KHNY30 Slit Collimator in the Proton Beam Line to SINQ.

In order to improve the safety of the planned MEGAPIE experiment at SINQ, an additional method has been proposed to avoid proton beam bypassing target E and perforating the MEGAPIE target with too much non-scattered beam [1]. The functioning of this method is based on the energy dispersion of the 90° bending system consisting of the magnets AHM and AHO and the field-lens duplet QHI29 / QHJ30. A model has been developed to compute the halo at the location of the maximum dispersion (inside QHJ30) as accurately as possible. The required Turtle Input File shows the necessary parameterization of the beam line between Target M and the SINQ target. Results from these computations are shown in Fig. 1 and Fig. 2 below.

Fig. 1: The histogram depicts the computed proton beam distribution in the middle of QHJ30 (the location of the slit) for a well centered beam at target E. The total halo is only of the order of 10 nA for a 1 mA proton beam intensity at the SINQ target. The low-energy halo tail (to the right = upper jaw) is longer than the high-energy halo tail (to the left = lower jaw). The prognosis is that a few nA of beam should already hit the upper jaw even if it is in the out-position. This could also be seen with the first tests with beam. If the lower jaw is put into the working position (-25 mm) only about 5 nA of the proton beam would hit it and produce some permanent but moderate losses around the slit. Fortunately, this region is well shielded by the quadrupole iron surrounding the slit.

Fig. 2: In this histogram the resulting beam at the slit position is shown, if in the model computations the beam on Target E is shifted by 1.5 mm from the center, so that 0.14 % of the protons are bypassing target E. Most of these protons would be hitting the lower jaw, if it is put into the working-position. The charge of the protons hitting the electrically insulated jaw is collected and measured with a MESON unit (see control diagram of Fig. 6). But it should be mentioned here, that the currents measured with the MESON units are at least twice the charge of the stopped protons per time unit, because a lot of electrons are knocked out of the copper jaws and flowing to the surrounding vacuum chamber [2]. The produced beam spill is also ionizing the air inside the closely down streams placed ionization chamber (MHI37) and producing a current signal registered by the LOGCAM2 unit (see control diagram in Fig. 6). Both signals are strong enough to produce already an interlock signal with only 0.1 % of the beam bypassing target E.

Fig. 3: This picture shows a simplified drawing of the cross section through the vacuum chamber of the quadrupole lens QHJ30 with the slit plug-in unit. Fortunately the vacuum chamber of the QHI20/QHJ30 field-lens was build as a so-called 'cross-slit' type chamber as commonly preferred for the pion/muon beam lines. The initial reason for this was the better transmission of the beam halo because of the presence of dispersion inside this lens. This space could now be exploited for the outer parking position of the slit jaws in case they are not used. First experience with the slit jaws in the out-positions showed, that the halo hitting the copper jaws is negligible.

Fig. 4: This 3-dim design drawing shows all the important components of the slit collimator unit. The support tube containing the jaws with the rod linkage and the cables (see Fig. 5) is connected to a vacuum flange with the 2 stepping motors and the vacuum feedthroughs attached to it. The whole unit is plugged into the quadrupole's 'cross-slit' vacuum chamber from the right side. In order to do this, the AHO bending magnet (weight = 60 t) and the adjacent vacuum valve to the right side had to be moved away temporarily.

Fig. 5: This photo shows the inside of the slit plug-in unit in opposite direction of the proton beam. Well visible are the 2 closed copper jaws with the gear needed to open and close them. Also visible are the coaxial cables of the 2 thermo-couples (wound to spirals because of the longitudinal movement of the jaws) and the ceramic-pearls-insulated leads for the current measurement at the jaws.

Fig. 6: This block-diagram shows the principle of the control-electronics for the slit. All units are connected with the control computer via the CAMAC bus. Currents, temperatures, voltages, interlock-limits etc. may be read or set via this bus. Not shown in this diagram are the cables needed for transmitting the interlock signals.

Fig. 7: This screen shot shows on the left side the signals from the ionization chambers along the proton beam line from target E to the SINQ target for a well centered beam. On the right side the same display for a proton beam shifted near the edge of target E so that about 0.1 % of the protons are missing the graphite. The lower KHNY30 jaw is put in working position. The amount of beam bypassing the target E was estimated with the profile monitor MHP56 (see [1] Fig. 3) The signal of MHI37 is very close to the interlock level (relative maximum).

Fig. 8: This screen shot is a control-room display of all relevant control parameters of the KHNY30 slit collimator. Both signals (red columns) MHI37 (=97.7 nA) and KHNY30u.ILOG (=80.4 nA) are higher than the usual interlock levels (they have been temporarily over-bridged for this screen shot).

[1] U. Rohrer, A Novel Method to improve the Safety of the planned MEGAPIE Target at SINQ. PSI Large Research Facilities Scientific and Technical Report 2001, Volume VI, p.34-35
[2] L. Rezzonico, private communication.

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Urs RohrerGraphic Transport Last updated by Urs Rohrer on 25-Jan-2006