MD456: Monitoring of abort gap population with diamond particle detectors at the BGI in IP 4

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1 CERN-ACC-NOTE February 15, 016 MD56: Monitoring of abort gap population with diamond particle detectors at the BGI in IP Oliver Stein / TE-MPE Keywords: abort gap, beam gas interaction, BGI, beam losses, diamond detectors Summary In this MD, diamond based particle detectors (dblm) were used for measuring showers of the beam interactions with the in the BGI induced neon gas. This setup was proposed in a feasibility study for using dblms at the BGI to measure the abort gap population by detecting the beam gas interactions. During the MD neon gas was induced in the BGI vacuum chamber to increase the interaction rate. Two nominal bunches were injected and accelerated up to 6.5 TeV. The measurements lasted for 10 minutes. The bunches could be clearly identified. But the resulting count rate of the beam gas interactions was a factor 70 lower than predicted by the feasibility study. In addition, a problem with the timing information lead to a widening of the histogram peaks. CERN-ACC /0/016 1 Introduction At 7 TeV and 808 bunches per beam, 36 MJ are stored in each circulating LHC beam. In case of a failure and at the end of physics ptoduction the beams have to be safely extracted without harming the machine. For this purpose fast extraction kicker magnets (MKD) are installed in IP 6 at the LHC which deflect the beams into the dump lines. At the end of these lines the high energetic beams are absorbed by the dump block. The extraction kickers have a rise time of about 3 µs. Particles passing the MKDs during this time are deflected with an incorrect angle. This causes beam losses, which can quench downstream magnets and could in a very unlikely case lead to damage of accelerator equipment. To avoid these losses a 3 µlong particle free gap, the so-called abort gap (AG), has been introduce into the filling pattern of the LHC beams. In general the beam abort is synchronised to this gap so that during the MKDs rise time the intensity of passing particles is very 1

2 small. To guarantee a safe beam abort the particle intensity in the AG is monitored constantly by the synchrotron radiation monitor (BSRA). If the abort gap population exceeds a certain threshold countermeasures are induced to reduce the AG population. An alternative method to monitor the abort gap population was proposed in a feasibility study in 015 [1]. Using diamond based particle detectors (dblms) for counting beam gas interactions at the beam gas ionisation monitor (BGI) in IP, the abort gap population can be calculated. At the BGI neon gas can be introduced into the BGI vacuum chamber which increases the beam gas interaction rate and leads to a higher count rate. The proposed setup was realised and tested in Spring/Summer 015. The installed diamond detectors provide nano-second time resolution which allows to detect intra bunch losses at a bunch spacing of 5 ns. Measurement setup and detector position Two dblm detectors were installed at the BGI in IP. One detector is positioned upstream of the BGI and used to measure the background of beam gas interactions. The second is placed downstream of the BGI vacuum chamber. The optimal detector positions were determined by FLUKA simulations [],[3]. nd. dblm 1st. dblm Figure 1: FLUKA dose simulation of the beam gas interaction and indication of the detector positions. In figure 1 the deposited dose of the beam gas interactions downstream of the BGI are displayed. It is clearly visible that the highest dose levels are close the beam pipe. To reduce crosstalk from the second beam the detector was placed beside the beam pipe at y = 0 cm. In this case the beam pipe partially shields the detector from losses from the second beam. The installed detectors are single crystalline CVD diamond detectors (BCM1FLHC) with an active volume of 5 x 5 x 0.5 mm 3 []. The detectors are operated with bias voltage of 1 V per µm. The signal is transmitted via a fibre optical link to the data acquisition system which is installed in bdg As DAQ the CIVIDEC ROSY system is used [5]. This system records the losses in a histogram like style by dividing the LHC turn (given by the LHC turn clock) into bins of 1.6 ns length. Every time the signal exceeds the set threshold the number of counts in this specific bin is incremented. This allows the accumulation of losses over a long time.

3 3 Measurements The goal of the measurements was to identify the circulating bunches and to verify predicted count rates. Before the start of measurements the gas pressure in the BGI was increased injecting the neon gas to.9e-9 mbar. One pilot (9.5e10 protons) and two nominal (1.15e11 protons) bunches were injected and accelerated to 6.5 TeV. The measurements lasted for 10 minutes. The resulting histogram is displayed in figure. The two circulating bunches are clearly visible. The small accumulation of counts around 69 µs indicates the position of the pilot bunch st. bunch nd. bunch Counts (#) 6 pilot bunch Time (µs) Figure : Histogram of the detected beam losses. In total 8 beam gas interactions were recorded during the measurement. This is about a factor 70 lower than predicted by simulations. Different effects can contribute to this discrepancy between feasibility study and measurements. Small differences between the geometries in the FLUKA model and reality may cause a reduction in the count rate. Furthermore, the beam ionises the neon gas which can lead to a pressure decrease close to the beam. This would then result in a drop of beam gas interactions. This effect had not been taken into account in the simulations. Further studies are needed to quantify these effects and identify other sources for these discrepancies. Following the specifications of the detector and the readout electronics the width of a nominal LHC bunch should be 3 - bins. In figure 3 the first peak of the histogram is shown. The peak has a width of about 100 ns (6 bins), which is much wider than expected. During the measurements the beam diagnostics did not show any irregularities. Thus, this broad peak is most probably due to a problem with the timing used for the histogram. Further investigations are onging to identify and mitigate this problem. 3

4 10 Counts (#) ns Time (µs) Figure 3: Zoom into the first peak. Conclusion The presented results show that the detection of the beam gas interactions at the BGI in IP are feasible. The two circulating bunches could be clearly identified. Nevertheless the count rates deviated significantly form the predictions. This discrepancy between feasibility study and measurements is still under investigation. Additionally it was seen that the bunch peaks were much wider than expected by the DAQ specifications. This indicates a problem with the timing to keep the histogram synchronous to the beam. The repetition of the measurements after fixing the timing issues of the DAQ will give a better picture of the bunch width and intra bunch beam gas interaction rates. The identification of intra bunch losses will be the next step towards the abort gap monitoring with dblms at the BGI. Acknowledgements Thanks to all people who helped preparing the MD. Especially to Daniel Wollmann who supported the MD actively. The MD was in parallel with the MD: TCDQ-TCT retraction and losses during asynchronous beam dump. This parallel work was absolutely flawless and well organised, thanks to C.Bracco et al. A special thank to Marcel Mursy who helped during the measurements and the data analysis after the MD.

5 References [1] O. Stein, F. Burkart, B. Dehning, R. Schmidt, C. Buhl Sorensen, and D. Wollmann. Feasibility Study of Monitoring the Population of the CERN-LHC Abort Gap with Diamond Based Particle Detectors. In Proceedings of IPAC015, Richmond, VA, USA, Richmond, VA USA, 015. [] T. T. Böhlen, F. Cerutti, M. P. W. Chin, A. Ferrari, P. G. Ortega, A. Mairani, P. R. Sala, G. Smirnov, and V. Vlachoudis. The FLUKA Code: Developments and Challenges for High Energy and Medical Applications, 01. [3] A. Ferrari, P. R. Sala, A. Fasso, and J. Ranft. FLUKA: a multi-particle transport code. CERN , INFN/TC 05/11, SLAC-R-773, 005. [] A. Bell, E. Castro, R. Hall-Wilton, W. Lange, W. Lohmann, A. Macpherson, M. Ohlerich, N. Rodriguez, V. Ryjov, R.S. Schmidt, and R.L. Stone. Fast beam conditions monitor BCM1F for the CMS experiment. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 61(3):33 38, mar 010. [5] E. Griesmayer. CIVIDEC,