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OVERVIEW OF M2 COMMISSIONING IN 1999 |
After the end of the NA47 and P297 runs in April 1997, the M2 beam was redesigned and partly rebuilt for the COMPASS/NA58 experiment. The last section of the beam, starting at the last vertical bend (BEND-6), was completely rebuilt. Upstream of Bend-6 the only hardware change was the addition of a new large-aperture collimator in position 858, which serves as a momentum slit for the hadron mode of the beam.
The main changes are related to the following requirements:
These requirements lead to a physical installation as described by a Beatch file called m2yr2000. The description of the beam optics comes as 4 different files:
The transport outputs can be found in the /afs/cern.ch/user/e/eagroup/database/transout directory on AFS. The corresponding optics drawings are available on the web as Postscript and PDF files with url http://home.cern.ch/~gatignon/optics/filename.ps, respectively .pdf.
The commissioning of the beam was done in parallel with various trigger and detector tests by the COMPASS collaboration. Two periods were available, namely a first run from 1 till 21 July (Run I) and a second period from 7 September till 6 October (Run II). From the 30th of September till the end, the M2 beam was constrained to-225 GeV/c in the front end, due to compatibility requirements with P61 operation. It was decided to run only the hadron beam in this last week.
It should be noted that all the beam and infrastructure installation was completed in time for the1st of July!
On the 1st of July the M2 operation was started in muon mode with m+ beams of 190 GeV/c from decays of 208 GeV/c p+. After an initial tuning a flux of 2.7 107 muons per cycle were observed with 2.5 1012 ppp impinging on a 500 mm Be target, with the hadron momentum slit (COLLs 1 and 3) opened to ±30 mm.
On the 6th of July the experimental scaler signals were changed to give the BH3 beam rate (40 x 40 mm2), V2 (70 mm diameter hole) and V3 (a50x50 cm2 counter with a 120 mm diameter hole). With these signals the halo was minimised in V2 and V3. In the end we achieved a flux of 2.1 107 muons per cycle in ionisation chamber 2, about 107 in BH3. The V2 rate was 1.8% of BH3 and the V3 rate about 17%. The scraper settings found were almost exactly identical to the ones found in a test in 1995.
Later BH3 was found to be saturating. Spot sizes in BH3 showed a RMS of about 8 mm in each plane, where the real waist seemed to be a few metres upstream of the target. This condition was left till the end of Run I.
In Run II the V2 counter was unfortunately no longer available. The beam was re-tuned with a better performing BH3 counter. The halo was optimised by a slight retuning of Q33 current. The current in Q36 was optimised to give smaller divergence by minimising the width of the distributions seen in H4 (30 m downstream of the target) and H5 (50 m downstream). Finally we ended up (on September 12th) with equal rates observed in BH3 and ION-2, namely 2 107 muons per burst for 1.5 1012 ppp on T6. The spot size (s) in BH3 was about 7.5 mm in X and 7 mm in Y for about 0.9 mrad RMS divergence in both planes. At that time the ratio in the V3 counter (120 mm hole) was about 4%.
On the 15th of September the intensity on T6 was increased to some 1013 ppp, resulting to some 1.5 108 muons per cycle seen in Bh3 and in ION2. When needed by the experiment, the intensity could be reduced to about 3 107 muons/cycle by selecting the 40 mm target head.
On the 23rd of September a new counter VS with a 50x50 mm square hole (approximate size about 70 cm horizontal and 30 cm vertical) was installed about 1 metre downstream of the target center. The beam was now tuned to go through the center of the hole in that counter (which turned out to be well aligned with BH3). Subsequently Q35 and Q36 were tuned to minimise the rate in VS, whilst keeping a reasonable spot in BH3. The hope was that this way the waist would be displaced more towards the target and hence the divergence reduced. This lead to a spot with 8 mm s in each plane. The divergence was then measured by COMPASS to be 0.4 mrad s in X and 0.8 mrad s in Y.
The halo was 4.4% IN V2/BH3 and 13% in VS/BH3. Then the halo in VS was reduced further by scraper tuning, mainly closing SCR7 to a gap of ±28 mm. At that stage V2/BH3 went down to 2.6% and VS/BH3 to 9% (target OUT). Note that inserting the NA58 target increased the VS/BH3 rate to 30%! The beam rate was not significantly reduced by this scraper tuning.
This was to be considered very adequate and was left as a final setting: Beam file M2.7. Improvements to the trigger were made that allowed to reject many of the interactions of muons in the iron of Q36, which produce a muon traversing H4 and H5 and at the same time a secondary charged particle traversing BH3. With the improvements mentioned, the trigger could indeed identify interactions in the target.
Final parameters of the muon beam at the end of Run II:
|
Parameter |
Measured |
Expected |
|
Beam momentum (p / m) |
+ 208 / 190 |
|
|
Proton flux on T6 (500 mm Be) |
1013 ppp |
|
|
Muon flux on COMPASS target |
1.4 107 |
1.5 107 |
|
Spot size at COMPASS target (sx x sy in mm) |
7.8 x 7.8 |
< 8 |
|
Divergence at COMPASS target (sx x sy in mrad) |
0.4 x 0.8 |
< 1 |
|
Halo/Beam in V2 (50 x 50 cm with 120 mm hole) |
2.4% |
£ 4% |
|
Halo/Beam in VS (50x50 mm hole) target OUT |
13% |
15% |
|
Halo/Beam in VS (50x50 mm hole) target IN |
30% |
- |
On the 13th of September we tried the +110/100 GeV/c muon beam and obtained a flux of 4 107 m+/cycle for 1.5 1012 ppp on T6. The spot size at BH3 was round with 8.5 mm s in each plane, again with a waist several metres upstream of the target. Beam file M2.8.
The +280 GeV/c hadron beam was first switched on on the 5th of July. After some painful tuning, we achieved some 107 hadrons per SPS cycle for 1.5 1012 ppp on a 40 mm Beryllium target in T6. The tuning was painful, mainly because the ionisation chamber and to some extent also TRIGGERs 1 and 2 were seeing lots of background / showers, probably from interactions in the beam hodoscope modules (four times 20 mm of scintillator!).
On the 6th of July, a second iteration was made by minimising the rates seen in V2 and V3. The V3/BH3 ratio went down from 9 to 1.1%! However, the wire chamber profiles downstream of the hodoscope modules showed some flat tails (muons and interaction products) and it was essentially impossible to see the beam in the FISCs, due to backgrounds from probably hadrons and muons.
On the 15th of July we tried again and got profiles from the BH3 counter, which showed a RMS of 3x4 mm. The V3/BH3 ratio was about 1.6% at a beam flux of 1.2 107. This was achieved with slits of 4 mm in COLL-1, 10 mm in COLL-3, 10 mm in COLL-2, 16 mm in COLL-4 and 20 mm in the momentum slit COLL-5. The flux on T6 (40 mm Be target) was about 1.7 1012 ppp. This situation was left for the remainder of Run I. Beam file M2.3.
As a consequence of the large background in the chambers and the FISCs, we decided to add two 4-jaw collimators in the upstream and downstream ends of the beam hodoscope section. This installation was done in the shutdown between Run I and Run II.
In Run II the only hadron running was done in the last week, when compatibility with the P61 beam imposed 225 GeV/c for the M2 beam momentum. As the first 6 quadrupoles and Bend-1 are in common with this beam, a different optics had to be designed. This new optics was tried the first time on the 1st of October. The setting up profited a lot of the new collimators, as the scans on ION-2 looked much cleaner. At the end of the day, we obtained a p- flux of 106 for 3 1012 ppp on a 100 mm target in T6. Collimator slits in COLLs 1-5 were respectively 8, 6, 10, 10 and 8 mm. The profiles in MWPC 15-18 looked much cleaner and the beam was clearly visible in the FISC profiles, though above a large flat background. The spot size in BH3 was 3 x 1 mm s, without significant tails. The halo rate in V3 was 5.4%. The VS counter was unfortunately no longer available. By switching on the vertical scrapers (SCR 1, 2, 4, 5, 7) and the MIBs this rate could be reduced to 1.6%.
On 4 October we prepared new files (M2.11 and M2.12) with a 2 x larger vertical spot and smaller divergence (mx=1.667, my=-3). This was a significant improvement for the experiment. Also we opened the collimators to achieve a flux of 107 p-/cycle. This reduced the V3 rate to 1.2%. The spot size was 3.9 x 3.7 mm s in BH3. This was left as a final setting for the 225 GeV/c hadron beam:. Beam file M2.12.
Please find below a summary of V3 vs BH3 and ION-2 fluxes. V3 is a 50x50 cm counter with a 120 mm diameter hole.
|
Beam condtion |
ION-2 |
BH3 |
V3 |
V3/BH3 |
|
Muon beam 190 GeV Run I |
2.1 107 |
107 |
1.7 106 |
17 % |
|
Muon beam 190 GeV Run II |
2 107 |
2 107 |
4 105 |
4 % |
|
VS tuning |
2 107 |
2 107 |
4 105 |
4 % |
|
Final status after scraper tuning |
2 107 |
2 107 |
2.6 105 |
2.6 % |
|
Hadron beam 280 GeV/c (90% p) |
1.2 107 |
1.2 107 |
1.3 105 |
1.1% |
|
Hadron beam 225 GeV/c (p-) |
1.2 107 |
1.2 107 |
1.3 105 |
5.4% |
|
Hadrons 225 GeV with scrapers |
107 |
107 |
1.2 105 |
1.2% |
It is worth noting that a p- beam without scrapers has 2 times more muon halo than the muon beam. Switching on the scrapers and MIBs reduced the muon halo in the pion beam to half that of the muon beam. The positive hadron beam contains only 10% pions and has therefore a muon halo which is only half that of the muon beam.