Proton Beam Line:Target station E to SINQ

This section of the 590 MeV Proton Beam Line Channel is 55 meters long and its main components are 4 bending magnets, 12 quadrupol lenses, 9 (horizontal and vertical) profile monitors (no beam position monitors available for this proton beam line), 5 horizontal steering magnets, 7 collimators and 2 pairs of slits. The goal is to reshape the proton beam after the passage through target E (4 or 6 cm [1]) with collimators and slits and transport the remaining part of the proton beam (57 % for 6 cm or 70 % for 4 cm carbon at the Target station E) to the SINQ target (see also proton beam line Target station E -> Beam Dump) with as little losses as possible in the second half of the proton beam line.
Optically this proton beam line consists of a long stretched "Z" and a flat "U". Because the protons have to enter the SINQ target station perpendicular from below the target, the whole proton beam line is turned into a vertical plane. Just under the target 3 collimators prevent the back scattered neutrons from activating the proton beam line components below and avoid the deposition of protons at the rim of the entrance window of the SINQ target. Electrodes mounted in front of all 3 collimators (MHB's) allow monitoring position and width of the proton beam.
This proton beam line consists of 2 locally separated parts, the first behind target station E is in the experimental hall, the second one in a 11 m deep channel below the SINQ target hall. Both parts are connected together via a 12 m long drift tube embedded in a tunnel inclined by 28 degrees against the horizontal. See Fig.1 (43 kB) for more details. The projected emittance in both directions are given by the blow up at target E and the subsequent trimming through an elliptical collimation system (14 mmmrad in x and 18 mmmrad in y). The trimming of the proton beam by the collimators can well be seen on the measured profiles (1 horizontal- and 2 vertical-cut). The shown profiles are typical for this proton beam line and all but gaussian in shape (specially MHP55 in x, Fig.2: 8 kB ).
The shown envelope fit displays the proton beam at 900 µA during the commissioning of this new proton beam line on Dec 4, 1996 (see Fig.3: 22 kB ). (Note that x and y half-panes are interchanged here.) In order to get this relatively good fit, the effective length of the first 4 quadrupoles (QHGs) had to be changed from 630 (stemming from field measurements) to 644 mm. This 2 % stretching of the field length is probably due to the iron of the steering magnets (SHGs), which are squeezed between the two quadrupoles forming a duplet. How well the parameterization of this proton beam line is understood is demonstrated in Fig.4 (23 kB) which shows the results of a proton beam centroid shift fit produced by varying the currents through the bending magnet AHL and the steering magnet SHG21x and by using the measured changements in proton beam position at the different profile monitor locations (+) as fit constraints.
The proton beam losses at the target station E and the nearby collimators sum up to about 43 % (for 60 mm C target) or 30 % (for 40 mm C target). After the slits KHN21x and KHN22y (which act as proton beam halo scrapers, see Fig.5 [25 kB]) and the long drift tube the sum of the losses along the remaining 25 m of the proton beam line are very low (10 ppm). The losses have been computed with the Monte Carlo program Turtle. The results stemming from 10 million particles passing through the proton beam line are displayed graphically in a logarithmic Excel diagram ( Fig.6: 15 kB ) . It should be mentioned here, that the blow up at target E and the subsequent cut-off at the collimators is required in order to achieve a proton beam diameter of 10 cm at the SINQ target and at the same time low losses down in the proton beam line cellar. Target E thickness reductions from 60 mm to 40 mm makes an operation under the above mentioned conditions still possible, but of course at the expenses of some of the Pion and Myon Physics experiments (of which the event rate is proportional to the length of target E).
Two wide angle pictures taken from the platform on one side of the big 64 degs bending magnet (AHO, weight = 50 tons) are shown in Fig.7 (33 kB) and Fig.8 (34 kB) . They both give some good impression of the component's 3D-arrangement. The bending magnets are blue and the QHI quadrupoles are red and the 3 last ones (QHJ) are nickel plated. In the background one can see the yellow 25 ton crane. Compare also with Fig.1 (43 kB) , where the same color code is used and where nickel plated magnetic components (radiation hardened) are colored in grey.
Some interesting historic facts about the PSI high current proton beam developments and the commissioning of the SINQ in 1996 were presented in an article at the PAC97.
A non-trivial problem of this proton beam line is the fraction of the proton beam, which may miss the target E (lateral dimension = 6 mm) caused by some occasional wrong steering of the proton beam leaving the 590 MeV ring cyclotron. This proton beam will be non-scattered by the graphite material of target E and therefore has more momentum (dp = 21/32 MeV/c for a 40/60 mm long target). Computations show, that this leads unfortunately to 10-20 times higher current density for the non-scattered proton beam component at the SINQ target. To avoid this, an electronic beam transmission device, which monitors the proton beam intensity before and after target station E (MHC4 and MHC5) computes the beam transmission permanently and triggers an interlock signal in case too much proton beam is bypassing target E. In order to get more redundancy in detecting the proton beam bypassing target E, a second method which stops the wrong proton beam at the dispersive focus inside QHJ30 has been realized recently (2003). This redundancy is considered as crucial for the safety requirements during the planned operation of the MEGAPIE experiment.

[1] K. Deiters, F. Foroughi, G. Heidenreich, U. Rohrer, Reduction of Target E Length from 60 mm to 40 mm. PSI Scientific and Technical Report 1999, Volume VI (Large Research Facilities), p. 26-27.

The proton losses along the proton beam line were computed with the computer code Turtle which is available electronically for various PC operating systems.

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Last updated by Urs Rohrer on 28-Jun-2006