Minutes RF structure meeting 14.11.2008

M. Schuh, M. Vretenar, G. DeMichele, S. Ramberger, R. Wegner, N. Alharbi

DTL

• First measurment results:
  • TM₀₁₀ found at 352 MHz without waveguide couplers
  • first Q measurements with TM₀₁₀ at only ~8500 (half drift tubes are made of aluminium, no RF contact in half drift tubes)
  • with post couplers: measured E0, a flat field is found at certain post-coupler lengths
  • First bead-pull measurment results (GiovanniDeMichele)
  • all futher measurement results can be found here
• avoid to adjust field with post couplers due to higher load on coupler and lower Q
• to do measurements without post couplers: moving only tuners to adjust field (see presentation from SNS)
• in order to improve the Q factor, half drift tubes in the end-caps should be welded in; for Q factor measurements additional end-plates with copper plated half drift tubes may be envisaged
• for power measurements water cooled end-caps are required
• perform high power test (LINAC4 duty cycle) without post couplers at CERN:
  • Measure X-Rays ( action: contact safety staff) to estimate field levels
  • test with magnets in drifttube
• SPL duty cycle as 2nd priority, there might be heat problems with the actual setup and a irreversible deformation should be avoided

PIMS, thermal drifts

• during operation with SPL duty cycle, PIMS modules will heat up considerably, the frequency will decrease of about 200kHz (worst case)
• if for any reason, one cell will get a small frequency offset, the power distribution within a PIMS module will change in a direction that will amplify this frequency offset and again the power distribution will change. Is this a drift (convergence) or an instability (divergence)?
• the strongest effect will occur for a frequency offset in one end cell. So assuming an initial frequency offset of df₀ = +10 kHz in one end cell,
• the power in this cell will fall down to p₁ = 99.2% of the nominal power level (more power will be dissipated in the other cells, the total power level is assumed to be constant all the time)
• as the power of the end cell is reduced, it cools, shrinks and its frequency increases further, in numbers, to df₁ = +11.57 kHz
• again the power drops, to and the frequency increases to df₂ = +12.09kHz
• after the nth iteration, the frequency offset will be with in the case of the PIMS hot prototype
• as q<1, we have convergence and the final frequency offset is . Plot thermal drift df
• curves show the relation between the initial frequency offset and the final frequency offset for each of the 7 PIMS cells. Furthermore, the Tilt sensitivity and max-min/mean voltage errors including the discussed drift effects are given for frequency offsets in each cell. Plot thermal drift Verr -- (RolfWegner)

-- MarcelSchuh - 20 Nov 2008