June 2022: Bunch-to-Bucket SIS18 -> ESR

Table of Contents

Introduction

A machine experiment is performed on 2 June. The aim of this experiment is to repeat the experiment from 2021 and check reproducibility. UNILAC delivers a beam of ¹⁹⁸Au⁶⁵⁺ @ 10.2 Mev/u which is accelerated to 147.2 MeV/u with SIS18. SIS18 is operated at 1st harmonics (h=1), thus the full beam is extracted in one single bunch. On its transfer to ESR the bunch passes a stripper foil to further strip the ions to hydrogen-like ¹⁹⁸Au⁷⁸⁺; this is associated with some energy loss down to 144.0 Mev/u. The bunch is then injected into ESR, that is operated at 3rd harmonics (h=3).

This setting is not well suited for the experiment, as the bunch from SIS18 is too long to fit into the ESR buckets (moreover, ESR has half the circumference of SIS18). However, this setting is a good test case for a new ability of the b2b system introduced with the 2022 beam-time: Calculation of rf-phase matching is done independently of the actual harmonic settings in the ring, and does no more restrict the choice of harmonic numbers.

Setup

See here.

B2B firmware and software v00.03.18, branch b2b_dietrich_2021-oct-14, commit 83eab29

Settings

• SIS18
  • h=1
  • revolution time 1.434 us
• ESR
  • h=3
  • revolution time 0.721 us
  • ca. 1e7 particles per bunch
• b2b
  • 'beating period' 119 us; this short period is due to the energy loss in the stripper foil
  • 'fine tune' yes ; FT improves extraction precision by checking neighboring rf-cycles around the theoretical beating time: Typically the beating time does not exactly coincide with an rf-cycle; thus one has to calculate which of the neighboring cycles fits better)
  • 'multi-beat tune' no ; MBT is a technique improves phase matching in case of very short beating time: Matching is calculated for a couple of beating periods (1, 2, 3, ...), the results are compared and the best match is chosen.

Data Acquisition

All diagnostic data of the b2b system has been recorded internally.

Diagnostic of the beam has been done using the FCT Ring system that has been developed by the beam instrumentation group (credits to A. Reiter, O. Chorniy, H. Bräuning). FCT GE02DT1FP has been selected as signal source. The FCT system triggers on the cavity signal by ring-rf system and aquires analog data from the FCT. Thus it provides a 'position' of the bunch relative to the rf-signal. This measurement is repeated many times and plotted. Thus, this system allows to monitor the shape and position of bunches relative to the rf-signal as a function of time. An example is given in the figure below.

Figure: Example of a image of the FCT system. x-axis ('bunch-time'): The data shows the position and shape of the bunch relative to the the rf-signal and 'x=0' marks a trigger by the rf-signal from the DDS. y-axis: development of signal with time. Details see text.

The above figure shows the FCT signal for about five revolutions in ESR and displays its evolution for almost half a second. This is the data of one injection into ESR, there is NO summing/averaging over many injections. The beam from SIS18 is injected as 'coasting' beam at an arbitrary phase of the ring-rf, thus every injection looks different. Here, one of the three rf-buckets is filled more than the other two. The ring-rf-voltage and the electron-cooler are switched-on shortly prior to injection. As can be seen, the ions are cooled into center of the buckets and the signal becomes more and more pronounced.

Log

It was discussed with the ESR team how-to and when to do the experiment. Changing the ESR injection from h=3 to h=1 would have required using a different pattern. This was considered out of scope due to the very tight schedule. Nevertheless, it was decided to dedicate a short period of time for a quick test experiment, although conditions were not optimal. The prime aim of this test to verify functionality of true bunch-to-bucket transfer at this beam-time.

The machine experiment itself was performed in the late afternoon and took about one hour only.

time [UTC]	phase difference [ns]	phase difference [°]	pic	remark
15:32:01				change injection at ESR 'coasting beam' -> 'bunch2bucket'
15:33:26	0	0.0	x	weak pattern
15:34:07	0	0.0	x
15:34:48	0	0.0	x
15:35:33	0	0.0	x	there it is
15:36:17	79	39.4	x	looks ok
15:36:54	159	79.4	x	looks bad
15:37:30	159	79.4	x	looks ok
15:38:07	238	118.8	x	looks bad
15:38:44	318	158.8	x	do things repeat every 120 degree (h=3)? yesss, looks ok
15:39:21	318	158.8	x
15:39:58	358	178.7	x
15:41:50	278	138.8	x
15:42:27	278	138.8	x
15:43:05	318	158.8
15:45:08	79	39.4	x	do things repeat every 120 degree? yesss, looks ok
15:46:17	99	49.4	x
15:46:53	99	49.4	x
15:47:30	99	49.4
15:48:06	99	49.4
15:48:44	219	109.3	x	things should look bad with 60 degree? yesss, looks bad
15:49:21	99	49.4
15:50:23	99	49.4	x
15:51:54	219	109.3	x	expected to be bad again, yes!
15:52:30	99	49.4	x	looks ok
15:53:07	107	53.4	x	looks ok
15:53:44	115	57.4	x	looks ok
15:54:27	115	57.4		looks ok
15:55:10	123	61.4	x	looks ok
15:55:53	123	61.4	x	looks ok
15:56:36	131	65.4	x	looks ok
15:58:49	131	65.4	x	looks ok
15:59:30	250	124.8	x	with 60 degree offset, this is expect to look bad, yes!
16:00:11	250	124.8	x
16:01:24	131	65.4	x	good again
16:02:07	131	65.4	x
16:02:49	131	65.4
16:03:32	131	65.4
16:04:16	131	65.4
16:04:57	131	65.4
16:05:40	370	184.7	x	with 120 degree offset, this is expected to look good again, yes!!!
16:06:22	370	184.7
16:07:04	370	184.7
16:07:46	370	184.7
16:08:29	382	190.7		let's do a series from one bucket to the next; 178 -> 298 degree
16:09:11	382	190.7
16:09:53	382	190.7	x
16:10:36	382	190.7		no beam from SIS18
16:11:17	370	184.7		no beam from SIS18
16:12:00	370	184.7		no beam from SIS18
16:12:43	382	190.7		no beam from SIS18
16:13:27	382	190.7		no beam from SIS18
16:14:11	382	190.7		no beam from SIS18
16:14:54	382	190.7		no beam from SIS18
16:15:34	382	190.7		no beam from SIS18
16:16:16	382	190.7		no beam from SIS18
16:16:59	382	190.7		no beam from SIS18
16:17:42	394	196.7	x
16:18:25	394	196.7	x
16:19:08	406	202.7	x
16:19:51	418	208.7	x
16:20:32	430	214.7	x
16:21:13	442	220.7	x
16:21:56	453	226.1	x
16:22:39	465	232.1	x
16:23:22	477	238.1	x
16:24:05	489	244.1	x
16:24:47	489	244.1	x
16:25:29	501	250.1	x
16:26:12	513	256.1	x
16:26:55	525	262.1
16:27:54	525	262.1
16:29:17	525	262.1		no beam
16:30:00	525	262.1
16:30:43	537	268.1	x
16:32:40	549	274.1
16:33:42	561	280.1	x
16:34:41	573	286.1	x
16:35:28	585	292.0	x
16:36:27	597	298.0
16:37:36				change injection at ESR 'bunch2bucket' -> 'coasting beam'

Table: Log of experiment on 2 June 2022. First column: deadline of message CMD_B2B_START. Second column: phase difference between SIS18 and ESR h=1 signals in nanoseconds. Third Column: Same as second column but expressed in degree of the ESR h=1 signal. Fourth column: marks if a screenshot exists.

The log of the experiment is shown above. In the first half it was tried to verify that bunch-to-bucket transfer works as expected. In the second half a systematic test was done, where it was tried to 'push' the bunch from one bucket to the next one.

Results

The main measurement is a series of 24 transfers within about half an hour and is summarized in the figure below.

Figure: Five different transfers from SIS18 to ESR. Revolution time in ESR is 721 us. Using third harmonics, there are three rf-buckets separated by 240ns. The beam is observed using the 'FCT ring' system. y-axis: each of the five figures shows the development of the bunches for 140 ms following the transfer into ESR. x-axis: each row measures the FCT signal for 3ms, which corresponds to roughly four revolutions in ESR. The value of 'phi' in the upper right is the phase difference between the SIS18 and ESR h=1 signals, normalized to the ESR revolution frequency. The five figures show a change of the ESR phase in a range of 178.7+6 to 178.7+114 degree (almost 120 degree). Top to Bottom: The bunch is 'moved' from bucket #1 to bucket #3.

The figure above shows a test measurement where the bunch is moved from one rf-bucket to the next one. Unfortunately, the available data only cover a phase window of 6..114 degree and not the full 0..120 degree. Please note, the the numbering of buckets is done according the the 'bunch time' given by the FCT system.

As the SIS18 is operated at h=1 and ESR at h=3, the bunch from SIS18 is too long and can not be nicely placed into one of the rf-buckets in ESR. Thus, the figures look quite ugly.

• phi = 184°; the bunch is mainly placed in bucket #1
• phi = 208°; the bunch is placed to the left of bucket #1, and starts smearing
• phi = 232°; the bunch is place into the gap in between buckets #1 and #3. It smears over a large region and the eye of the observer is only guided by the empty bucket #2
• phi = 256°; the bunch is placed to the right of bucket #3, and is still smearing
• phi = 286°; the bunch is mainly placed in bucket #3

Remark: The above figure also shows that bunches become better defined for longer storage times. This due to beam cooling by the electron cooler as shown on the first figure to the very top of this page.

Summary

First, the main new feature is the ability to achieve phase-matching without a dependency on specific harmonic numbers. This new feature of the bunch-2-bucket system has been demonstrated successfully.

Second, it is shown that extracted bunches can be placed into specific buckets of the injection machine. This can be achieved by changing the phase difference between the h=1 DDS systems of the two rings.

-- DietrichBeck - 29 Jun 2022