Sliding Seal FMA Target Chamber

Description

It is impossible to completely describe all the intricacies of this versatile target chamber, but this document should answer some basic questions. To know more you should talk to Cary Davids or Birger Back.
This scattering chamber has a sliding seal which allows the FMA to be rotated with respect to the beam axis from -15 to +45 degrees.
The target height and angle can be adjusted with the aid of stepping motors and shaft encoders on the ends of the rotating feedthroughs. There are 3 concentric rings on the floor of the chamber which rotate. They are also controlled by stepping motors and shaft encoders. Keep in mind that they rotate with the chamber when the FMA rotates so this must taken into account in determining the angle of any detectors on the rings.
The left side of the chamber has a 12"x12" flat opening for access or for attaching large detectors.
A rotating target wheel can be used in place of the standard target ladder.

Getting to Know Your Sliding Seal

There is a bellows upstream of the chamber with one end connected to the seal and the other end fixed to the beamline. The seal is at the front of the chamber so the chamber rotates with the FMA. Two wrenches are connected by a turnbuckle to take up any slack in the metal band and keep it pressed againt the double o-ring seal. A mechanical pump evacuates the volume between the 2 o-rings. It is important to keep tension on the seal and to keep the bellows straight. Otherwise it will leak.
When you rotate the chamber, you must turn the wrenches in the direction which will straighten out the bellows (look at it from above). You'll figure it out by trial and error. Keep an eye on the seal pressure while you turn the wrenches. There is a thermocouple gauge which should be connected somewhere. If you are moving the wrenches correctly, the pressure should not go up much and should come all the way back down to its original base value when you stop moving things.
If you are rotating the FMA to a large angle, you may have to remove and reposition the wrenches. Loosen the turnbuckle, remove the wrenches, put them back on, and tighten the turnbuckle as quickly as possible so the seal does not leak too much.

Specifications

diameter: 16"
height: 19"
# of targets on standard ladder: 5 (4 w/ FC, 3 if also want a blank)
# of feedthroughs: 24 bnc
radius of inner ring: ~3 7/8"
radius of middle ring: ~5 3/8"
radius of outer ring: ~6 1/2"
FMA entrance apertures: 4 choices mounted between chamber and quadrupole

Rotating FMA

close valves before chamber, after chamber, above cryopump, before PPAC
turn key to bypass so turbo stays open even if ion gauge trips off
unplug battery for FC suppression (it is on beamstand upstream of chamber)
turn off monitor HV
release FMA rotation brake
crank 1/2 deg (readout is LED and is in degrees)
check sliding seal
- is seal pressure still ok
- adjust wrenches if bellows upstream of chamber is not straight.
move any equipment and racks at the focal plane which is not attached
repeat the previous 3 steps until you are at the desired angle
set parking brake
turn on ion gauge if it tripped off
open cryo valve if pressure is OK
when pressure is normal, open valves before and after chamber
close valve above turbo
turn key to protected and turn alarm on if it was off
adjust the angles of things inside the chamber (they move with the FMA): movable FC if any, monitor, target, reset foil
plug in FC suppression battery
turn on monitor HV
open valve in front of PPAC

Venting Chamber

turn off monitor HV
close all valves to chamber: before, after, turbo, cryo
turn off ion gauge
alarm off
turbo off
unplug battery for FC suppression (on beamstand upstream of chamber)
nitrogen flow on (valve on beamstand near wall upstream of chamber)
place hose near vent valve
open manual vent valve slowly (black needle valve above turbo)
watch neddle gauge to see when at atmosphere
turn off nitrogen at wall
remove lid slowy so you don't create any suction and damage targets
place lid on its holder with the rubber stoppers

Changing Targets

bring containers for targets over to chamber
carefully reach into chamber, slide the top target out, put in container
when sliding targets up and down, do NOT let them drop
move slowly to avoid disturbing the reset foil
replace the reset foil if it looks bad
check big o-ring and lid for hairs, scratches, etc.
replace chamber lid slowly to prevent rupturing foils

Pumping Out Chamber

check o-ring and lid, replace lid slowly (if not already done)
close vent valve
connect roughing pump fitting on end of hose and hand tighten
turn on roughing pump, open valve above pump
slowly open vent valve (now it's a roughing valve). the first sign of pumping is smoke from the pump exhaust. if the needle gauge does not start moving soon, you may have to push down on the lid to compress the o-ring.
at ~2K on the chamber thermocouple gauge turn key to bypass, open turbo backing valve, open valve above turbo
close roughing valve
at ~500 microns turn turbo on
when turbo light is green, turn switches from on to protected
when thermocouple pressure =0, try to turn ion gauge on
open cryo valve when pressure is low enough
degas ion gauge when ~1e-6
close valve above turbo (cryo alone is enough)
turn key to protected, alarm switch on
plug in FC suppression battery
turn on monitor HV (make sure windows are covered)
open beamline valves before and after chamber

Last Updated: May 24, 1996 (D.J. Blumenthal, [email protected])