Emittance blow-up and loss maps in LHC using the transverse damper as exciter

Transcription

1 CERN-ATS-Note MD (LHC) September 5, 2014 Emittance blow-up and loss maps in LHC using the transverse damper as exciter W. Hofle, D. Valuch, R. Assmann, S. Redaelli, R. Schmidt, D. Wollmann, M. Zerlauth Keywords: Transverse damper, blow-up, loss maps Summary CERN-ATS-NOTE /03/2012 The transverse damper in LHC can be used to excite transverse oscillations in both planes for various purposes. A dedicated firmware has been developed that permits to injects a band limited noise signal into the damper feedback loop that can also be gated synchronously with the revolution frequency in order to act only on a short portion of the beam, for example a single batch. The method has been developed with the aim of blowingup transversely the beam principally for the purpose of efficiently generating in a well controlled way so called loss maps which are an integral part of the verification of the collimation system. The method can also be used to shape the transverse emittance for other purposes and, at low intensity, to probe the machine mechanical aperture or for producing fast and high losses to probe the loss level at which magnets quench. The present MD report summarizes the results of a test on August 26, 2011, demonstrating the method and its selectivity to batches, showing the equivalence of the loss maps with loss maps obtained using the classical method of crossing the 1/3 order resonance and testing a blow-up up to the transverse aperture limit. Contents 1 Introduction 2 2 Experimental Setup 2 3 Beam conditions 2 4 Emittance evolution 2 5 Loss maps 3 6 Next steps 6 1

2 1 Introduction It is highly desirable to replace the current method of checking the collimation efficiency, the production of losses by crossing the 1/3 order integer resonance, by a more controlled method that permits to obtain similar loss maps in a controlled and safe way in the presence of the full LHC beam at top energy. An initial test on May 7, 2011 [1] showed that with noise, band limited in frequency to 2.5 MHz, and applied to all dampers of one plane (beam 1, vertical dampers were used in these tests) beam can easily and rapidly be lost in LHC with residual intensities below what can be detected. Following these encouraging tests a dedicated firmware was developed to generate the excitation file inside one of the damper FPGAs which also opened the possibility to gate the excitation to only act on part of the circulating beam using the knowledge of the bunch One position available on the FPGA [2]. A procedure had been written prior to the MD to obtain approval from rmpp [3]. In the following we report on the results of the MD that was carried out on August 26, Experimental Setup Due to a limitation with the front-end hardware a stand-alone VME crate had to be used, with a dedicated damper signal processing VME board and associated software for the generation of the excitation signal. The excitation signal was added to the damper feedback loop in the analog domain. Due to these limitations the blow-up was only available for one beam at a time, and beam 2 had been chosen for the set-up of the correct timing delay with respect to the bunch One position. Following the successful tests during the MD and the subsequent migration of the damper front-end software to the new LINUX standard, an operational system for the blow-up is planned to be deployed in all four damper VME crates for the 2012 run. Linear shift registers are used to generate a 14 bit pseudo random number every 40 MHz clock cycle. The resultant time sequence has a flat frequency distribution and can be shaped with bandpass or low-pass filter. During the MD two sections of 2 nd order IIR filters were used which limited the bandwidth effectively to 500 khz, well within the frequency limits within which a high kick strength is available from the damper system. Note that gating of the noise signal to part of the beam creates side bands in the spectrum (folding with sifunction). MD results and some details on the algorithm have been presented at the LHC Studies Working Group meeting on September 13, 2011 [4]. 3 Beam conditions The circulating beam current during the beam tests is given for both beams in Fig. 1. The inital setup period with up to 2 individual probe beam injections (top graph) was followed by measurements with two 6-bunch trains (50 ns spacing) and individual bunches for loss map tests (bottom graph). The filling pattern used for the measurements with bunch trains is shown in Fig. 2, where the single bunch intensities from the fast beam-current transformers are given as a function of the RF bucket number. The evolution of the single bunch intensity throughout the test duration is given in Fig. 3. Relative losses of the bunches in the first train are shown in Fig. 4. The figures of beam losses per bunch proof the efficiency of the gating. 4 Emittance evolution The analysis of emittances during the experiment also shows, as depicted in Fig. 5, that the bunches not targeted by the blow-up preserve their transverse emittance, while the emittance of the bunches 2

3 Total beam intensity [ p ] Beam 1 Beam :00 02:00 03:00 04:00 Time [hh:mm] Total beam intensity [ p ] Beam 1 Beam :00 05:00 06:00 07:00 Time [hh:mm] Figure 1: Circulating beam current during the MD of August 26 th, 2011: initial setup with up to two individual probe bunches (top) and tests with 12-bunch trains and single nominal bunches for loss map tests (bottom). The blow-up technique was available for beam 2 only. targeted is increased by a large amount. It should be noted that the gap between batches has been chosen to correspond to the normal spacing of 1 µs. With smaller gaps the selectivity would decrease due to the limited bandwidth up to which the full damper kick-strength is available. Fig 6. shows emittances at the end of blow-up as measured by the synchrotron light monitor. 5 Loss maps Table 1 summarizes the different loss maps that were performed during the MD. Figures 7, 8 and 9 show the absolute loss distribution around the whole LHC for case (1), case (7) and case (4). Taking into account the different loss rates during the experiments the loss pattern look very similar. This 3

4 15 Single bunch intensity [ p ] RF bucket number Figure 2: Single bunch intensity as a function of the RF bucket number for the blow-up measurements with 50 ns bunch trains. A special filling pattern with 2 trains of 6 bunches was achieved by using one PS-booster ring only out of the 2 used for the standard 12b trains (double-batch mode). The spacing between trains was 975 ns. The probe beam in bucket 1 was kept in the machine during the tests. Bunch intensity from fast BCT [ p ] Bucket 01 Bucket 21 Bucket Bucket 61 Bucket 81 Bucket 11 04:30 05:00 05:30 06:00 06:30 Time [ hh:mm ] Bunch intensity from fast BCT [ p ] Bucket 1491 Bucket 1511 Bucket Bucket 1551 Bucket 1571 Bucket :30 05:00 05:30 06:00 06:30 Time [ hh:mm ] Figure 3: Single bunch intensity as a function of time for the 6-bunch train affected by the blow-up (left) and for the one un-affected (right). result is confirmed by a comparison of the normalized losses around IR7 for case (7) (horizontal blow-up of 1 nominal bunch with the ADT) and case (4) (crossing of the horizontal third integer tune resonance) in Figures and 11. The variation of the normalized losses in the locations with the highest losses to cold magnets in the DS downstream of IR7 (MB.19L7.B2, Q8L7.B2, Q19L7.B2) were 15%. This can also be seen in Figure 13, which compares the normalized losses. It has been therefore shown that the loss pattern generated by blowing up the bunch with the ADT reproduces well the loss pattern achieved by crossing the third integer tune resonance. Figure 14 compares the beam loss signals measured at the horizontal primary collimator versus time for the cases (4) to (8) in Table 1. It can be seen that the time structure of the losses is very similar for the tune resonance and the cases with damper off. The losses reach comparable absolute values with a very similar delay after the start of the excitation. The two cases with damper on show lower 4

5 1 Relative bunch intensity losses Bucket 01 Bucket Bucket 41 Bucket 61 Bucket 81 Bucket :30 05:00 05:30 06:00 06:30 Time [ hh:mm ] Figure 4: Relative bunch losses for the 6 bunches of the train affected by transverse blow-up. The step in intensity that occurs at 05:54 for the last bunch corresponds to an attempt to generate loss maps. Figure 5: Evolution of bunch emittance as measured by the synchrotron light monitor during the blow-up measurements with two 6-bunch trains. Horizontal (top) and vertical (bottom) emittances are given (Courtesy of F. Roncarolo, BE-BI). 5

6 Figure 6: Bunch emittance as measured by the synchrotron light monitor at the end of the blow-up measurements with two 6-bunch trains (Courtesy of F. Roncarolo, BE-BI). Table 1: Relevant times of loss maps performed during the blow-up studies and beam and blow-up parameters. Num. Time Plane Method Duration N b Amplitude (1) 05:54 H Blow-up 5 s 12 Full excitation (2) 06:08 V Blow-up 8 s 12 Full excitation (3) 06: V Blow-up 8 s 12 Full excitation (4) 06:45 H Resonance 8 s 1 no excitation (5) 06:56 H Blow-up 8 s 1 Full excitation, FB on (6) 07:12 H Blow-up 8 s 1 Half excitation, FB on (7) 07:17 H Blow-up 8 s 1 Full excitation, FB off (8) 07:29 H Blow-up 8 s 1 Half excitation, FB off absolute losses and a slower build up of these losses. 6 Next steps Following the successful MD a fully operationally system is planned to be deployed for both beams for the 2012 run. A software control interface needs to be defined and implemented that will permit to control the essential parameters and allow initiations of a loss map with unsafe, full intensity beam at top energy. This will certainly require additional tests for validation with low intensity beam. Similarly procedures have already been set-up [5] for a quench test. For aperture measurements a dedicated MD could not yet be performed but is planned for Results from the present MD, as reported in the LHC studies group [4], demonstrated that a low intensity beam indeed can be blown up to the aperture limit a promising outlook. 6

7 0 IP2 IP3 IP4 IP5 IP6 IP7 IP8 IP1 Absolute losses (noise subtracted) Beam losses [ Gy/s ] Figure 7: Loss maps obtained with the blow-up method with 12 bunches in the machine (case (1) in Tab. 1, gated to six bunches) 0 IP2 IP3 IP4 IP5 IP6 IP7 IP8 IP1 Absolute losses (noise subtracted) Beam losses [ Gy/s ] Figure 8: Loss maps obtained with the blow-up method with 1 nominal bunch in the machine (case (7) in Tab. 1) 7

8 0 IP2 IP3 IP4 IP5 IP6 IP7 IP8 IP1 Absolute losses (noise subtracted) Beam losses [ Gy/s ] Figure 9: Loss maps obtained by crossing the third-order resonance with one nominal bunch in the LHC (case (4) in Tab. 1) References [1] LHC-RF and LHC-OP log book, May 7, 2011, [2] D. Valuch, Transverse feedback: high intensity operation, cleaning, lessons for 2012, 2011 Evian Workshop on LHC Beam Operation, Evian (2011) [3] W. Höfle et al., controlled transverse blow-up with ADT, EDMS Document No , [4] W. Höfle, Controlled transverse blow-up with ADT, LHC Studies Working Group, September 13, [5] S. Redaelli et al., Quench Margin at 3.5 TeV, EDMS document No , 8

9 1 0 IP7 Relative beam loss rate IP7 Relative beam loss rate Figure : Relative losses in IP7 for the cases (7) and (4) in Tab. 1, normalized to the peak loss at the primary collimators: blow-up (top) and resonance (bottom) methods. 9

10 Relative beam loss rate Relative beam loss rate Figure 11: Relative losses in the dispersion suppressor left of IP7 for the cases (7) and (4) in Tab. 1, normalized to the peak loss at the primary collimators: blow-up (top) and resonance (bottom) methods.

11 0 0 Relative losses Relative losses TCP.A6R7.B2 TCHSV.6R7.B2 TCP.B6R7.B2 TCP.C6R7.B2 TCHSH.6R7.B2 TCHSS.6R7.B2 TCSG.A5R7.B2 TCSG.E5L7.B2 TCSG.6L7.B2 TCSG.A4R7.B2 TCLA.A6L7.B2 TCSG.B5R7.B2 TCSG.D5L7.B2 TCSG.B4R7.B2 TCSG.A4L7.B2 TCSG.B5L7.B2 TCSG.D4R7.B2 TCSG.4L6.B2 TCSG.4L6.B2 TCP.D6R7.B2 TCLA.B6L7.B2 TCLA.D6L7.B2 TCLA.C6L7.B2 TCLA.A7L7.B2 TCDQA.B4L6.B2 TCHSH.6R3.B2 TCP.6R3.B2 TCDQA.A4L6.B2 TCSG.5R3.B2 TCSG.B5L3.B2 TCSG.A5L3.B2 TCSG.4L3.B2 TCLA.A5L3.B2 TCLA.B5L3.B2 TCLA.B7L7.B2 TCP.A6R7.B2 TCHSV.6R7.B2 TCP.B6R7.B2 TCHSH.6R7.B2 TCP.C6R7.B2 TCHSS.6R7.B2 TCSG.6L7.B2 TCSG.A5R7.B2 TCLA.A6L7.B2 TCSG.E5L7.B2 TCSG.A4R7.B2 TCSG.B5R7.B2 TCSG.D5L7.B2 TCSG.B4R7.B2 TCSG.B5L7.B2 TCSG.A4L7.B2 TCSG.D4R7.B2 TCSG.4L6.B2 TCSG.4L6.B2 TCLA.B6L7.B2 TCP.D6R7.B2 TCLA.D6L7.B2 TCLA.A7L7.B2 TCLA.C6L7.B2 TCHSH.6R3.B2 TCP.6R3.B2 TCDQA.B4L6.B2 TCDQA.A4L6.B2 TCSG.5R3.B2 TCSG.B5L3.B2 TCSG.A5L3.B2 TCSG.4L3.B2 TCLA.A5L3.B2 TCLA.B5L3.B2 TCLA.B7L7.B2 TCLA.7L3.B2 Figure 12: Fractional distribution of losses at the B2 collimators measured during loss maps with the resonance (left) and blow-up (right) methods (cases (4) and (7) of Tab. 1). Relative losses Relative losses 06L7.B2I 06L7.B2I20 06L7.B2I30 07L7.B2I 07L7.B2I20 07L7.B2I30 08L7.B2I 08L7.B2I20 08L7.B2I30 09L7.B2I 09L7.B2I21 09L7.B2I22 09L7.B2I30 11L7.B2I 11L7.B2I20 11L7.B2I30 12L7.B2I 12L7.B2I20 12L7.B2I30 13L7.B2I 13L7.B2I20 13L7.B2I30 06L7.B2I 06L7.B2I20 06L7.B2I30 07L7.B2I 07L7.B2I20 07L7.B2I30 08L7.B2I 08L7.B2I20 08L7.B2I30 09L7.B2I 09L7.B2I21 09L7.B2I22 09L7.B2I30 11L7.B2I 11L7.B2I20 11L7.B2I30 12L7.B2I 12L7.B2I20 12L7.B2I30 13L7.B2I 13L7.B2I20 13L7.B2I30 Figure 13: Fractional distribution of losses in the magnets of the dispersion suppressor left of IR7 measured during loss maps with the resonance (left) and blow-up (right) methods (cases (4) and (7) of Tab. 1). 11

12 Beam losses at the TCP-H [ Gy/s ] 3rd order resonance Damper ON, full blow-up strength Damper ON, half blow-up strength Damper OFF, full blow-up strength Damper OFF, half blow-up strength Time [ s ] Figure 14: BLM signal as a function of time measured at the primary collimators during different types of loss maps (see Tab. 1). 12