Dorn, C.

Forck, P.

Kester, O.

Kowina, P.

Zwicker, B.

GSI Repository

Retrofitting of a non-invasive Bunch Shape Monitor for GSI LINACs∗B. Zwicker1,2, C. Dorn1, P. Forck1,2, O. Kester1,2, and P. Kowina11GSI Helmholtzzentrum, Darmstadt, Germany; 2Goethe Universita¨t, FrankfurtIntroductionWithin the FAIR-Project a proton LINAC [1] is sched-uled as a new injector for the SIS18. A non-invasive BunchShape Monitor (BSM) is foreseen to determine the longi-tudinal bunch structure with a phase resolution of 1◦, withrespect to the 325 MHz acceleration frequency. It is in-tended to ensure proper longitudinal matching of the ac-celerating structures. The presented device is based on thecreation of secondary electrons by the ion beam passinga section of high local nitrogen pressure. The secondaryelectrons are accelerated by an external driving potential to-wards a time-resolved imaging system to obtain the bunchtime structure [2].Compensation of Beam DeflectionThe applied driving potential has unwanted effects onthe ion beam. For an 11.4 MeV/u beam with an appliedvoltage of -31 kV the beam deflection goes up to 0.7 mrad(for protons) according to CST based calculations. For thepresent monitor location at the transfer channel a signifi-cant fraction of the beam is lost [3]. To overcome this flaw,two additional electrodes have been designed and build tofit in the existing vacuum chamber. To design an appropri-ate electrode geometry a field simulation was performed,using CST finite element code.Figure 1: The upper image shows a CST simulation of thepotential distribution at the symmetry plane along the beamaxis. The lower image shows a calculation of the E-fieldalong the beam axis.∗Supported by EU-Project CRISP, WP3 T1 Non-intercepting BunchShape MonitorsThe most advanced design for optimal compensation is anadditional Field-Box with the same geometry split in themiddle into two and to put one in front and one behindthe original Field-Box. Due to the lack of available in-sertion space inside the BSM vacuum chamber, this solu-tion is not an option. A more sophisticated design basedon CST simulations was chosen, which stays as close aspossible with the optimal layout (see Fig. 1). Finally theselected design is a compromise of two contrary objec-tives, namely achieving a sufficiently homogeneous highfield strength with a fixed voltage of -31 kV and leavingenough space in a symmetry axis coaxial to the beam axis,at least 55 mm in diameter (iris size in front of the BSM).This does forbid two capacitor plates with a fixed distance.For sufficient compensation the distance between the ca-pacitors plates is below 40 mm which allows a maximumfield strength of 620 V / mm in comparison to the Field-Box with 420 V / mm. In addition the capacitor plates arebended in the middle to allow the beam free pass within a60 mm diameter (see Fig. 2).The achieved compensation at 5 mm around the beamaxis is about 99 %, decreasing with distance to the axis.Within a 40 mm diameter the deflecting is still suppressedby 90 %, which corresponds to a remaining deflection an-gle of 0.07 mrad.Figure 2: Two compensation electrodes installed in frontand behind of the original Field-Box, the red arrow indi-cates the beam path.References[1] L. Groening et al., LINAC’12, Tel-Aviv, THPB034, p. 927[2] P. Forck et al., DIPAC’05, Lyon, p. 48 ,[3] B. Zwicker et al.,IBIC’13, Oxford, MOPC36GSI SCIENTIFIC REPORT 2013 FG-UNILAC-13doi:10.15120/GR-2014-1-FG-UNILAC-13 FAIR@GSI 309

Retrofitting of a non-invasive Bunch Shape Monitor for GSI LINACs

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Retrofitting of a non-invasive Bunch Shape Monitor for GSI LINACs

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