3","Target-4","Target-5"
(numbering from "Left limit" to the "Right limit").
- Each of the 7 positions is determined by switches that are connected to the stepper motor
controller and by the encoder (position transducer)- see the specs.
Output signal from encoder is a positive
voltage potential in a range of 0-4V.
- Range of motion from the "Left Limit Switch" to the "Right Limit Switch" is: 440 mm
- Travel distance per one stepper motor revolution is: 1.2 mm/revolution
- The number of steps per revolution is: 200
- In microstepping mode there is 10 microsteps/step
- The movement speed of the target frame is:~ 8-10 mm/sec
Linear Limit Switches (see Table 1):
- All the limits switches are "Normal Open" (gives logical(TTL) "1" to the controller)
when they are not pressed.
When the limit switch is pressed (a target in position or in one of limit position) it is closed
and the logical "0" appears on the input of the controller.
- Left Limit switch is connected to the "Extended Limit Switch" of the T-Channel
(pin 23C of backplane VME44 connector)
- Right Limit switch is connected to the "Retracted Limit Switch" of the T-Channel
(pin 24A of backplane VME44 connector)
- All the targets switches are connected in parallel and look like one switch, which is connected
to the "Home Limit Switch" of the T-Channel (pin 24C of backplane VME44 connector).
In order to determine which target is in the beam, the software should count the target switch
crossings.
To enable the stepper motors operations the "T Auxiliary Output"(22C) should be set to logical "0"
Lifting Moving System
The stepper motor and the limit switches of this system are connected to
Y-Channel of the motor controller
The basic parameters are:
- The number of positions: 3,
- Center (0mm)- center of the beam,
- Top (+30mm) - above the beam axis by~ 10.3 mm,
- Bottom (-30mm) - below the beam axis on 10.3 mm.
Each of this positions is determined by a switch, connected to the stepper motor controller,
and by the encoder (transducer with resistance ~5 kOhm). Output signal from the encoder is a positive
voltage potential in a range of 0-4V.
- The travel range from Bottom Limit Switch to Top Limit Switch is: 60 mm
- The travel distance per one stepper motor revolution is: 2 mm/revolution
- The number of steps of the stepper motor per revolution is: 200
- In microstepping mode there are 10 microsteps/step
- The motion speed of the target structure is: ~ 1 mm/sec
Lifting Limit Switches (see Table 1):
- The limit switch is "Normal Open" (gives logical(TTL) "1" to the controller)
when it is not pressed. When pressed, it is closed and the logical "0" appears on input
of the controller.
- Top Limit Switch is connected to the "Extended Limit Switch" of the Y-Channel
(pin 15C of backplane VME44 connector)
- Bottom Limit Switch is connected to the "Retracted Limit Switch" of the Y-Channel
(pin 16A of backplane VME44 connector)
- Center Limit Switch is connected to the "Home Limit Switch" of the Y-Channel
(pin 16C of backplane VME44 connector)
In order to enable the stepper motor operations a "Y Auxiliary Output"(14C) should be set to logical "0"
Signals to FSD System
- There is no position of the target frame that could put a thick piece of material
into the beam.
- We suggest two modes of FSD operations:
- Normal mode FSD signal is generated if the target frame is not in a "Hole"
position (any linear limit) or not at the "Center" (lifting center limit switch).
- Moller mode The Moller FSD signal is masked. This mode should be used
only during Moller measurements. The target can be moved in both projections
with the beam turned on.
Software Requirements
- Initialization (software startup after an ioc reboot)
- Move the linear steppermotor to the retracted limit switch ("Right Limit" of target frame)
and set this as the linear HOME position(hole).
- Move the lifting steppermotor to the home limit switch ("Center") and set
this as the lifting HOME position.
- Normal Operation Requirements:
- Move to Target 1-5 : a routine that moves the requested target in the beam.
- Move to Hole Position (a linear limit): a routine that moves the target
frame "hole" in the beam
- Linear Jog: a routine that allows the user to jog the linear stepper motor +/- 15 mm
- Lifting Jog: a routine that allows the user to jog the lifting stepper motor in
the full travel range +/- 30 mm
- Software interlocks: there are no interlocks for movement of the target frame in any direction.
