IOTA Beam Position Monitor System for ElectronsOverviewThe IOTA Beam Position Monitor (BPM) system consists of four button electrode pickups, cables, custom analog signal conditioning electronics, and custom readout electronics. The system is designed to measure single bunch of electrons injected from the FAST Linac into the IOTA storage ring. The IOTA ring has a 30MHz RF system and is an h=4 ring with a 7.5MHz revolution time. The system was designed to be able to measure each of the 4 bunches on a turn by turn basis. Orbit measurements are supported by averaging the turn by turn data.

PickupsThe IOTA ring has 21 Beam Position Monitors. After the injection Lambertson magnet, there is one 76mm inner diameter housing with four 20mm diameter button electrodes located at A1C. The 20 other pickups are 47.5mm inner diameter housings with four 11mm button electrodes as shown in Figure 1. The ring layout can be seen in IOTA_Layout.pdf. The 2” pickups are the same pickups and button electrodes used in the FAST Linac. The pickup is installed with two buttons aligned vertically and two buttons aligned horizontally to provide beam position in both planes. One pickup of each type is mapped on our BPM wire test stand and the response corrected with a 2d 7th order polynomial to linearize the pickup response over +/-12mm.

Figure 1: A standard 47.5mm housing and 11mm button feed through.To provide a test button response, a test fixture was made by attaching a strip line to one of the 11mm button electrodes. This is also provides a straightforward method for measuring the button response as shown in Figure 2. For stored beam in IOTA, the bunch length is expected to be 1-2ns which equates to 500-1000MHz and should have minimal loss due to the button. The signal is relatively flat after 1GHz and for very short bunch lengths at injection, the peak signal is very high and loss is not a concern.

Figure 2: Drawing of the button test fixture and measured frequency response using the test fixture.

The signals from each electrode are brought up to the service building via ¼” Superflex Heliax cable. The length of the cable runs is matched for each bpm and

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varies from 150’ to 225’. A list of all pickup locations and measured cable lengths can be found in the supporting files as the IOTA_BPM_List.pdf.

Analog Transition ModulesThe analog signal conditioning is performed by transition modules located in the service building in the same rack as the readout electronics. The system consists of one control board and one 4 channel module for each bpm pickup as shown in Figure 3. The bpm modules consist of three sections – an RF input section, an RF envelop detector section, and output section.

Figure 3: Right is the control module, and left is bpm signal conditioning module.

The RF input section consists of a low pass filter, a directional coupler for a test signal input, 20db of gain, programmable attenuation from -2 to -33db in roughly 0.5db steps, and another 20db of gain. The RF envelope detector section consists of an Analog devices ADL5511 chip operating in envelope mode with in input bandwidth of xx and an output bandwidth of 130MHz and a Bessel Low Pass filter of 37MHz. The output section consists of three amplifiers with a total gain of 17db and bandwidth of 260MHz with the ability to offset the output from approximately 0V to -1V. The attenuation of each channel can be controlled by a control module over CanBus. The offset can be adjusted for each bpm module via a Pot adjustment on the front panel. A simplified block diagram is shown in Figure 4. The full schematics for the transition modules can be viewed in IOTA_Transition_RevA_PgX.pdf where X=1..5.

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Figure 4: Block diagram for a single channel of the analog transition module.

Each transition module was bench tested using a network analyzer and a beam mockup using a fast pulse generator into a strip line attached to an 11mm button electrode as shown in Figure 2. The pulse generator is able to generate 1ns pulses up to 5V which produces a doublet of up to 50mVpp out of the button. Test results for initial module testing are shown in Figure 5.

Figure 5: Left, RF section frequency response. Middle, pulse response for button after RF section, RF envelope detector, and final transition output. Right, linearity check.

Data Acquisition SystemThe data acquisition system consists of three components –a timing and trigger module, eleven digitizer modules (2 bpms/module), and a system processor. The entire system resides in a single VME/VXS crate and is currently using VME protocol for readout and control 0f the timing module and digitizers.

Timing and Trigger ModuleThis is a custom module developed for bpm applications. It has an RF clock input along multiple inputs for triggers and control signals like TCLK or BeamSYNC for

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providing machine synchronous clocks and triggers. It has 12 clock and 12 trigger outputs which are fully customizable and configurable. For IOTA, the timing board distributes the 30MHz ring RF reference to each digitizer module. It also provides trigger information to each digitizer. The trigger is currently a serial stream synchronized to the RF which provides timing information along with what type of trigger is requested. Currently, this consists of Injection Trigger, Orbit Trigger, and Kick Beam Trigger. The current design supports up to 8 different trigger types. This allows arming and generating all trigger types through a single source point rather than setting up each digitizer module individually. The current IOTA implementation block diagram and control section is shown in Figure 6.

Figure 6: Timing board overview and control block diagram. The BES input is used for injection. The BSync1 and BSync2 inputs are for kick triggers. The software trigger is used to take orbit measurements.

Digitizer ModulesThe analog signals from each button are digitized and processed by custom digitizer modules developed at Fermilab. The module for IOTA is a 4th generation digitizer shown in Figure 7. It is a VME/VXS module which also has a GigE connector on the front panel. It has 8 input channels which can be either AC or DC coupled to 250MS/s, 16bit ADCs. It In the latest generation, the ADCs provide data to the Alter Aria V FPGA via a JESD-204b multi-gigabit serial links. The module can be readout over VME bus, VXS serial link, or ethernet. The IOTA bpm system makes use of the external clock input so that the board operates locked to the 30MHz IOTA ring RF reference clock. The ADCs are clocked at 8*RF resulting in 8 samples per RF bucket or bunch. The boards also have two independent banks of DDR3 with a total of 512MB of RAM and enough bandwidth to store all raw ADC data as well as processed data in parallel.

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Figure 7: Digitizer block diagram. The firmware makes use of Altera’s QSYS system running a 32-bit ARM processor for interface and control of the sub-systems. The DSP block handles all IOTA specific logic and signal processing.

The Digital Signal Processing (DSP) module in the digitizer firmware is custom designed to measure the magnitude of the button signal for each bunch. The DSP section performs three main tasks – data alignment via ability to shift each channel up to 32 samples (2 turns), recording raw ADC data on request for diagnostics, and calculating the magnitude for each electrode referred to as DSP data henceforth. For the RF Detector output, the magnitude is proportional to the area of the pulse. The initial implementation was very simple and performed a background measurement on each channel before beam was injected then did an eight sample integration on each bunch with background subtracted. In this implementation all

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system timing was set off the injection trigger and was handled on a channel by channel basis by a combination of trigger delays out of the timing module and single sample ADC delays on each channel. This worked fine for injection but proved insufficient for stored beam due to a combination of background fluctuations, noise, and small signal amplitudes of the initial beam. The current implementation measures the magnitude of each bunch by subtracting two baseline samples between bunches and two samples at the peak of the pulse as shown in Figure 8. This was chosen track maximize signal to noise for very small signals observed in the first stored beam.

Figure 8: Raw ADC samples for all four button electrodes from a single bunch in the IOTA ring. Selected in green are the two background samples and two signal samples used to measure the magnitude.

On any trigger, the digitizer can store raw data up to 16M samples and magnitudes for each bunch up to 4M turns. The memory allocation is fully configurable so that for each trigger type, the data size and location in memory can be specified. The VME readout time is currently 8MB/s allowing readout of 2048 turns of magnitude data for all 4 bunches in under 200ms. To readout 65k raw ADC samples for the entire system takes just under 1 second. This could be improved by an order of magnitude by utilizing the GigE to readout data. Currently, three trigger types are implemented – injection, kicked beam, and orbit. To facilitate the orbit measurements, the magnitudes can be averaged for up to 255 turns. In the current firmware, this feature is enabled at the board level and applies to all triggers.

System ProcessorThe system processor is an x86 multi-core processor running real time Linux rather than VxWorks. It handles all system configuration, data readout, calculation of position and intensity from magnitude data, and interface to the ACNET control system as well as provide backdoor diagnostic access. This is an entirely new framework which was first implemented for the new Booster BPM system.

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Current ResultsThe IOTA bpm system has gone through numerous growing pains in all areas. There have been numerous issues since stored beam with RF was established in mid-October. A lot of progress has been made in the last 2 weeks of beam. During this time, the focus has been on providing orbit data by averaging 255 turns in the digitizers and then further averaging this data for up to 2048 turns to provide orbit data with sufficient resolution. This was done to overcome the very small signals we were seeing in the presence of the system noise floor. This allowed orbit measurements to made with beam currents down to ~50A. During this time, a scheme to provide amplification of the signal right at the button electrodes in the tunnel. Currently, 7 BPMs are instrumented with pre-amps in the tunnel which provide 32db of gain and a few ns of ring to the signal.The data presented here was taken on Dec 21st after verifying the timing for each bpm. The raw digitized data for a single bunch from a BPM with and without pre-amps is shown in Figure 9. This data was taken with approximately 500A beam current stored in the IOTA ring. For this beam current, the attenuation for the bpm with pre-amp was set to 30db while the attenuation for the other was set to 0db. Note, the pickup with the pre-amp sees nearly double the signal on the pickup without the pre-amps for the same bunch. At this time, it is not completely understood why we see nearly 6db difference between the signals rather than just 2d gain difference but it may be due to the tunnel pre-amp lowering the bandwidth of the signal and reducing the effective cable attenuation.

Figure 9: Raw digitized signals for BPM E1R without a pre-amp (left) and BPM A3R with a pre-amp (right).

For the same data, the Turn by Turn response for 1024 turns after the kick is shown in Figure 10. To estimate the resolution of the TBT for each bpm, the RMS of the position post kick where it is expected the variation is almost entirely noise is calculated. For BPM E1R without a pre-amp the estimated position RMS for both horizontal and vertical is 140m for 500A stored beam. For BPM A3R with a pre-amp the estimated position RMS for both horizontal and vertical is 70m for 500A

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stored beam at nearly full attenuation. As expected, the resolution goes linearly with the signal amplitude. For the BPMs with pre-amp we can maintain the resolution at lower beam currents by reducing the attenuation. In this case, down to less than 20A of beam current theoretically.

Figure 10: TBT position data for BPM E1R without pre-amp (left) and BPM A3R with pre-amp (right).

The improved BPM response from recent changes has brought some new issues to light. While looking at orbit data over 10s of minutes of stored beam, a position dependence on intensity became apparent as shown in Figure 11 over a 30 minute period where the stored beam intensity went from 600A to 200A. This behavior was noted in all BPMs (with and without pre-amps) at some level.

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Figure 11: Position dependence on intensity for stored beam from 600A to 200A for BPM E1R without a pre-amp and BPM A3R with a pre-amp.

This observation prompted a closer look at the linearity of the analog transition modules. It was found that the RF envelope detector does not linear especially for small signals. While the initial tests shown in Figure 2 looked reasonable, when looked at as the ratio of input/output as in Figure 12 the issue is immediately obvious. In principle, but to do so requires estimating the true input to the RF detector from the observed magnitude. The best way to accomplish this is currently under study.

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Figure12: Ratio of output of the analog transition module to the signal input to the RF envelope detector as a function of the input signal size.

Under DevelopmentCurrently, new firmware for the Timing Board and the Digitizers is under test which simplifies the system timing. It makes use of the turn marker produced by the low level RF system to synchronize all triggers removing the current need for an injection trigger and several levels of delay to set the bpm timing. It also provides the ability to configure each the number of turns to average for orbit measurements while still providing TBT data for injection and kicked beam triggers.Correct for the non-linearity in position response. The plan is to apply a quick correction as part of the position calculation with the final solution being done as part of the DSP in the digitizers. Improving and optimizing the controls interface and data acquisition for the users is also being addressed. A few items are time out errors when requesting data, insuring synchronization of all data from all BPMs, providing user control for the attenuation settings, status readback to alert of potential readback errors, saturation, or timing issues, and providing means to access larger data sets.The current pre-amp test looks very promising for delivering the required resolution over a wide range of intensities. Ideally, this requires a design specific for the IOTA application and some thought as to whether it can also be made applicable for protons as well.

Reference MaterialsIOTA_Layout.pdf Full layout of the IOTA ringIOTA_BPM_List.pdf List of all BPM locations, names, cables, etc47.5mm_BPM_Housing.pdf Drawing for standard BPM housing11mm_Button_Drawing.pdf Drawing of standard BPM feed through76mm_BPM_Housing.pdf Drawing for large aperture BPM housing20mm_Button_Drawing.pdf Drawing for large aperture BPM feed throughIOTA_Transition_RevA_Pg.1.pdf Schematics for signal conditioning (1 channel)IOTA_Transition_RevA_Pg.2.pdf Schematics for control and test signalAdl5511_datasheet.pdf Data sheet for RF envelope detector

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