E-201 fiber alignment
Summary:
The fibers sit on an aluminum sample holder (see Figure 1) which is positioned on a motorized stage
capable of 5 axis movement.
The tilt of the sample holder is corrected using the reflection of a laser beam from mirrored surfaces.
The image of the electron beam is detected on a target foil with a video camera.
With the beam off, the center of the fiber is moved to the position where the beam was seen by the
camera.
Therefore, both tilt and x y positions are adjusted correctly for the beam to pass through the fiber.
Apparatus:
Items marked in bold are part of the alignment procedure. Some items refer to numbered objects in
Figure 2.
One vacuum chamber with eight ports.
Sample holder (50 mm aluminum) (item 2)
Motorized stage (vacuum compatible from Newport and capable of 5 axis movement. (item 1)
Additional long movement in x possible with an additional linear stage that traverses 2 inches). (item
8)
Adaptor between stage and holder (item 1)
Adaptor between stages (item 7)
Adaptor between stage and baseplate (item 6)
CCD camera for alignment with microscope objective to see the fibers (100um bore size).
Low resolution CCD camera for general viewing of set up.
Two cubes upstream and downstream with actuated glass pellicles.
HeNe laser.
Pellicle for inside chamber. (item 3)
Actuator for inside pellicle. (item 9)
OTR (Opticle Transition Radiation) screen- 1um thick titanium mounted on holder.
Reflector mounted on sample holder.
Off-axis parabola mirrors (one inside vacuum with a hole (item 4) for the beam and one outside)
Flat mirror (item 5)
Beam expanders
THz detector
Alignment in detail:
The chamber contains a 5-axis motorized stage for fine tuning in positioning. The fibers for the wakefield
studies and the beam will be in good alignment after following the procedure below.
Please monitor secondary electron detectors when inserting devices into the beam.
1. Insert glass Pellicles
These pellicles are inserted using air-driven plungers in an arrangement identical to the OTR
screens. The cubes for these are upstream and downstream to the vacuum chamber.
2. Mark beam vector and switch beam off for an access.
3. Establish HeNe laser on beam vector
This is a class 3a laser.
4. Establish 0th order alignment
The stage can be moved to a position where the laser is striking close to the correct position.
5. Correct tip/tilt
A reflector on the sample block is used to correct the tilt of the sample holder. If correct, the
laser beam should be reflected along the same beam path that it enters. After this stage, the tilt
will not need to be adjusted again.
6. Move OTR foil into laser beam.
The laser is now turned off. Access is over.
7. Image beam on OTR foil
Beam needs to be brought back. The mirrored pellicle needs to be inserted into the path of the
beam at 45 degrees such that a camera at 90 degrees can see the OTR. The position of the beam
is noted (eg by making a note of the pixel on the CCD or more likely by marking the monitor).
8. Position Fiber
Turn off the beam and move the stage such that the fiber is in the position that the beam was
in. Remove the mirrored pellicle from the beam path.
9. Turn beam on.
The fiber will be perfectly aligned.
These alignment techniques were developed for, and eventually proven in T-481. In this test beam the
fibers were aligned to the beam vector on the first try, with no measurable beam loss for the duration of
the test beam.
Illustrations:
Figure 1: The fiber holder assembly is shown with the 10 V-grooves for the fiber tubes visible, and the top clamping plate
drawn in transparency. The OTR screen is shown on the right side.
Figure 2: The experimental assembly including the “Kraken” vacuum chamber which replaces the “octopus chamber” from
FFTB. In dark yellow, there is the approximate optical path of the THz CCR signal, an off-axis parabola mirror, and a flat
mirror to direct the signal out of the chamber and onto a detector (not shown). The beam enters from “12 o’ clock”.
Figure 3: Photograph of the Octopus vacuum chamber and surrounding equipment for the T-481 experiment installed at
FFTB.
Figure 4: A sketch of the Kraken chamber layout for E-201.