X-Ray Facility 414-416
Institute of Molecular Biophysics Florida State
University Tallahassee, FL
32306-4380 Telephone:
(850) 644-6448 E-mail: soma@sb.fsu.edu URL: http://www.sb.fsu.edu/~soma |
X-Ray Facility Hardware Manual
Filament
Change
Procedure for
changing burnt X-Ray Generator Filament and replacing it with a new filament
Table of contents
Figure 1 Vacuum release knob located beneath the rotating anode
chamber
Figure 2 Front vacuum cover assembly with screws highlighted
Figure 3 Cathode assembly & anode chamber upon removal of front
vacuum cover plate
Figure 4 Cathode assembly with KimWipes placed beneath the assembly
Figure 5 High voltage wires visible after terminal cover plate has
been moved down
Figure 8 Replacing a new x-ray filament in side the cathode
assembly
Figure 10 Vacuum controller panel
ã 2000-2001 Thayumanasamy Somasundaram
405 Institute of Molecular Biophysics, Florida
State University,
Tallahassee, FL 32306-4380
E-mail: soma@sb.fsu.edu • URL: http://www.sb.fsu.edu/~soma
URL: http://www.sb.fsu.edu/~raxis
Phone 850.644 6448 • Fax 850 561 1406
August 05, 2001
Photos courtesy of Rigaku/MSC
Filament Change
Procedure for
changing burnt x-ray filament and replacing it with a new filament*
During the regular use of the x-ray generator (Rigaku generator is only covered in this write-up) the filament that produces the electrons burns out and needs to be replaced. The useful life of a typical x-ray filament is approximately one thousand hours of operation (assuming continuous generator operation this means filament needs to be changed once in forty days). The symptoms of a burnt out filament are one or combination of the following:
§ Alarm at the x-ray generator with a lit red LED next to FC
§ Needle at the filament current meter shows no measurable value
§ Hour meter does not change value even after several minutes of generator operation
§ No diffraction pattern after eight to ten minutes of exposure
§ Absence of water ring and/or absence of beam stop shadow in diffraction pattern.
This notes describes how to remove the burnt filament and replace it with a new one. Due to the fact, we have to break and regenerate high vacuum, the whole procedure can take eight to ten hours.
1.
Switch the x-ray generator off, close the
chilled water supply to the Haskris water chiller[1],
however, maintain the water circulation to the generator and wait for ½ hour.
2.
Shut the rotating anode (RA) off, if it is
not already off, and shut down the vacuum pumps. Switch off the main powers at the gray breaker box on the wall,
switch off the Haskris water chiller, and wait for another ½ hour.
3.
Using the special tool (flat steel plate with
a wedge[2]),
open the vacuum release valve[3]
(see Figure 1) underneath the front covers to the anode
chamber and let in air slowly. Once the
anode chamber is equilibrated with outside pressure, tighten the vacuum release
valve back to the original position (hand tight).
Figure 1 Vacuum release knob located beneath the rotating anode chamber
4.
Using a Phillips screwdriver carefully remove
the four screws (see Figure 2) found on the front vacuum
cover assembly of the x-ray generator head, ensuring that the removal of the
screws proceed uniformly.
Figure 2 Front vacuum cover assembly with screws highlighted
5.
Remove the front vacuum cover and its
'o'-ring making sure NOT to touch the interiors of the cover or the
chamber. Place the cover and the
'o'-ring aside. Now you can see the
interior of the anode chamber (see Figure 3).
Figure 3 Cathode assembly & anode chamber upon removal of front vacuum cover plate
6.
Now place a long lint-free Kimwipes â EX-L[4]
inside the space made available with the removal of front cover. The idea behind the Kimwipes is to protect
any screws, shims, and such falling into the vacuum chamber located directly
underneath the cathode assembly while one is removing the burnt filament (see Figure 4). So, stretch
the wipe underneath the cathode assembly and let part of it hang outside the
chamber. Essentially all operations are
done above the wipe and anything falls will be caught by the wipe.
Figure 4 Cathode assembly with Kim Wipes placed beneath the assembly
7.
Visible in front will be the back of the
cathode assembly (it may be VP3 cathode, i.e., 0.3 x 3.0 mm2 fine
focus cathode). The terminal
cover-plate[5] protecting
the filament terminals is located directly beneath the cathode assembly. Carefully undo few turns of the two Phillips
screws holding the plate, with out completely removing the screws.
8.
The terminal cover plate should automatically
slip down revealing behind it the two terminal posts for the cathode (see Figure 5). Now
tighten the Phillips screws back with the terminal cover plate down.
Figure 5 High voltage wires visible after terminal cover plate has been moved down
9.
Using a metric hex wrench undo the two hex
heads holding the high voltage wires to the bottom legs of the cathode
assembly. These are captured-screws and
can’t be removed. Remove the wires away
from the legs of the cathode.
10.
Using one gloved hand, hold the cathode
assembly with an electronics-lab glove [6](made
of white nylon). Now with the other
hand, unscrew the hex heads at the bottom corners of the cathode assembly. Once the screws are completely undone, you
can remove the whole cathode assembly away from the chamber retaining the two
hex screws (captured-screws[7]).
This should reveal the rotating anode, filament terminal and cathode holding screws.
11.
Place the cathode assembly on a clean
Kimwipes sheet with the Wehnelt window facing up. Careful examination will reveal that the filament is broken. Most of the time filament breaks at the
middle. This breakage is accompanied by
bend in the filament as well. (See Figure 6 for the frontal & backside view of the VP3
cathode assembly).
12.
Now turn the cathode such as way that the hex
heads are facing up on a clean Kimwipes sheet.
Now undo the two Phillips screws found on the top corners of the cathode
assembly completely.
13.
The back cover of the cathode should now come
off completely, revealing inside the burnt filament (see Figure
7).
14.
The filament itself is held to the Wehnelt
window (i.e., the front of the cathode assembly) with two more hex screws[8]. Removing the two hex screws will release the
filament from the cathode assembly. Now carefully store the hex screws. The removed filament will look similar to
one of those shown in Figure 8 below except for the
missing filament.
15.
Using very fine Emery polishing paper clean the
inside and Wehnelt window[9]
of the cathode assembly taking care not to create big scratches.
Figure 8 Replacing a new x-ray filament in side the cathode assembly
This completes the removal of the burnt filament. Now we will proceed to installation of a new filament in the next section.
16.
Select the appropriate filament. For example, VP3 cathode can take Rigaku Cat
# CN 4892V2 if one requires 0.3 x 0.3 mm2 fine focus at 6° take off
angle or VP5 cathode can take Rigaku Cat # CN 4892V1 if one requires 0.5 x 1.0
mm2 fine focus at 6° take off angle. Some of the new fine focus and line focus filaments are shown
below in Figure 9.
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Figure 9 New Rigaku filaments: CN4892V2 fine focus (0.3 x 3.0 mm2) CN4892V1 fine focus (0.5 x 1.0 mm2) & CN4892X2 line focus (0.2 x 0.2 mm2).
17.
Place the filament inside the cathode
assembly. Place the hex screws in place
and engage them slightly.
18.
Observe under an optical microscope and align
the filament appropriately making sure that it is not bent. The filament should not be too close to the
windows either.
19.
When the optimal position is reached, tighten
the hex screws fully.
20.
Place the back cover of the cathode assembly
and hand-tighten it using the two Phillips screws.
21.
Holding the cathode assembly with an
electronics-lab glove place it back in the chamber and engage the two hex
screws at the bottom corners of the cathode to the holes inside the chamber.
22.
A small guiding pin[10]
found inside the chamber will help you position the cathode in the appropriate
place.
23.
After ensuring that the cathode is placed
evenly, hand-tighten it fully.
24.
Carefully place the captured-screws attached
to the high voltage wires back to their respective legs at the bottom of the
cathode and tighten them one by one. While tightening, make sure they do not come in contact either
with themselves or with the walls of the cathode assembly[11].
25.
Carefully release the Phillips screws holding
the cover plate and at the same time move the plate upward to hide the high
voltage terminals behind the plate.
26.
Once the plate covers the terminals
completely, fully engage the two Phillips screws.
27.
Place the vacuum chamber cover back in place
making sure that the 'O'-ring is still in place and tighten the cover evenly.
28.
Ensure the vacuum release valve is
hand-tight. Due to minor differences in
the placement and tension of various screws the exact position of the new
filament is has changed. Therefore,
complete beam optimization procedure should be done following the successful installation
of new filament.
29.
First, switch the vacuum control from AUTO to
MANUAL. Flipping the vacuum controller
panel switch to MANUAL mode can do this.
See Figure 10 below for the location of the
switch.
Figure 10 Vacuum controller panel
30.
Start only the rotary pump by flipping the RP
switch to ON position. Run the pump
several hours (~6-8 h) to ensure degassing and removal of adsorbed gases and
moisture.
31.
Then start the turbo molecular pump (TMP) by
flipping the switch to ON position. Run
this pump together with RP pump for few (~2-3 h) hours. Then switch on the ion gauge (IG).
32.
Monitor the vacuum with the digital multi meter. When the vacuum is better than 0.200 V,
switch on the TARGET motor. Due to
degassing of adsorbed gases in the magnetic seal, there will be some
degradation in vacuum but soon it will recover.
33.
Now you are ready to start the x-rays. This completes the procedure for new
filament replacement.
34.
Now proceed to monochromator adjustment
procedure.
If one follows this note, it is relatively easy to replace the burnt filament. As noted in the beginning, steps 30 to 32 make take several hours. At times, it may require you go back and forth between steps 30 and 32 few times for uninterrupted generator operation. Please send your suggestions and comments to Soma.
[1] Haskris water chiller is located beside the generator and connected to the building chilled water supply.
[2] For a picture of the special tool, consult magnetic seal replacement procedure Figure 1.
[3] Vacuum release valve has a knurled circular head with a '-' slot in the middle.
[4] Long Kimwipes are better due to their extra width.
[5] Terminal cover plate is very thin silvery plate with two oval-shaped slots at the bottom corners.
[6] It is essential that fingerprints or oily residues not left anywhere inside the vacuum chamber.
[7] Terminals hold the captured-screws and they do not come off once undone.
[8] The filament is spot welded on the top of two metal posts. The metal posts are secured to a ceramic base.
[9] Wehnelt window seen in Figure 6a determines the focus even if the filament is the same.
[10] The guiding pin is located between two cathode terminals in side the vacuum chamber.
[11] Holding the terminal with a self-locking tweezers will be helpful