GTS meeting 1 15 16

Attendees: V. Vylet, J. GRames, R. Suleiman, J. Hansknecht, M. Poelker, Y. Wang, and C. Hernandez-Garcia.

• The GTS will be utilized to characterize electron beam using a beamline connected to an inverted insulator electron gun and terminating at a beam dump. We are presently focused on R&D aimed at building a new inverted gun that employs an alkali-antimonide photocathode, and to make magnetized beam as part of an LDRD proposal

• Besides tune-up beam in pulse mode, two high current modes of operation are foreseen:
  • a) 350kV at 5 mA DC
  • b) 250 kV at 32 mA DC

• We discussed two radiation scenarios that might pertain to Vashek’s radiation calculations: 1) chronic loss at some relatively low level, representing a small fraction of the total

beam current delivered to a locally shielded dump, and 2) acute short term loss of ALL the beam produced by the gun due to some unanticipated mishap. Of these two conditions, Vashek was most concerned about long term chronic beamloss, item#1

• For calculating the shielding requirements of condition 1, Vashek needs to know the expected beam loss current during nominal operations, that means, how much current do we expect to have in halo once the beam has been steered to hit the dump. After discussion, we do not have a clear answer on the expected beam current loss, particularly because we will be using a “new” photocathode, CsK2Sb. New to JLab, at least.

• J. Grames proposed the following: We anticipate having no more than 10 micro-Amps of beam loss during normal Ops (Halo). Then let’s do the following test: On purpose, set the beam current to 10 micro-Amps, then steer the beam to specific positions along the beamline. Then the Radiation Control Group would take radiation measurements at various locations, like above the penetrations in the GTS Mezzanine, the FEL back door, etc. Based on the results, Vashek can then calculate the necessary additional shielding. One area of concern is the FEL back entrance door, as it appears to be in line of sight of the GTS gun through the vent that communicates the FEL back staircase with the GTS, serving as the FEL HVAC plenum.

• The beam loss under normal operations is 10 uA (chronic loss). Vashek will calculate the radiation under this condition. He will present the results to the Shielding Configuration Management Board (SCMB) along with the plan to measure the radiation dose at 10 uA once GTS gun is ready. Depending on this measurement, shielding requirements for GTS will be determined: what to shield and the thickness needed.

• It is expected to be ready to deliver beam in GTS sometime between mid-March to mid-April, 2016. We will start with the 10 uA beam loss test. After delivering 10 uA to dump - we will use magnets to dump the beam at different locations along the beamline while monitoring radiation in and around the GTS. We will also monitor vacuum in beamline.

• For accidental loss of the full beam current (before the dump). Engineering solutions to terminate the beam must be implemented: CARM trip circuit, current comparator circuit (compares supply current vs dump current), and ion pumps vacuum current trip circuit.

• Action Items:
  • Vashek to present the shielding request (that includes the test run) to the Safety Configuration Management Board.
  • Carlos to provide Vashek with GTS drawings indicating the position of the gun, and position of penetrations in the Mezzanine.
  • Carlos to ask if the FEL back door is lead shielded.
  • Carlos to coordinate a GTS walkthrough with Vashek, Melvin Washington, and Riad Suleiman.