CERN-ACC-NOTE-2017-0013 21 February 2017 [email protected]
MD1228: Validation of Single Bunch Stability Threshold &
MD1751: Instability Studies with a Single Beam
L. R. Carver, D. Amorim, N. Biancacci, X. Buffat, G. Iadarola, K. Lasocha, K. Li, T. Levens, E. M´etral, B. Salvant, C. Tambasco (CERN, CH-1211 Geneva 23, Switzerland)
Keywords: CERN LHC, Accelerator Physics - Beam Instabilities, Impedance, Electron Cloud, Beam-Beam Effects
Summary
Instabilities were being routinely observed in B1V during ADJUST. The timing of the instabilities has been localised to shortly after the TOTEM bump has been implemented. The result is emittance blowup which can negatively effect the luminosity output of the fill. This MD aimed to rule out possible sources of the instability (i.e. beam-beam effects or electron cloud) by only taking one single beam to 6.5TeV and going through the full machine cycle. After the implementation of the TOTEM bump, a reduction of the octupole current was performed in order to determine if there was a discrepancy in the threshold between simulations and measurement. As a precursor, the results of the End of Fill MD: Validation of Single Bunch Stability Threshold will also be described.
Contents
1 Introduction 2
2 MD1228: Validation of Single Bunch Stability Threshold 3
2.1 Fill 4804 . . . 3
3 MD1751: Instability Studies with a Single Beam 6
3.1 Fill 5143 . . . 6 3.2 Fill 5144 . . . 10
4 Discussion 12
1
Introduction
Since increasing the number of bunches to 2076, many instabilities have been observed during the ADJUST phase in B1V. With data acquired from the BSRT and the ADTObsBox, it appears that the emittance blowup observed is related to the TOTEM bump that is implemented at the beginning of ADJUST. Figure 1 shows the BSRT data during ADJUST for fill 5093 [1]. Fill 5093 was intentionally delayed in order to allow the time for a full BSRT scan between each stage in the ADJUST process. It allowed any effect from the TOTEM bump to be separated from the separation collapse in the interaction points (IP’s). It is not yet known what is the cause of this instability.
Figure 1: Bunch by bunch emittances for B1V going through ADJUST for fill 5093, which had a slow ADJUST in order to gain more emittance points to separate the effect of each stage. It can be seen that the blowup occurs at a time after the end of the TOTEM bump. Due to the low sampling rate of the BSRT it is difficult to pinpoint exactly when this emittance blowup occurs in the other fills.
The LHC is now operating with a BCMS beam, which has reduced transverse emittance (target emittance of 1.5um/1.5um in H/V). The current machine settings at flat top have not been changed since the emittance of the BCMS beam was reduced. These settings are Q=15/15 and Joct=470A. In the most recent fills prior to the MD (fills 5108-5116) over the weekend of the 23rd July 2016, emittance blowup during ADJUST has been observed in only two of the fills (5110 and 5112). This is troubling, as it means that the instability is not reproducible.
MD1751 conducted two fills to try and determine the nature of the instability observed in ADJUST. In the first fill we filled only B1 with 2076 BCMS bunches and 12 bunches in B2 and performed a full machine cycle with reduced octupoles. This removes beam-beam effects from the model, and if the instability is still observed then it can be concluded that it is not a beam-beam effect. We then performed an octupole scan at the end of this fill which is a continuation of the instability threshold studies performed in 2015 and at the beginning of 2016. In the second fill we filled B1 and B2 (as much as was possible due to problems with the injectors) and then went to ADJUST, performed the TOTEM bump but did not collapse the separation. We then re-performed an octupole scan to try and see if there are any discrepancies.
During commissioning at the beginning of 2016, some time was made available to verify the single bunch stability threshold at flat top. The results of this measurement will be presented first, then the
results from each of these measurements will be described.
2
MD1228: Validation of Single Bunch Stability Threshold
2.1 Fill 4804
Originally scheduled as an end of fill MD, some time was made available during commissioning for a dedicated measurement of the single bunch stability threshold with the new collimator settings for β∗= 3m at flat top.
The measurement was carried out in fill 4804. A single nominal bunch with Nb = 1.2e11 and
x,y = 2um/2um was taken to 6.5TeV and the chromaticity was trimmed to Q0 = 9/8. The octupoles
were reduced from Joct = 280A towards Joct = 0A to see when an instability develops, a technique
described in detail in previous MD notes [2, 3]. An instability was observed at Joct = 76A in both B1H
and B2H, which gives a normalised octupole current of Joct = 63.3A (when considering Nb = 1e11 and
x,y = 2um/2um). This is shown in Fig. 2 and compared to results from DELPHI [4].
Figure 2: Measured single bunch instability threshold compared to DELPHI predictions for flat top in 2016.
Figure 3 shows the acquisitions from the headtail monitor for both B1H and B2H at the time of the instability. It clear that one can infer 2 nodes in the headtail motion in both cases.
The rise times were calculated by fitting an exponential to the amplitude of the BBQ data, and the azimuthal tune shift can be calculated by looking at the waterfall plot. For both cases, it is difficult to ascertain the azimuthal mode number (it is predicted to be 0 but it is not clear as there is no distinct tune line), but rise times of τ = 38s for B1H and τ = 33s can be calculated. The instabilities are of a very similar for both beam 1 and beam 2. These can be seen in Fig. 4 and Fig. 5.
Figure 3: Headtail Monitor acquisitions in B1H (top) and B2H (bottom). Both acquisitions occurred at 05:00:46. In both cases, | l |= 2 (2 nodes) can be seen.
Figure 4: Left: Turn-by-turn data from BBQ with a fit of the rise time of the instability in B1H. Right: A waterfall plot of the FFT of the BBQ data for B1H. This instability could be consistent with a mode (0,2) however a clear tune line cannot be seen.
Figure 5: Left: Turn-by-turn data from BBQ with a fit of the rise time of the instability in B2H. Right: A waterfall plot of the FFT of the BBQ data for B2H. This instability is very similar to the instability in beam 1.
3
MD1751: Instability Studies with a Single Beam
3.1 Fill 5143
The first ramp started at 23:15 on 30-07-2016 and was fill 5143. An overview of the key parameters during this fill can be found in Fig. 6.
Figure 6: Overview of the BBQ activity, octupoles and bump corrector knob (RR57) for B1 (top) and B2 (bottom) during fill 5143. Also labeled are the start and end (S/E) of the squeeze, totem bump and separation collapse in IP1 and IP5.
In this fill, 2076 bunches were injected in injections of 96 bunches (2 x 48) with the standard physics filling scheme. 12 bunches were injected into beam 2 in order to avoid issues with the orbit feedback. The LHC then accelerated the beams to 6.5TeV. Once at flat top, the octupole current was reduced from the usual operational value of 470A to a 376A. This was to make the machine more sensitive to the small perturbations that could cause an instability. The machine then went through the full physics cycle, including the ’separation collapse’ in IP1 and IP5 (despite there being no collisions). After the BSRT had given been given enough time to perform a full scan, it was observed that there had not been any emittance blowup in B1V. Figure 7 shows the emittances in horizontal and vertical for beam 1 after a few BSRT scans in ’Stable Beams’.
At this point, the octupole currents were slowly reduced in the machine in order to determine the octupole threshold. The first octupole reduction was performed at 02:16, and it can be seen from Fig. 8 that there were small losses during the first step. This could be due to losing particles in the tails that
Figure 7: Beam 1 horizontal and vertical emittances after waiting for approximately 10 minutes after reaching ’Stable Beams’. No emittance blowup occurred during ADJUST.
Figure 8: Losses from the FBCT for beam 1. The first step in octupole reduction was performed at 02:16. No further losses were observed at all throughout the remainder of the octupole reduction.
have had time to populate after having constant octupole currents for some time, then when the first step occurs, some particles are lost. No more losses were observed at all throughout the rest of the octupole reduction.
In steps, the octupole current was reduced to 0A without losses being observed and without significant instabilities. Upon reaching 200A, the octupole currents in beam 2 were not reduced further through a
fear of dumping both beams due to instabilities in beam 2. Due to time constraints it was not possible to wait for full BSRT scans after each step as the octupoles approached 0A (approximately 5 minutes for one BSRT scan of 2076 bunches). After reaching 0A, the BSRT scan revealed that there had been some blowup in the horizontal plane. This can be seen in Fig. 9. As the blowup can be seen only in the final scan, it can be traced back (using the approximate 5 minute sampling of the BSRT) to conclude the blowup happened at sometime between 66A and 0A.
Figure 9: Beam 1 horizontal and vertical emittances after the octupoles currents have been reduced to 0A. Some emittance blowup can be seen on several bunches in horizontal.
Throughout the fill, the ADTObsBox was constantly triggered in B1. Figure 10 shows a plot of the amplitude of the tune peak for each acquisition and it is plotted for the bunches with the highest cumulative bunch amplitude over all acquisitions. The black dashed line represents the start of the TOTEM bump. It can clearly be seen that for a period of about 10 minutes after the TOTEM bump, there was no activity on the individual bunches. This is also shown by the overview plot seen in Fig. 6, shortly after the separation collapse there is BBQ activity.
We remain at 0A for approximately 8 minutes, before dumping the beams in order to prepare for the next fill.
Figure 10: ADTObsBox data showing the tune amplitude of the most active bunches as a function of time. The dashed lines are the start and end of the TOTEM bump.
3.2 Fill 5144
The plan for fill 5144 was to now fill the machine with 2076 bunches in both beams, go to end of squeeze, perform the TOTEM bump and then reduce the octupoles without collapsing the separation in any IP. The reasoning for this was to create a condition that is unique and can not be repeated unless given dedicated time. However, due to limited time and issues with the injectors the machine was filled as much as possible (924b per beam) and the MD was continued with these settings.
A full overview of the fill can be found in Fig. 11. Standard settings for physics were again used (Q0 = 15/15 with slightly reduced octupole of Joct= 376A).
Figure 11: Overview of the BBQ activity, octupoles and bump corrector knob (RR57) for B1 (top) and B2 (bottom) during fill 5144. Also labeled are the start and end (S/E) of the squeeze and totem bump. After performing the TOTEM bump, the BSRT performed a full scan and it was observed that there was no emittance blowup in B1V. The emittance scans for both B1 and B2 can be found in Fig. 12 and Fig. 13 respectively.
The octupole currents were then reduced in order to see if there is a discrepancy in the stability thresholds for single beam versus two beams. The octupoles were reduced in small steps until they reached 0A, at which point both beams were still stable with no emittance blowup or losses being observed at any point.
Figure 14 shows the amplitude of the tune peaks found in the ADTObsBox data, and is plotted for the bunches with the highest cumulative amplitudes over all acquisitions. In this case, it can be seen that shortly after the TOTEM bump there is activity on some of the bunches in the 6th injected batch
Figure 12: Beam 1 emittances for horizontal and vertical from the BSRT after the TOTEM bump. No blowup was observed.
Figure 13: Beam 2 emittances for horizontal and vertical from the BSRT after the TOTEM bump. No blowup was observed.
of 96b. However, care must be taken not to draw strong conclusions and say the B1V blowup is caused by beam-beam effects, as no blowup was observed in this case.
Figure 14: ADTObsBox acquisitions during fill 5144. Plotted are the tune amplitudes as a function of time for the most active bunches. The black dashed lines are the start and end of the TOTEM bump.
4
Discussion
There are two possible stabilising mechanisms at EOS that are not present at flat top. The first is that the lattice contributes a non-negligible Q” from the lattice. Q” creates a tune spread that depends on the momentum deviation of the particle which can provide Landau damping, but is also a source of chromaticity so it can also change the interaction between the beam power spectrum and the machine impedance, which changes the coherent tune shift.
Another possible mechanism is the non-linearities in the IR’s which are probed to high amplitudes when at β∗ = 40cm. This provides a source of amplitude detuning and can introduce a spread in the bunch even with Joct = 0A. Measurements have shown that the equivalent spread is proportional to
approximately 100A in the Landau octupoles, which is enough to stabilise the single bunch impedance instabilities.
Future MD’s will aim to try and disentangle between the two effects, but it is clear now that we are in a regime where Q” and non-linearities are not negligible effects.
5
Conclusions
The single bunch stability threshold was measured at flat top and shown to be in good agreement with the prediction from DELPHI [2, 5].
The operational instabilities in B1V could not be reproduced during the MD, so the nature of these instabilities is yet to be determined. However, it was found that at β∗ = 40cm the full single beam and 924b in both beams were stable for Joct = 0A. This exact stabilising mechanism requires further
investigation. Activity was seen in some of the bunches shortly after the bump only in the presence of 2 beams, but this is not enough alone as no emittance blowup was observed.
Acknowledgments
The authors would like to thank the MD coordinators and operators for their help in carrying out the measurements.
References
[1] L.R. Carver, C. Tambasco, ’ADJUST instabilities’, HSC Section Meeting, 08-08-2016
[2] L.R. Carver et al, ’MD 346: Summary of single bunch instability threshold measurements’, CERN-ACC-NOTE-2016-0002, 08-01-2016
[3] L.R. Carver et al, ’MD 751: Train Instability Threshold’, CERN-ACC-NOTE-2016-0004, 08-01-2016 [4] N. Mounet, ’DELPHI: an Analytic Vlasov Solver for Impedance-Driven Modes’, HSC Section
Meet-ing, 05-07-2014
[5] L.R. Carver et al, ’Instabilities and beam induced heating in 2015’, 6th Evian Workshop, .
