1. Experience with beam instrumentation at Linac4
Prepared by Jean-Baptiste Lallement and Alessandra Lombardi

2. Thank you BI colleagues for
Instrumentation that overall worked very well
An outstanding support during the whole commissioning period (keep it up, we are not done)
An open mind to our feedback

3. List of permanent instruments
Low energy (0-3MeV)
Medium (10-160MeV)
160 MeV
SEM grids
1/1
4/4
6/6
Wirescanners 2 4 6
Beam transformers 3 5
BLMs 1 9
Pick-ups 15 10
BSMs

4. Avant-propos
The Linac4 beam instrumentation layout was frozen 7 years ago.
We had permanent diagnostics and dedicated test benches for the staged commissioning
Different diagnostic types and measurement techniques development and commissioning spread over the last 5 years…
Not much time in the last commissioning stage to used and measure what we would have liked…
The reliability run is the good occasion

5. Beam Current Transformers
The first thing we look at (and not only when something goes wrong)
Essential for operation (watchdog)
Necessary for setting up the beam up to 30 MeV
LEBT and MEBT magnets
DTL1 transmission curve
Generally reliable
Issues with amplifiers required access to the tunnel (reliability…)
Calibration still an issue ?
BCTric app is not very user friendly
Synchro between beam and acquisition
Overall OK
Watchdog OK

6. Beam Loss Monitors
Never really used them – But they do clearly show losses – Is there an application ?
Could be complementary to other diags (profile, BPMs) to help in setting up steerers and quads.
We did see losses when playing with settings in the high energy part (above 100 MeV)
The one between L4D – L4C is sensitive to the RF in the CCDTLs. It look like its signal only depends on the presence and level of the RF in the CCDTLs
Not yet used enough to give feedback at this point

7. Bunch Shape Monitor(s)
Used the first one at 3 and 50 MeV on the temporary bench
An expert tool, not for everyday operation
Excellent tool to measure the longitudinal emittance.
Will be used in the Linac4 (after the last PIMS) and in the transfer line (after the debuncher)
Will be the only measurement between the linac and PSB and it will take the function of the present LBS.
It will be of fundamental importance after the connection
Not much time to test and use it at 100 and 160 MeV.
Need of a BI expert to make measurements.
Needs an application

8. Beam Position Monitors
Started to use at 50 MeV – multipurpose tool
Only tool for on-line steering (trajectories) and setting up the cavity phases and amplitudes (ToF).
Essential at each restart
Used during the different commissioning stages above 50 MeV.
Need of a BI expert to set it up (ToF).
Application needed

9. Transverse profile measurements at 45 keV(tunnel and Test stand )
Very difficult regime, with many physical processes in action (space charge, neutralisation, uncontrolled emittance increase, losses)
The SEM grids are completely inadequate under every aspect :
Resolution
Size (we cannot measure under operating conditions)
Effect of the bias
Effect on the neutralisation and RFQ transmission
We need to find (another?) solution

11. SEM and wire scanners
Used to verify beam size, set up buncher phases and measure transverse emittance from beam sizes
Not essential for operation but essential for setting up
Used extensively up to 100MeV, measured values are generally in very good agreement with expectations.
Time resolved measurements are very useful (e.g. chopper set-up) and work well
Overall OK but issues with vacuum to be addressed.
Not all of them available yet.
Application is very heavy and blocks the computer quite often.

12. Emittance meter: slit and grid
Direct emittance meter possible at the low energy end 45keV, 3 MeV, 12 MeV
At 3, 12 MeV (temporary bench) we had excellent measurement.
Control and interface is ok for short-term measurement campaign but definitely not suitable for source test stand.

13. Summary Red : instrument not usable
Yellow : instrument ok, interface missing
Green : all good.
Low energy (0-3MeV)
Medium (10-160MeV)
160 MeV
SEM grids
1/1
4/4
6/6
Wirescanners 2 4 6
Beam transformers 3 5
BLMs 1 9
Pick-ups
0- wish a couple of pickups 15 10
BSMs

14. Concluding remarks
The reliability run is a good occasion to bring the instrumentation in an operational state (=usable by operators)
The instrumentation that was more useful is the one one that is more user-friendly coupled with a validated machine model.
Post-processing of data is insufficient in general and it added a lot of strain during commissioning both on BI and on the commissioning team. Let’s avoid this -if possible- for the connection to the PSB.
Laser based instrumentation : so far it was like an independent experiment and I wish that this paves the way to new diagnostics for the linac accessible also to us and the OP team.