Hello,
On Friday afternoon we started the ABS to measure the
performance of the dissociator at full BLAST field with
the additional magnetic shielding in place.
We observed:
- The same baseline that we had before we disassembled
the ABS.
- A loss in intensity measured as a drop in target chamber
pressure from 1.6*10-7 torr to 0.9*10-7 torr. The loss
is linear in the BLAST coil current from 1000A up.
- No change in the tune of the dissociator. The change in
the impedance is about 1 Ohm.
- The same dependence of the intensity on dissociator rf
power at full field as is observed with no BLAST field.
That means we have saturation at the same power level
of about 270W at 60sccm and constant intensity in the
optical monitors over a larfe power range.
- an increase of the heat load into the nozzle when the
Blast field is ramped down to zero.
The intensity loss that we observed now is significantly
different from the previous measurements that were taken
with no magnetic shielding. In the current range 0-3000A
we now have a linear dependence on the magnet current
whereas the previous measurement showed a slight increase
up to 2000A and then a decrease above 3000A.
My conclusion is that with the present configuration the
dissociator performance is as good as without the BLAST
field. The loss in intensity is probably caused by a shift
of the nozzle in respect to the skimmer. We now have a big
piece of steal as part of the dissociator frame that can
pull the dissociator can sideways, causing the shift of
the nozzle and the intensity loss. An intensity loss of
this size would correspond to a shift of the nozzle position
of 1.5mm at full BLAST field. We should be able to measure
this with the optical plummet installed on top of the
dissociator.
Hauke
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Hauke Kolster MIT * Bldg 26-551 * Cambridge, MA 02139
hauke@lns.mit.edu Tel (617) 253-2693 Fax (617) 253-9599
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