NMR Einzel Lens EL3 Summary of Tests

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ßNMR Einzel Lens EL3Summary of Tests
Andrew MacFarlane, March 19, 2004
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Problem When the platform BIAS is at high
positive potential, the solenoid is above about 1
Tesla AND EL3 is on above about 3 kV Electrons
are generated and accelerated and focussed
onto Sample, damaging it. As a consequence we
cannot run EL3 in high field/high bias
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To address this problem we performed some tests
Conditions
the gate valve ILE2A3IV2 closed which
isolates EL3 and the ßNMR experiment from the
rest of the beamline. the EL3 cryocooler ON the
B detector HV OFF, cryopump ON the Ion Gauges
IG3 and IG5 OFF some convectron gauges
on pressure at the experiment about 3.910-9 Torr
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The I-V characteristic of the Platform w/ Magnet
off, EL3 off
Note these are all EPICS readbacks
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The I-V characteristic of EL3 w/ Magnet off,
Platform off
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The I-V characteristic of EL3 w/ Magnet at 3T,
BIAS 30kV
Note The effect is clearly visible (by eye)
above 3 kV on EL3
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To confirm the effect was there at this time we
installed the BC412 scintillator and the CCD
camera and took a series of images
9mm aperture behind scintillator
Ion Gauge Light
Dark Image
3kV, 30kV, 3T
11 sec exposure, no contrast adjustment, all
images treated equally
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Note There seemed to be a time lag (10 sec)
for the formation of the glow. The first camera
image was dark, then subsequent pictures showed
thte light.
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Then we swept the Platform BIAS down to see the
effect
30 kV
20 kV
10 kV
EL3 3kV and B 3 T
All images have one linear contrast stretch
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Then we swept the magnetic field with EL3 3kV,
BIAS 20 kV
3T
2.5T
1.5T
2T
0.5T
1.25T
Zero Field
1T
All images have one linear contrast stretch
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Here is a map of the fringe field in EL3 for 1
Tesla in the solenoid
Solenoid Centre
EL3
Field Contour (Gauss)
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Conclusions

• The electrons are not from upstream of EL3
• The problem is reproducible
• The electrons are probably not field emission
  from EL3 (3kV too small) and I-V is linear
• The light intensity switches on nonlinearly with
  field

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Plans

• Check the pressure dependence of the glow
• Reverse bias EL3
• Seek expert advice