Beam extraction - 50th anniversary of the CERN Proton Synchrotron

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Fig. 35: Transverse profile of the beamlets before extraction (left) and intensity signal of a pick-up in the PS–SPS transfer line of the extracted beam The rest of the commissioning period in 2008 was devoted to the preparation of the operational magnetic cycle, the study of the extraction with multi-bunch beam, and the determination of the most suitable longitudinal structure to be injected in the SPS. This has been another crucial point of the study. The beamlet formation, in fact, is not sensitive to the number of bunches in the machine, but it is sensitive to the beam momentum spread, since via chromaticity a large momentum spread creates a large tune modulation, inducing trapping/de-trapping phenomena and reducing the capture efficiency. The losses at extraction, however, depend on the bunch spacing due to the finite rise time of the kickers. In particular, the kicker rise time is longer than the h=8 PS bunch spacing, the rise time being about 350 ns and the bunch spacing about 260 ns. This implies that a fraction of the beam will be intercepted by the extraction septum during the kicker rise. The situation becomes worse if the h=16 harmonic is preferred or if the beam is de-bunched: the losses are nearly doubled going from 0.6% of the circulating intensity for h=8 up to 0.9% and 1% for the h=16 and de-bunched cases, respectively [44]. A detailed series of studies has been made by injecting a beam in the SPS with different longitudinal structures from the PS and assessing the dependence of the losses as a function of the harmonic number and of the RF voltage at extraction. The conclusion was that a de-bunched beam is the most suitable for the SPS, even if this choice does not minimize the losses in the PS. Hence, a MTE de-bunched extraction, see Fig. 36 (left), was prepared after the end of the 2008 SPS run, increasing the extracted intensity to 1.4×10 13 protons. The extraction efficiency, expected to be up to 97–98 %, turned out to be on average about 93%, but with peaks up to 99%. These fluctuations were correlated with beam instability due to a slightly negative value of the chromaticity just prior to the resonant crossing. For the nominal extraction case, with an efficiency of about 98%, the beam loss pattern was compared with the five-turn extraction currently in use [48]. As expected, (see Fig. 36, right), the MTE beam losses are concentrated in the extraction region, whereas for the CT, losses of typically 5–6% of the circulating intensity are spread out over the entire machine circumference [48]. These losses are generated by the interaction of protons with the blade of the electrostatic septum in SS31. Moreover, particles scattered at the septum generate losses in the region between SS40–45 and SS72–76. During the last part of the CNGS run, it was possible to inject a MTE, h=16 bunched beam in one of the CNGS SPS cycles. The total intensity injected was about 1.4×10 13 protons. The two batches, each one of about 0.7×10 13 protons, had the last PS ejected turn with a larger intensity than the other four. The proton beam in the SPS could be injected, accelerated, and extracted towards the CNGS target. Neutrinos were also produced during the last night of the 2008 CNGS run. 30
Fig. 36: Intensity of a MTE de-bunched beam extracted over five turns measured by a transformer in the PS–SPS transfer line (left). Beam losses at each PS straight section for the MTE beam compared to a CT extracted beam (fixed target physics) for the same intensity (right). The sizeable reduction is apparent. The target for the 2009 physics run was to complete the commissioning, in particular cure the instability and the ensuing emittance blow-up, prepare a debunched version of the MTE beam, and improve the beam sharing among the five beamlets. In the second half of the 2009 run it was possible to deliver a beam to the SPS for one of the CNGS cycles. The intensity delivered was about 1.6×10 13 p per PS extraction. By the end of the 2009 CNGS run about 4.7×10 17 p were delivered via the MTE beam. The intensity sharing was improved continuously during the 2009 run, introducing the correction of the non-linear coupling generated by the MTE octupoles, using the string of octupoles normally used to combat beam instabilities. Then, the last bit was achieved by means of the transverse damper that improved the sharing so that the target of 20% was reached. In Fig. 37 the impact of the beam excitation via a signal generated synthetically and sent to the damper kicker is clearly seen. Intensity (10 10 ) 20 18 16 14 12 10 8 6 4 2 Single frequency (average over 16 data sets) No excitation (1 data set) Noise (average over 9 data sets) 0 836.001 836.003 836.005 836.007 836.009 836.011 836.013 Time (ms) Fig. 37: Extracted spill profile for no beam excitation, excitation with a single frequency at the tune value, excitation at the tune value with noise. The difference is apparent. It is worth mentioning that a number of instabilities (microwave, and coupled-bunch longitudinal of quadrupolar mode) were observed during the MTE commissioning in 2009. The first one required the beam to be debunched only at the very end, just prior to extraction, as can be seen in Fig. 38. 31
Page 1 and 2: Beam extraction 1 Introduction In t
Page 3 and 4: high-quality cable used for the PFN
Page 5 and 6: 3.1 Integer resonance extraction 3.
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Beam extraction - 50th anniversary of the CERN Proton Synchrotron

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