Week of March 27th, 2000

Monday - 27 March 00
Worked at repairing the Pierce Glassman
Vented the accelerator, removed the ion Gun.
The extractor electrode (3.3mm) was replaced with one having a 6.2 mm
aperture. The 4.85mm aperture pierce was left in. Accelerator back under
vacuum.

Tuesday - 28 March 00
Lei MST Operator
Glassman repair continued thanks to Jim Adney.

Wednesday - 29 March 00
Glassman repair complete to Jim's credit. (Paul, don't worry, I've
promised him a beer/breakfast at the Terrace.)
Glassmans reinstalled and tested
HV controller having a problem - removed for repair

Thursday - 30 March 00
Testing operation of 4.85mm pierce aperture and 6.2mm extractor aperture,
with a spacing of .9" between two triangular
electrode supports (note that I do not know what the electrode spacing
is for this configuration) and source #2 (3 turn sent on 3-17 from
John) installed.

Despite the larger extractor aperture, we are still unable to maintain
control over the extractor voltage.

Vented the accelerator to reestablish the electrode spacing of 3/8". Also
changed ion source - installed source #3 (rcvd 3-17-00).

Pumped down, slowly warmed up source, confirmed emission from source, end
of run.

Friday - 31 March 00
4.85mm aperture pierce, 6.2mm aperture extractor, electrode spacing of
3/8", source #3 installed.

This run was quite detailed and systematic. The notation used in the
follwing table and the remainder of the email is as follows.

Fi - Filament current as measured from the analog meter mounted on the
pierce stack (units of Amps).
Psk - Pierce voltage as read from the silver control knob on the HV
controller. The scale factor is 1=20V, or 100=20kV
Esk - Extractor silver knob - Extractor voltage as read from the silver
control knob on the HV controller. The scale factor is 1=20V, or 100=20kV
Ask - Accelerator silver knob - Accelerator voltage as read from the
silver control knob on the HV controller. The scale factor is 1=20V, or
100=20kV
CmPi - Circuitmate Pierce current - Pierce current readback as displayed
on the digital circuitmate meter. Scale factor for each V, divide by 4,
multiply by 1000 to get current in uA.
Pi - Pierce current - Pierce current readback as displayed on the HV
controller analog meter. Multiply by 1000 to get current in uA.
UBi - Unbiased shutter current - current levels read off of the
oscilloscope, connected to an I/V that's connected to the shutter and
last ungrounded wire while the beam is being swept across the shutter.
Bi Biased shutter current - current measured on an analog meter that's in
series with a 6.5Mohm resistor that's in parallel with a multimeter
(that's displaying the bias voltage). (The circuit is shutter to parallel
DVM/resistor to analog meter to ground.)
FBW - Full beam width - Full width of the base of the beam profile as
measured by the BPM and displayed on an oscilloscope.
FWHM Full width half max - An approximate reading of the beam's full width
half max profile as measured by the BPM and displayed on the scope.

Try to extract current in a 10kV Extraction voltage range at various Fi
Fi Psk Esk Ask CmPi CmEi CmEv Pi UBi Bi FBW FWHM
5A 228 177 105 84.9mv 1.68V .02 0uA
6A 228 177 74 1.09V 1.04V .255 65uA 1.5cm
7A not holding off 10kV, dropped Fi to allow CmEv to drop, brought Fi
back to 7A and CmEv immediately rose to 2.0V (40kV) and CmEi dropped to
33.8mv
It should also be noted that the focus in both the 5 and 6A cases was
questionable and the base of the BPM was high.

Attempt to get in the range of 7kV Extraction voltage at various Fi
Fi Psk Esk Ask CmPi CmEi CmEv Pi UBi Bi FBW FWHM
5A pierce analog meter only showing .019 (19uA), only 1uA measured on
unbiased shutter. Abandoned 5A and moved onto 6A
6A 228 193 135 2.0V 33.8mV .590 80uA very unstable
7A 228 193 135 2.6V 33.7mv .63 120 2cm 1.5cm
again we've lost control of the extraction voltage and beam focus

Attempt to get in the range of a 5kV Extraction voltage at various Fi
Fi Psk Esk Ask CmPi CmEi CmEv Pi UBi Bi FBW FWHM
6A 228 205 153 .333V 1.77V .082 40
7A 228 205 135 1.3V 1.06V .325 60 1.5 .75
beam is stable
7A 228 205 136 1.5V .977V 1.96 .347 100 2cm 1cm

We tried and recorded many other settings, but these are a somewhat
representative set.

We then tried filament pulsing. The extracted current levels were no
better than what we saw without pulsing. With the filament current at
7.7A, we first focused the beam, getting following characteristics
Fi Psk Esk Ask CmPi CmEi CmEv Pi UBi Bi FBW FWHM
7.7A 228 214 177 1.1V 1.19V 2.1 .270 80

We then let the source/filament idle at 3.8A Pi=.014 for five minutes. We
pulsed the filament back upto 7.7A and recorded the following time
history from an unbiased shutter as read from the oscilloscope
17s=20uA, 38s=40uA, 1m=60uA, 1m 43s=30uA, 2.5m=70uA, 3min=20uA
The beam shape was okay, but the current was very unstable. The current
never rose higher than the 70uA reading at 2.5m.

We repeated at 8A
15s=20uA, 22s=40uA, 28s=60uA, 33s=75uA, 40s=60uA, 46s=40uA

Conclusions of run - we are still experiencing charge exchange
between the pierce and extractor. This seems worst at high extraction
voltages. The current levels are good, the beam is a bit broad, and we
cannot always maintain control over the electrode voltages.

At this point vented the accelerator, remove the gun and noticed that
the source cup had moved approxminately 3mm toward the extractor electrode
(the filament had moved with it). In this case it's likely that the collet
was just not tight enough.

We swapped out the pierce electrode and reinstalled the 2.48mm pierce
aperture along with a new source alignment disk that sits between the
pierce electrode and the collet assembly. We left the 6.2mm extractor
aperture in, kept the electrode spacing at 3/8" and reinstalled source #3.
Brought the beamline back under vacuum, and later that evening checked to
make sure we could extract a beam.

Saturday 4-1-00
Testing of the original 2.48mm aperture pierce, with the alignment disk,
6.2mm aperture extractor, 3/8" spacing and source #3.

Fi Psk Esk Ask CmPi CmEi CmEv Pi UBi Bi FBW FWHM
6A 228 182 81 .715V 1.141 1.814 .174 150 2cm 1.5cm

HV Test, 200MOhm resistor from extractor donut to ground.
6.2 353 317 220 .313 1.813 3.12 .076 130 1.2 .8

Next made some measurements on a self biased shutter at the high voltage
setting shown above.
Fi=?, 33.6V Bias, Pi=0.76, Bi=8uA
104V Bias, Pi=0.75, Bi=25uA
139V Bias, Pi=0.95, Bi=34uA
Fi=6.8A, 148V Bias, Pi=.104, Bi=36uA

At Low Voltage
Fi Psk Esk Ask CmPi CmEi CmEv Pi UBi Bi FBW FWHM
6.2 228 187.5 97 .434 1.069 1.856 .106 27
7 29
8 28

We noted during these runs that the BPM has a higher DC offset at the
lower voltages than at the higher voltages.

We then decided to try a mid-range voltage of approximately 55keV.
The results were similar to those at HV
Fi Psk Esk Ask CmPi CmEi CmEv Pi UBi Bi FBW FWHM
6.5 278 243 159 35
7 35
We then switched back to the unbiased shutter configuration so that we
could see the beam as we swept it on the shutter
7A 297 268 174 .721 1.313 2.64 .184 200
348 415 215 .722 1.621 3.1 .178 200-250

Conclusions of run - the cross talk between the two electrodes is no
longer a problem. The extracted current levels are in the range needed to
again begin looking for secondaries, though we wouldn't mind them being a
bit higher. The beam is larger in size than we'd like it to be.

We then pulled the gun back out of the accelerator. We made two changes
during this vent. We increased the pierce aperture from 2.48mm to 3mm.
We also modified the mid sized extractor aperture, increasing it's
aperture to 4.55mm and installed this in place of the 6.2mm aperture.
The modification of the pierce was to expose more of the emitting
surface area of the source. And the smaller extractor was to improve
beam focus and reduce the beam size.

We installed this modified gun, with source #3 Saturday night. We
verified emission of the source and then called it a day.

Sunday 4-2-00
3mm aperture pierce, 4.55mm extractor aperture, 3/8" spacing, source #3.

We did a bit of testing with this configuration, but the source seemed to
be getting a bit old. With a filament current Fi=7.2A, we measure
Bi=25uA. 8A did not give us any more current on the biased shutter. We
also tried leaving the filament current high, tripping the HV with the
panic button and resetting it with the HV controller in an effort to
simulate gun pulsing rather than filament pulsing. The results were
similar to filament pulsing.

In preparation for the MST scheduled clean run, we did a bit of comparison
between beam signals detected at the shutter and at the BPM. This test
was to refamiliarize ourselves with the beam profile measured by the sweep
plate in contrast with that measured by the shutter. The
refamiliarization was precautionary - so that we could continue to do beam
work in the event that we could not open our PBL gate valve during the clean
run.
(This is not a current restriction, but it is a possible restrictions)

We then vented the accelerator and changed the source in anticipation of
the clean run. While opening source number four, a thin circular disk of
source material was found in the paper towel. The filaments of source #4
were fully visible. Not being able to reinsert the material and not
knowing how the source might emit, we opted to install source #5. Source
number five had a full layer of source material covering the filament
turns. The turns were not visible through the material, and the top layer
appeared a bit crusty. In hind sight, I think this crust layer prevents
the source from emitting well and that we might have been better off with
source number four. More testing with the source might confirm or refute
this inkling. With source #5 installed, the accelerator pumped for a
couple of hours and the source slowly brouht up to full current over 1
1/2hr time span we began extracting current at 6.8A. This is a much
higher level than the other sources which began emitting in the range of
4.5A. In addition, we extracted only 8-12uA (at 45keV) onto the unbiased
shutter - the one positive feature was that it was very stable and
relatively well focused. We've observed that a source that appears to be
a poor emitter on day one, can improve considerably over subsequent days,
so we left the source to outgas overnight.

Monday - 3 April 00

We spent about and hour and a half or so slowly warming the source and
45min or so playing with it's emission.

Fi Psk Esk Ask CmPi CmEi CmEv Pi UBi Bi FBW FWHM
7A 228 212 142 134mV 1.222 2.08 .034 32 1.5 .7
40-52
7.1 298 274 184 .192 1.595 2.70 .047 56 1.5 .8

It was interesting to note that although the filament current is high, the
pierce current is very low. We did not push the filament currents to high
values, but decided to let it outgas a bit more and try it again in
subsequent days.

Conclusion of run - the beam size close to what we'd like it to be.
Unfortunately, the current levels with this source are not yet what we
need to look for secondaries with plasma. I should note that even if
current levels were sufficient with this source, we have a bit more work
to do before looking for secondaries. Due to the modified gun
configuration, we need to realign the beamline and do a bit of sweep
plate to primary detector pin calibrations.

Questions/comments?

Diane