May 2025: Bunch-to-Bucket SIS18 -> ESR with Beam Using the Phase Shift Method

Table of Contents

Introduction

This was the first time the Low-Level-RF System for all involved harmonics was used for shifting phases. Bunch-to-Bucket transfer from SIS18 -> ESR was successfully tested with beam. It was demonstrated, that the phase can be shifted in the extraction ring (here: SIS18) as well as in the injection ring (here: ESR).

Setup

Figure: Schematic view of data flow for bunch-2-bucket transfer SIS18 -> ESR with phase shifting in SIS18. Boxes are hardware modules, each an FPGA connected to the White Rabbit network with a host system added: Central B2B Unit (CBU), Phase Shift Module (PSM), Optical Mixer Unit (OMU), Direct Digital Synthesis (DDS), Phase Measurement (PM) and Kicker Trigger (kick). Arrows indicate data flow. Black: timing messages sent via the White Rabbit network as broadcast. Red: phase shift telegrams via dedicated optical links. Blue: two different phase measurements of one clock output of the same DDS. Data flow for monitoring and diagnostic is not shown.

The setup above shows the data flow for bunch-2-bucket transfers from SIS18 to ESR. Here, the phase shift method is employed with phase shifting in SIS18 and SIS18 is operated at fourth harmonics. The OMU outputs the relevant data to the group DDS of h=1 and h=4. As an example, when h=1 is shifted by 10 degree, h=4 will be shifted by 40 degree.

The setup used for this machine experiment is still in a prototype stage.

• the signal rate from the PSM ist just 10 kSamples and phase shifts words are sent only every 100 microseconds
• the OMU only has two outputs supporting fixed harmonics; the outputs can be configured using a serial connection from a notebook to the OMU

The setup shown in the figure above is not complete as it does not show the setup for phase shifting in the ESR. A second set of PSM, OMU and higher harmonics DDS was installed for phase shifting in the ESR. Basically, the setup is identical for phase shifting in the ESR with the only difference that the h=2 was used as harmonic number.

Settings

• beam ¹²C¹²⁺ @ 400 MeV/u, v/c ~0.71
• SIS18
  • h=4, about 10⁹ particles
  • revolution frequency ~988447.5 Hz
  • only two out of four bunches were extracted
  • cooled to a total emittance of 0.19 eVs/u
  • 'EMIL', emittance 0.0475 eVs/u per bunch
  • 'bucketfill' 3.43 rad
  • 'URF' 2156 V
  • synchrotron frequency ~656 Hz
  • bunch length ~98 ns or 21 m
  • FCT for diagnostics GS01DT1FB
• ESR
  • h=2, about 10⁸ particles
  • revolution frequency ~1974344.2 Hz
  • total emittance of 0.095 eVs/u (only two bunches from SIS18)
  • electron cooler OFF
  • 'EMIL', emittance 0.0475 eVs/u per bunch
  • 'bucketfill' 3.43 rad
  • 'URF' 766 V
  • synchrotron frequency ~477 Hz
  • bunch length ~98 ns or 21 m
  • FCT for diagnostics GE02DT1FP

No stripper foil was used between SIS18 and ESR. For bunch-2-bucket transfer using the frequency beating method, the (half) beating frequency was close to 400 Hz.

Log

2025-May-23, 0800-1700

• machine experiment of phase stabilization at Ring-HF
• using beating method
• the b2b phase difference between both machines was determined optimal at 300 degree

Figure: Extraction from SIS18 with frequency beating, h=4. Two out of four bunches are extracted.

Figure Injection into ESR with frequency beating, h=2. Two bunches are injected and stored int the ESR.

2025-May-23, 1700-2000

• as no follow up was planned at ESR, we have used the opportunity for a first shot using the phase shift method
• phase shift in SIS18, shift time set to 10 ms
• 
• Figure: First phase shift with beam in the SIS18. Shift time is 10 ms. All four bunches are shifted and two bunches are extracted. The phase shift is reverted at the end of the extraction flattop.
• unfortunately, it was not possible to inject and store the beam into the ESR. The following things were checked
  • correct kicker timing
  • correct synchronization of SIS18 and ESR h=1 group DDS
  • correct synchronization of SIS18 h=1 and h=4 group DDS
  • correct phase shifting of the SIS18 h=1 and h=4 group DDS
  • correct phase shifting of SIS18 h=4 gap voltages
  • correct phase shifting of SIS18 FCT signals
  • all of this was found to be correct
• 
• Figure: Injection into ESR with frequency beating. yellow: SIS18 kicker signal, green: ESR kicker signal, magenta, orange: ESR FCT signals, red: SIS18 FCT signals.
• 
• Figure: Injection into ESR with phase shift in SIS18. yellow: SIS18 kicker signal, green: ESR kicker signal, magenta, orange: ESR FCT signals, red: SIS18 FCT signals.
• it was quite late and we did not have an idea what we could still check, gave up for this day

2025-May-26, 0730-1800

• over the weekend: had the idea, that the problem is with ramping down of equipment in the transfer line; however, experts confirmed that all settings stay constant even in pre- and post phase of the transfer - this is a time window of 40 ms
• 0730: started pattern with SIS18 -> ESR transfer again; beam is back immediately; b2b transfer with beating method works immediately
• 0745: try phase shift in SIS18 again and reduce time for phase shift from 10 -> 5 ms (just to see what happens) -> works immediately, uff!
• 
• Figure: First successful transfer of beam from SIS18 into ESR using the phase shift method. Shown is the FCT signal in ESR. This is a breakthrough, however the figure shows a rather clear signature of dipole oscillations.
• It was concluded that some settings have changed over the weekend. Tuning till 0945
  • till 'Frequenzverstimmung ESR': set to 0.272
  • 'b2b Phasendifferenz': set to 300.5 degree
  • the beam is already oscillating a significantly in SIS18, some improvements due to changed settings in SIS18
• again: measurement to determine correct phase difference using the FCT application from the launcher
  • 095510, 295 degree
  • 095530, 296 degree
  • 095621, 297 degree
  • 095730, 298 degree
  • 095800, 299 degree
  • 095840, 300 degree, this was best
  • 095930, 301 degree
  • 100000, 302 degree
  • 100050, 303 degree
  • 100318, 304 degree
  • 100527, 305 degree
• 1030: with the help of the ESR crew the reason for failed transfer at longer shifting times was found.
  • 
  • Figure: Injection into ESR. Magenta, orange: FCT signal of ESR indicating the time of injection, blue: level of ESR injection septum. Time of phase shift is 6.5 ms.
  • the injection septum remains only for 10ms at the correct level; a shift time of 6.5 is just at the limit
  • this was further confirmed by
    • setting the shift time to 10 ms ... (= transfer when septum ramps down)
    • ... and increasing the correction on the septum from 8 mrad to 28 mrad (to bring the value on the ramped down back to a good value); this was done as 'bypass trim'
    • --> beam is back in ESR
  • the fix for this beam experiment was to shift/delay the event 0d521 'CMD_SEPTUM_CHARGE' by 10 ms ...
  • ... this was done by ParamModi -> Trim -> Injection -> Timing Master -> increase time for event 0d521 by 10 ms -> trim
• 1145: try phase shift in ESR, identified two issues
  • some injections are good, some are bad, there is a real problem
  • as in SIS18, the b2b system performs a 2nd phase shift after the flattop
    • this is good with the extraction machine after the extraction flattop
    • but nonsense with the injection machine after the injection flattop
    • this must be fixed but not in this machine experiment
  • give up on phase shifting in ESR, continue with phase shift in SIS18 after lunch
• compare quality of phase shift with beating method
  • 1015, 6.5 ms shift time, 10x measurements
  • 1100, frequency beating, 10x measurements
  • 1135, 6.0 ms shift time, 10x measurements
  • 1227, 10.00 ms shift time, 10x measurements (still using the trick with 28 mrad correction of the septum)
  • 1240, 4.0 ms shift time, 10x measurements
  • 1348, 3.0 ms shift time, 10x measurements
  • 1401, 2.5 ms shift time, 10x measurements
  • 1416, 14.0 ms shift time, 10x measurements (event 0d521 shifted by 10 ms via trim)
  • 1424, 2.0 ms shift time, 10x measurements (event 0d521 at original value)
  • 1434, 6.0 ms shift time, 10x measurements (event 0d521 at original value)
  • 1445, 8.0 ms shift time, 10x measurements (event 0d521 shifted by 10 ms via trim)
  • 
  • 
  • Figures: Top: Long shift time of 14ms. The remaining two bunches in SIS18 are not disturbed by the large phase shift of almost 4*pi @ h=4. Bottom: Shift shift time of 2ms: The phase shift is non-adiabatic and the the remaining two bunches start violent oscillations. Both figures show some artifacts in the measurements. One period of the synchrotron oscillation is about 1.5 ms.
• 1500, try phase shift at ESR again
  • still erratic behavior
  • found defective module at synchronization h=2 group DDS <-> cavity DDS
  • module replaced
  • 1555, now bunch-2-bucket transfer SIS18 -> ESR using the phase shift and shifting the ESR phase looks as good as the other methods
  • three measurements with phase shift in ESR
• 1600 end of experiment, give ESR back to the ESR crew

(Immediate) Lessons Learnt

Successful bunch-2-bucket transfer is only possible up to ~8.0 ms after the beginning of the beam process 'Extraction Fast'. This is an issue for

• phase shift method: shift times larger than 6.5 ms
• beating method: transfer times due to long beating or 'multi beating algorithm' larger than 6.5 ms

In the future either the time for beating or phase shift must be limited or the ESR injection septum (event no 0d521) must be triggered by the bunch-2-bucket system.

We should monitor the cavity DDS systems to detect possible synchronization issues.

When using phase shifting with the injection ring, the bunch-2-bucket systems shifts back the phase after the injection flat top. This is a bug and should not be done.

It seems that phase shifting with beam in the ring starts to trigger stronger oscillations if the time for phase shift is shorter than two periods of the synchrotron periods.

The bunch-2-bucket so far only performs phase shifts in a range of 0 .. 360 degree (h=1).

• This could be reduced by a factor of 2 by shifting in a range from -180 .. +180 degree.
• In principle the range of phase shifts could be reduced by shifting in a range defined by the appropriate harmonics number (h=4 in this experiment), but then both h=1 DDS systems would not be synchronized any more. In principle this could be corrected by correct book keeping of bucket numbers. This might be ugly. Hm ...

-- DietrichBeck - 2025-05-30