Bubble Chamber Beam Test August 2015

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Emittance Measurements, September 3, 2015

  • We measured (normalized) emittance and Twiss parameters for the horizontal and vertical planes as a function of momentum (6.7, 7.7 and 8.3 MeV/c).
  • The results from a previous measurements is summarized in JLAB-TN-14-004 (February 2014): Transverse Beam Characterization in the CEBAF 5 MeV Region

media:TN-14-004.pdf media:TN-14-004.docx



Beam Charge

Maximum ¼ Cryo-unit Gradients

Calibrate ¼ Cryo-unit Gradients

Beam Energy

  • Approximate energy of the beam coming into 0L02 is 500 keV.
  • Cryomodule 0L02 adds the rest of the beam energy. The cavities in 0L02 (7 and 8) are 0.5 meters long. Their "GMES"s are measured in MV/m.
  • Energy contributed by 0L02, then, is 0.5 * (GMES of cavity 7 + GMES of cavity 8), assuming both cavities are on crest.
  • Beam kinetic energy estimate is T_estimate = 500 keV + 0.5 * (GMES-0L02-7 + GMES-0L02-8).


  • To measure the beam energy:

1. Use the above T_estimate formula to determine initial setpoint for MDL0L02 into 5D (bubble chamber) line.

BdL in 5D line [G-cm] = 1412 * T_estimate

2. Make sure 5D horizontal correctors are all set to zero BdL.

3. Send beam and center it on viewers 5D00 and 5D01 by adjusting MDL0L02 and vertical correctors in 0L and 5D region.

4. Once beam is centered on both viewers, use the formula below to determine beam energy: 


Beam momentum p [MeV/c] = (MDL0L02-BdL – 372) / 1412. The earth's field (and other environmental fields) contribution is about 372 G-cm. 

Total Energy E [MeV] = Sqrt( p2 * c2 + m2 * c4 ) ≈ p * c.

Beam Kinetic Energy T [MeV] = E – m * c2 ≈ p * c – m * c2.


  • To change beam energy by a known increment:

Assume we want to increase beam energy by 0.1 MeV, then follow these steps:

  • Increase GMES of cavity 8 (GMES-0L02-8) by 0.2 MV/m.
  • Increase MDL0L02 field by the ratio 0.1/p0
  • Send beam and center it on viewers 5D00 and 5D01 by adjusting MDL0L02 and vertical correctors in 0L and 5D region.
  • Once beam is centered on both viewers, use the formula above to determine beam energy.



Bubble Chamber Digital Camera with Beam in Injector

  • On Tuesday August 11, around 9:30 - 10:05 pm, we tested the bubble chamber sensitivity to gammas with beam (6.3 MeV/c?) to Faraday Cup 2 and 5D line. We did not see any thing on camera, none of the pixels showed any activity, nothing.

These were the beam conditions:

  1. 30 µA CW at CEBAF Faraday Cup 2
  2. 10 µA CW at 5D Line radiator/dump
  3. 10 µA CW at 5D Line Faraday Cup
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