Present: Fritz Caspers,
Elias Métral, Federico Roncarolo, Benoit Salvant.
Collimator jaws measurement:
Agreement between the theory and the
measurements with a 1.2 m long coil in the range 5 kHz to
500 kHz.
at higher frequencies, coil resonance
is dominant
at lower frequencies, very low
signal: (Zcoil with graphite) - (Z coil with copper) ~ -
0.01 Ohm below 20 kHz For comparison, below 20 kHz, absolute
value of the coil impedance is of the order of 3 Ohm.
Alignement error can lead to 0.01 Ohm variations.
Measurement noise from the LCRmeter itself seems to be in
the 1e-4 Ohm to 1e-3 Ohm region.
various reproducibility measurements
with bad coil alignment (translation or horizontal tilt) in
the gaps is underway.
the coil height (0.8 cm) is too high
to be safe measuring a 1 cm gap. We should do 2 layers or go
to bigger gaps.
Also, a coil with less turns should
be made to reach higher frequencies with both LCRmeter (up
to 2 MHz) and then with the VNA.
we could try to rigidify the coil by
wrapping it in a round tube, which could touch the jaws
(issue for fragile graphite!!!)
the calculation of the resonant
frequency of the coil is very tedious in our case (distributed
frequency dependant inductance and capacitance).
we should check the resonant
frequency of the coil in vacuum.
PIMS
nothing new
collaboration with SLAC
nothing new
physics behind the low frequency
impedance drop
"Geese have legs, Girls have legs..."
inductive bypass is misleading, and a
new term should be found
when the frequency decreases, the
radial distance between the beam and the effective image
currents in the wall increases, thereby decreasing its
action back on the beam.
also, an azimuthal redistribution
effect can be seen (I will discuss with Fritz to get a more
substantial explanation)
questions were raised about the
impossibility of the space charge to have any effect at DC
and low frequency.
More generally, the treatment of
phenomena at low frequency using TEM modes is hazardous as
they are not valid in DC.