Version 1April 17, 2020

AMPUR Run 2b summary for retreat on 4/17/2020

Evan Angelico, Ihar Lobach, Sergei Nagaitsev, Giulio Stancari

Contents

1 Comments on operation of IOTA 11.1 Positive notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Negative notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Comments on detector/experimental operation 12.1 Positive notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2 Negative notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.3 Thoughts and plans for the next phase . . . . . . . . . . . . . . . . . . 4

1 Comments on operation of IOTA

1.1 Positive notes

1. The IOTA ring was able to consistently deliver single electrons on command.These single electrons would survive longer than needed by the experiment. Theability to measure how many electrons were circulating was dependent on GiulioStancari being present to operate sync-light PMTs.

2. The IOTA ring was able to consistently deliver 100s to 1000s of electrons forcalibration of the AMPUR detector system. This was usually done by scrapingfrom higher currents. See e-log entries for operator notes on how to inject 1, 100s,and 1000s of e-.

1.2 Negative notes

1. There was one instance where accelerator issues delayed the experiment by a fewhours. See https://www-bd.fnal.gov/Elog/?orEntryId=170749. The shift on3-8-2020 started with gun modulator problems. Power supply to gun modulatorwas failing and not recovering with power cycles1. Once this was resolved, CC1video pulse problems arose. After many VME resets and a NIM bin power cycle,the CC1 modulator and Kylstron was able to turn on.

2 Comments on detector/experimental operation

2.1 Positive notes

1. The addition of the optional data acquisition system based on the 4-channel“HydraHarp” event timer allowed for the measurement of single, double, and

1see https://www-bd.fnal.gov/Elog/?orEntryId=170746

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Figure 1: The signal collection DAQ configuration for AMPUR, located in ESB.

tripple channel coincidence rates to be measured on a turn-by-turn basis. Datawas taken in two modes: one acquisition per RF marker, one acquisition if RFmarker is in coincidence with at least one pulse from the detector.

2. The motion stage that controls the position of the MCP-PMT was reliable and al-lowed for quick cycling between AMPUR and URSSE detectors. It also provideda method of testing detector operation while other experiments were running inany beam mode, i.e. by moving the detector out of the undulator optical path.

3. Inserting the undulator and setting endpoint positions to 25 µm precision waseasy thanks to the placement of dial indicators and thoughtfully constructedthreaded shafts. It required some level of composure late at night or early in themorning, and requires a controlled access.

4. Systems for monitoring VME count-rates of pulses from the MCP-PMT wereuseful for ensuring that the detector and beam systems were in the expectedconfiguration.

5. The addition of a remote controlled high voltage output for the MCP-PMT wasmuch appreciated.

2.2 Negative notes

Some lighter notes on operation of the experimental apparatus include

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1. The AMPUR and URSSE detectors have photo-sensitivity in different wavelengthspectra: AMPUR being 300-700 nm and URSSE being >800 nm. The URSSEexperiment used mirrors that do not reflect < 700 nm light. This meant swappingtwo mirrors at experimental changeover. This has the potential to introducelight-leaks, mirror misalignment, and operational delays.

2. The MCP-PMT charge readout, consisting of 8 anode pads funneled into 4 chan-nels on shielded cables, needs to have a charge drainage resistor on each channel.This is standard practice with MCP-PMT readouts. Attach a 10 kΩ resistorto each channel on the detector back-plane to drain any charge buildup in thegap between the MCP and the anode. Without this resistor, the anode outputs(sometimes unplugged or plugged into digitizer inputs) are allowed to float tohigh voltages on the order of 200V.

3. The data acquisition system had critical dependency on a four channel Tektronixoscilloscope that could not be accessed remotely on the Fermilab network. Man-ual data acquisition was not an inconvenience. However, a future iteration of theDAQ system should move away from this general-purpose instrument as a maindigitizer. Multiple aspects of the DAQ system were flawed, discussed below, andwould involve the replacement of this scope.

The data acquisition configuration is designed around the particular method usedto count photons and to measure their position in an unbiased way. A diagram of theDAQ system is shown in Figure 1. Counting the number of photons detected by anMCP-PMT is performed by measuring the charge deposited in all detector channelsand comparing to a calibrated dataset that is known to contain <1 photo-electron perevent 2. For this run, the strategy was to digitize waveforms so that pulses may beintegrated off-line to reconstruct charge.

Waveform digitization introduces dead-time on an event-by-event basis. Eventsshould only be digitized if they are known to contain a photon from undulator emission.This occurs every 1000 turns or so. Therefore the effectiveness of the system to collecta sample of 1 and 2 photon events depends heavily on the trigger configuration.

The results of this run are majorly constrained by the following aspects of the DAQsystem

1. The front-end electronics system was not so front-end, in that it was locateddownstream 30 ft of BNC cable. The level of attenuation observed was about 3-5dB, putting strain on the constant threshold triggering system. A next generationdigitization system should be located right next to the detector, as is often thecase in MCP-PMT front-end systems.

2. The DAQ relied on constant threshold discriminators that have a minimumthreshold of about 12 mV. These thresholds drift by ±1 mV over time. Sin-gle photo-electron pulse amplitudes without cable attenuation are on the level of7-10 mV for HV settings with total noise rates of 4 kHz. The solution was to

2Standard practice in PMT data acquisition

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use amplifiers. Pre-amplifiers close to the detector were not constructed/testedin time, so we settled with a x10 NIM amplifier located in ESB. The use of am-plifiers smears the reconstructed charge distributions and the timing resolutionof the system.

3. Waveform digitization was triggered by a combination of a discriminator andcoincidence unit. Both have non-negligable timing jitter relative to the timingjitter of the detector. The AMPUR experiment is not necessarily designed tomake timing measurements; however, a next iteration of the DAQ system couldbe designed to allow for timing measurements.

4. The system that triggered the waveform digitizer was gated by the main-bucketRF clock. The width of the gate was set to be 20 ns to cover all possible arrivaltimes of undulator photons. In hindsight, this gate was too wide. At a 133 nsrep-rate, this does not provide much rejection of 4 kHz dark-noise. In the future,the gate should be set to include a very small portion of arrival times - onlyenough to capture the rising edge and amplitude of pulses.

2.3 Thoughts and plans for the next phase

The next phase of the AMPUR experiment may or may not involve a Large AreaPicosecond Photodetector (LAPPD). The continuous position resolution of LAPPDsover 400 cm2 could allow for about an order of magnitude better angular correlationsensitivity. Even if an LAPPD is not employed, a next iteration experiment shouldhave a DAQ system that is designed to alleviate the problems discussed in Section 2.2.

1. The size of an LAPPD-based system implies the use of a much larger dark-box.I suspect that a next iteration URSSE experiment would also require larger dis-tances for interferometry measurements. These two systems would probably needto be decoupled, in separate boxes, but with the ability to transition smoothlybetween the two. I could imagine two dark-boxes with a remote-controllableoptical gate to select which dark-box the undulator photons propagate to (seeFigure 2).

2. The front-end electronics system should be as close to the detector as possible sothat signals that influence trigger inefficiencies are not attenuated.

3. Waveform digitization must have a more sensitive and efficient trigger system.At the moment, this is up for discussion (not sure of the solution, dependent onthe readout configuration and detector used).

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Figure 2: Sketch of a multi-darkbox configuration coupled to the output of the undu-lator optical feedthrough. Dark-boxes would sit on tables inside or outside of the ring,and would be selected using remote controllable mirrors.

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