Transcript RF studies at Fermilab MuCool Test Area Dazhang Huang, Yagmur Torun, Illinois Institute of Technology Alan Bross, Alfred Moretti, Zubao Qian, Fermi.

RF studies at Fermilab MuCool Test Area
Dazhang Huang, Yagmur Torun, Illinois Institute of Technology
Alan Bross, Alfred Moretti, Zubao Qian, Fermi National Accelerator
Laboratory
James Norem, Argonne National Laboratory
Derun Li, Michael S. Zisman, Lawrence Berkeley National
Laboratory
Robert Rimmer, Jefferson Laboratory
Fermilab MuCool Test Area
Magnetic field effect
MuCool Test Area (MTA) at Fermilab is a dedicated
facility at the end of the LINAC built to address the
needs of Muon Cooling experiment. The main task of
it is to test the RF cavities for muon acceleration. It
includes:
The major difference of MuCool cavity and others is
that the MuCool cavity must be working in external
magnetic field, thus it is very important to study how
the cavity behaves in magnetic field. In general
external magnetic field degrades the M.S.O.G.
(maximal stable accelerating gradient) rapidly for a
pillbox cavity as magnetic field goes higher. And it can
be described by universal curves. Tests were done a
few years ago, and repeat tests indicate reproducible
results
• RF power (13 MW at 805 MHz, 4.5 MW at 201 MHz)
• Superconducting magnet (5T solenoid)
• Large coupling coil (under construction)
• X-radiation data display exponential growth: ~E13,
follow Fowler-Nordheim field emission law
• 805 and 201 MHz pillbox cavities
• Radiation detectors
201 MHz cavity curved Be window test
• Cryo plant
• Tests were done w/. w/o external magnetic field.
• 400 MeV/c proton beamline
• Obtain magnetic field by moving the cavity inside the
fringe field of the SC solenoids
• Choice of materials to reduce surface strain damage
(BNL)
• At zero magnetic field, obtained stable operating
gradient of 19 MV/m, and the maximal gradient of 21
MV/m, note that the design operating gradient is 16
MV/m
• Use atomic layer deposition (ALD) to synthesize and
analyze cavity surfaces (ANL)
• At non-zero magnetic field: achieved ~14MV/m
gradient at 0,37 T in center of the cavity.
Possible solutions to magnetic effect
• RF cavities filled with high pressure gas (Muons. Inc
& Fermilab)
805 MHz cavity “button” material test
SC solenoids
201 MHz cavity
• “Button” system in the 805 MHz pillbox cavity
designed for easy replacement
• Tested so far: TiN coated Cu & Mo., bare Mo and W
• To be tested: Cu (w/, w/o electric polishing), & more
to come
MTA Experimental Hall inner-view
• Use radiation detector (NaI, scintillator paddle,
“chipmunk” to measure X-radiation from cavity, which
is proportional to the field emission current on
surfaces
201 MHz RF cavity beryllium
windows, tested at Fermilab
• Scatter plots show number of breakdown events,
highest gradient and magnetic field during testing
Molybdenum buttons
Rectangular box cavity for EB effect study
• RF breakdown is field emission current which is
proportional to number of field emitters
MTA RF cavities
There are two cavities in the MTA hall. One is 201
MHz pillbox cavity similar to the one use in the Muon
Ionization Cooling Experiment (MICE: mice.iit.edu);
another is the 805 MHz button pillbox cavity used to
do cavity material test.
201 MHz cavity
805 MHz cavity
• TiN_Cu: less stale than rest, maybe due to loss of
TiN coating
• Mo: generally above W curve, appears withstanding
higher surface gradient
• When there is a cross angle between E and B, a
 seen in the Cherenkov Detector
torque will be generated and it may
the
April 2 break
2008
asperities on surface and thus created more field
emitters. Therefore ideally, as E turns more parallel
to B, higher accelerating gradient we can reach
• TiN_Cu coated by LBNL: data appear more stable,
and has better performance at high magnetic field
• A rotatable box cavity has been designed to test this
idea: it can be rotated in 90±15 degree
nd