Post date: Jun 24, 2015 2:06:41 AM
Today from 11am the team achieved a good circulating beam in the machine and proceeded to optimise the injection using the fluorescent screens. (Will add a screenshot of this...). To optimise they had to change the power supply on the final steerer in the injection line to be a bipolar one rather than a unipolar one, to give a full range of possible values.
After lunch we took data using the 5 probes for fixed energy COD measurement for two working points:
D995, F814
D825, F814
The corrector was set to 700A for both of these measurements.
This data was taken to compare to the COD data without the rf cavity to have a look at the effect of the cavity itself. As previously noted there was ±1cm COD without the cavity present, and once the cavity is in the pattern now looks very similar but with a larger amplitude of roughly ±2cm (images/plots to be added).
Ishi-san did a quick analysis using SAD code and studied the difference between the COD patterns with RF_in and RF_out, and compared this to the pattern that a single angular kick from the rf cavity would provide of 12.5mrad (nb: quite small). It matches surprisingly well, so our thoughts at the moment are:
- there is now a COD of ±1cm WITHOUT the RF cavity present that was not there before the additional magnetic material was removed (ie. when we took that data in Nov 2013). This is rather confusing and we are not sure why removing the additional magnets etc would cause a COD.
- the COD without the RF cavity may arise from random horizontal alignment errors of the main magnets, but this would need to be studied. Alternatively, re-measuring the alignment of the main magnets may be necessary at some point in the future.
- On a positive note, the corrector appears to be working pretty well and the addition to the COD from the RF cavity with the corrector on is now relatively small compared to before. The corrector was set to 700A but can go higher, so when we do the accelerated COD measurements later, we will turn it up higher and see if the COD is further corrector.
- However, we don't expect now in any case to be able to reduce the COD to better than ±1cm without a major re-alignment campaign. Unless we have misinterpreted there results somehow.... so we may just have to live with it.
Toward the evening we switched on the RF to check that it was working OK, but the linac tube tripped a couple of times. It was left overnight for conditioning, but the RF looked OK (although we didn't actually use it to accelerate). This will be the first item on Wednesday.