Figure 5 - uploaded by Micha Dehler
Content may be subject to copyright.
Source publication
Following the cessation in the production of the com- mercial ADC/DAC boards adopted by the ELETTRA/SLS digital multi-bunch feedback systems, a new family of 500 MS/s data conversion boards with an 8 bit resolution has been developed. The ADC and DAC circuits are separate modules containing analog and digital electronics provid- ing a data rate of...
Context in source publication
Context 1
... with a programmable ratio of one to five to one to twelve, which are sent out to the FPDP ports. For demultiplexing ratios in the excess of 6 an additional board is necessary purely to carry the required connectors. The board can be triggered either via a software trigger or an external DECL trigger signal. A photo of the main board is shown in figure 5. The DAC works in the reverse manner. The FPDP board, being triggered via the Data Valid (DVALID) line of the FPDP ports, multiplexes the input data into a 32 bit 125 MS/s data stream for the main board FPGA, where it is buffered into an internal FIFO register. A START DAC trigger (external DECL or software) launches the second one to four multiplexing stage in order to produce the 500 MS/s data for the DAC circuit. Analog to the ADC, a 2 ways data redirector is incorporated into the chip, which allows storing FPDP input into the internal RAM and even using the DAC for data playback, where a signal written into the ZBT RAM is fed to the DAC. An important parameter determining the efficiency of any feedback application is the latency of the data through- put, which was reduced to a total of 88 ns for ADC and DAC board combined. With laboratory testing for the ADC board completed, a first commissioning for the front end part of the transverse multi bunch feedback has been started. The system consists of the RF front end, the new ADC board used with a demultiplexing ratio of one to six connected to the six DSP board. Equally included was another new development, a four channel 500 MHz DECL clock generator/shifter. Without the DAC, the system was used as a passive diag- nostic device, reading out bunch by bunch vertical position data via the diagnostics DSPs. For testing, we used a non standard homogeneous fill with out any gap with an average current of 75 mA. Since there is no gap in the fill pattern, coupled bunch modes due to daisy chain like effects as resistive wakes and ions will show up quite clearly despite the comparatively low current. Varying the chromaticity of the ring produces a mix of combinations of these effects. With a high chromaticity setting just below the stabil- ity threshold at three times the nominal value, the spectrum in figure 6 was obtained, showing high peaks a Q = − 1 , − 2 , − 3 . . . + Q y , Q y = 0 . 17 typical of an instability dominated by resistive wakes. Lowering the vertical chromaticity further to the nominal value leads the CBM spectrum in figure 7, where resistive wall instabilities are still recognizable, but the beam motion is dominated by other effects, probably of the ion type. As also the Digital to Analog Converter boards will be- come available in the near term, the main application for the new boards will be to commission and get into operation the bunch by bunch feedback systems for all three planes at SLS and for the longitudinal plane at ELETTRA (Two systems are in operation for now.). Extremely in- teresting is the strongly reduced latency of the new development in comparison to the original boards, which will allow to reduce the latency of the overall digital filter by one whole bunch circulation period (960 ns in the case of SLS), so that higher efficiency and better performance can be expected. Another system planned now is a bunch by bunch current feedback, optimizing the behavior of the SLS top up system [3]. The feedback RF front end together with a stand alone ADC board is going to be used to read out the sum signal of a BPM pickup. This information will be used for selective refilling of empty buckets in order to have a highly controllable fill pattern. A third application will be taking bunch by bunch data from the wide band BPM and microwave front end installed in the SLS storage ring, which will allow to measure intra bunch charge distributions for individual bunches within a multi bunch filling ...
Citations
... Due to the cessation in the construction of the commercial 500 MSample/s ADC and DAC boards adopted by the system, the digital processing electronics of one of the two TMBF systems running the appropriate software is used for the commissioning of the LMBF. A new family of conversion boards developed in collaboration with SLS will be ready soon [5]. ...
Bunch-by-bunch feedback systems have been installed at ELETTRA to counteract coupled-bunch instabilities. Following a novel approach both the transverse and the longitudinal systems rely on the same type of programmable digital processing electronics executing the proper software. After a description of the overall machine scenario in which the transverse systems are operated, the status of the longitudinal feedback commissioning is given
... As yet only the sum signals were used for longitudinal phase space measurements.Figure 3 While in the long term, a read out of individual bunch signals within the train is planned using a dedicated fast ADC ([3]), these measurements were done in single bunch mode using a GPIB controlled scope for readout. For various delay setting, the signal average in the vicinity of the bunch position was taken, as in figure 4. Two measurement problems can be seen here, the first being irregularities in the setting of the delay line leading to jumps in the 6 GHz output. ...
A direct way of obtaining intra bunch charge distribu- tions is to measure the amplitude roll off as well as the phase behavior of the spectrum of the single bunch self field. To that effect, a microwave pickup together with a microwave front end has been installed in the storage ring of the Swiss Light Source (SLS). As pickup, button type bpms are used, which have been designed for a broad band behavior in the excess of 30 GHz. Three bpms together with their individual front ends are used in order to sam- ple the beam spectrum at frequencies of 6, 12 and 18 GHz, which compares to the standard spectrum of a 1 mA sin- gle bunch extending to approximately 12 GHz (13 ps rms bunch length). The signals are mixed to base band in loco using the multiplied RF frequency as a LO. By shifting the LO phase, simultaneously the amplitude roll off as well the complex phase of the beam spectrum can be obtained. Where using a resonator as a pickup would smear out the response over several bunches, allowing only the determi- nation of average values, the current setup has a band width of approximately 2 GHz, so that individual bunches in the 500 MHz bunch train can easily be resolved.
... Already when launching the Swiss Light Source (SLS) project, we anticipated problems with transverse coupled bunch instabilities due to resistive wake fields (for earlier measurements see e.g. [1]) as well as possible longitudinal instabilities due to higher order modes in the main RF cavities. For the time being, transverse stability is ensured via a beam optics with a high chromaticity setting, at the cost of a decreased dynamic aperture, plus the use of an inhomogeneous fill pattern leaving 90 of the 480 available RF buckets empty. ...
Within the frame of the project for a multi bunch feed- back system for the Swiss Light Source (SLS), a new fam- ily of 500 MS/s analog to digital and digital to analog con- version boards with an 8 bit resolution has been developed, containing on-board MUX and DEMUX circuitry to reduce data rates to approximately 20 MS/s using up to twelve Front Panel Data Ports (FPDP). Using six quad processor DSP boards, full bandwidth bunch by bunch feedbacks in the transverse and longitudinal planes are set up to provide bunch by bunch correction kicks with a 2 ns resolution. We report on the hardware setup and properties as well as feed- back performance in the SLS storage ring.
