KLF beamline meeting - October 17, 2024

Agenda

• Announcements

• Beam tests in 2025 and beam bleed-through (Riad, Edy)

1. MOLLER related Beam Tests: (Note: laser is back to 249.5 MHz, need to switch to low-rep laser (need one shift))
   1. Study photocathode effects from KLong beam
   2. Deliver Hall D Low-rep Rate and High Bunch Charge Beam to Hall
2. Hall D Bleed-through:
   1. RF bleed-through results from bad RF Separator phasing in Beam Switch Yard. This can be eliminated.
   2. Bleed-through that originates at injector because each drive laser produces some dc beam that bleeds across all three chopper slits. We measured bleed-through in injector using different lasers (Mode-Locked laser vs. Gain-Switched laser), but found the same level of bleed-through as our Gain-Switched based drive lasers. All Hall D empty RF buckets will be filled with bleed-through current from Hall D laser in addition to the bleed-through from the other lasers. Other halls worried about effect on polarization, otherwise no issues. KLong should worry about accidentals and timing. Accelerator wants a requirement document. Bleed-through can be measured in injector. We will need to submit a beam test request. KLong DAQ should also receive a 15.6 MHz timing signal from Injector.

• FLUKA simulations (Pavel)

• Updates from GEANT4 simulations (Richard)

• Hypernuclei station (Mikhail)

• Engineering update (Tim)

• Any other business

Minutes

Present: Vitaly, Beni, Tim, Josh, Hovanes, Igor, Riad, Pavel, Edy, Mikhail, Marshall, Sashi, Moskov

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• Riad discussed the plans for the beam tests during 2025 run. He thinks he would need two "beam study days" to get reasonably conclusive results. One day of studies would be limited to the injector, and the other day will require beam delivery to Hall D.
• Riad pointed out that these measurements were planned earlier but were not done due to a lower priority assigned to them. The priority of these tests is affected by the Physics Division input, and we would need to be a high priority item on the Physics Division list so that the beam studies do not get canceled and that we do not get bumped off the schedule for the beam studies.
• Edy attends these "BTeam" meeting and can help some of us be included in the meetings.
• Riad expects that the bleed-through will be on the level of 1%. Hovanes asked why it should not stay at ~3nA as measured during KLF instead, if the slit aperture is not changed to raise the beam current. Beni commented that for KLF the DC beam-current will be the sum of the other halls, as for GlueX, plus leakage from the KLF laser. Riad expects that Hall D laser for KLF may substantially change the DC-beam-current and increase the bleed-through. The dark current level of KLF laser can be measured at the injector.
• Riad also asked if KLF is worried about the bleed-through from the other halls or about the bleed-through from Hall D laser. Hovanes said that any out-of-time bleed-through electrons arriving in-between the 64ns-separated bunches will be a problem as the kaons produced by them will show up to the experiment as low energy kaons in the beam.
• Injector group would like to have specifications for the bleed-through for KLF. This could be a slide describing the impact of the bleed-through on the experimental results and the maximum bleed-through current that could be tolerated by KLF. Moskov and Vitaly commented that they have estimated the impact of 0.1% bleed-though on one of the physics channels, and they think that KLF can tolerate such a bleed-through level. At this time, KLF does not have clear specifications for the out-of-time bleed-through current. Once KLF provides the specs, the injector group can devise a beam test in order to estimate the bleed-through levels for KLF and to try to satisfy these specifications.

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• Pavel described his recent work on simulating KLF background in the hall with FLUKA.
• According to Pavel, the low energy photons scattering off the target do not contribute much the the high rates in the GlueX detectors.
• Pavel also sees large contributions in the rates from high energy forward streaming particles around the beam pipe. He is able to reduce the dose rates due to these particles by introducing heavy materials around the beam pipe in the collimator cave walls.
• The contributions of the high energy participles that hit the beam pipe and keep producing radiation has not been addressed so far. Pavel does not thing that it will be easy to collimate these particles. The effect from the beam pipes seems to be difficult to quantify without simulations with large statistics. Pavel also thinks that these particles produced in the beam pipe could be the source of the high CDC rates.
• In his most recent models, Pavel limited the CuW tungsten collimator to the KPT assembly and the first half of the labyrinth wall right after KPT ("heavy core"). For other walls, he uses iron absorbers around the beam pipe. Pavel removed the collimator after the KFM in his recent model. The tungsten plug of KPT is 10cm long.
• One of the versions of his recent model has the beam pipe walls after the collimator cave thickened to 3cm all the wall to the target assembly to absorb the radiation created by particles hitting the pipes ("thick wall" version).
• In order to quantify the results, Pavel introduced a scintillator detector plane just upstream of the LH2 target and a couple of meters downstream of the LH2 target, where he scores the dose rates that can be related to the rates in the detectors.

• Pavel sees considerable improvement (an order of magnitude) in the dose rates using the model with the thick beam pipe with respect to the "heavy core" model and the "initial model". At R=40 cm just upstream of the LH2 target, the "thick wall" version of the "heavy core" model has approximately five times less dose rate than the "heavy core" model. If compared to the "initial model", the improvement is about factor of 10. The relative improvement is considerable less at the z-location after the cry-target due to radiation produced inside the target.

• Hovanes asked if having a thicker pipe with multiple narrow collimators could replace the very thick 3cm pipe with a similar effect. Beni had similar idea too. Pavel did not think that collimating this beam would help with the noise produced in the beam pipe.

• Richard pointed our that his model is not equivalent to either of the three versions presented today by Pavel, but is probably closer to the "heavy core" model. Hovanes asked if it wold be easy to try to increase the pipe thickness to simulate the effect on the CDC in the GEANT4 model as the improvement shown by Pavel might be sufficient to get to the acceptable rates in CDC. Richard would prefer to first see where the tracks creating the noise hits originate and try to target them rather than to "randomly" try new parameters. Pavel argued that, although the 3cm thickness of the pipe was not optimized, the beam pipe was made deliberately thicker to suppress the radiation from the pipes. Pavel and Richard did not seem to agree on this approach at this time.

• Pavel also presented his projections for the particles content and their spectra in the beam pipe with larger statistics than before. He estimates about 6.7KHz K-longs in the beam. He also sees around 1GHz rate for high energy photons (E>0.5GeV). Muon rate is flat and stayed suppressed (<1KHz in total). (Note that this is with 10cm KPT plug). The neutron spectrum slope is not very steep, the neutron and kaon rates at 4 GeV momentum are still about the same. At 2 GeV momentum, there are quite a bit more neutrons than kaons (approximately factor of x5). Moskov asked if using low-Z materials, like Lithium, to suppress neutrons rate would work for KLF. Pavel thinks that it would help with neutrons with momenta of a couple of hundred MeV. For high momenta, low-Z materials will likely suppress kaon flux as well, but it would need to be studied.

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• Richard did not have updates from GEANT4 to show. He is currently working on adding a facility to his simulations that would enable him to track down the origin of the hits in the CDC (and other detectors). He might be able to present results at the next meeting, as well as the 2D plots for the hit rates in FDC and TOF , as asked by Lubomir and Hovanes.

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• Mikhail was having trouble with network connection. We decided to postpone his presentation until next week.

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• Tim reported that Keith Harding is back and is working on CPS shielding.
• Stephanie is working on the support structure.
• There were no news from the magnet. We are still expecting the delivery of the magnet and the power supply in the spring. The mapping of the magnet and test of degaussing procedure are expected to be done in either spring or summer of 2025.

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• The next meeting will be held on October 24th.

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