BNL-CERN Technical Meeting:
Dressed SPS Double Quarter Wave Crab Cavity
Minutes submitted for revision on April 17, 2014
Minutes approved on April 21, 2014
Date
Time and location
Minute taker
Revised by
Attendees attendant
Attendees absent
April 15, 2014
16:00-18:00 (CERN, Genève) / 10:00-12:00 (BNL, Upton)
Silvia Verdú-Andrés
Luis Alberty, Rama Calaga, Binping Xiao.
BNL: Giorgio Apollinari (FNAL), Ilan Ben-Zvi, Sergey
Belomestnykh, John Skaritka, Silvia Verdú-Andrés, Binping
Xiao, Qiong Wu.
CERN: Luis Alberty, Kurt Artoos, Rama Calaga, Federico
Carra, Raphael Leuxe, Thierry Renaglia.
LBNL: Alex Ratti.
CERN: Ofelia Capatina, Norbert Kuder (excused).
FNAL: Tom Nicol (excused).
Subject: Technical meeting to update, identify and coordinate the BNL and CERN
efforts on the dressed SPS Double-Quarter Wave Crab Cavity (DQWCC) design.
Agenda:
1) Cavity prototyping: port length (S. Verdu-Andres, BNL)
2) Latest FPC hook design (S. Verdu-Andres, BNL)
3) Update on thermal studies for FPC hook (F. Carra, CERN)
4) Review of helium vessel and tuning system (R. Leuxe, CERN; discussion BNL-CERN)
Discussion:
 Materials presented during the meeting are uploaded in Indico:
https://indico.cern.ch/event/314198/

The power loss due to Joule effect is about 7W for a 3mm-thick copper
gasket that is located 230mm away from the cavity central plane in the HOM
port tubes. This loss has been evaluated for a nominal deflecting voltage of
3.34 MV and taking into account the anomalous skin effect of copper at 2K
and the additional losses due to surface roughness (about 30% of the total
losses). R. Calaga proposed to make a pedestal for the first inductive rod of
the filter to reduce current density. He also suggested that the HOM filter
angle can still be optimized, but according to B. Xiao the RF performances of
the filter also depend on this angle. In any case, Q. Wu recalls that these
optimizations will not decrease significantly the losses in the gasket.
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BNL proposed some possible solutions to reduce the losses. The first
option is using Nb-coated gaskets. This solution has been studied by other
people in the past and results were not satisfactory. BNL will try this option
with the Nb-coated gaskets by Alameda Co. An alternative in the same
direction under consideration by BNL would be to use gaskets made of two
concentric rings: the inner part in niobium to reduce thermal losses and the
outer part in copper for good vacuum tightness. The different thermal
expansion coefficient of niobium and copper would lead to a gap between
both rings after cool down. The niobium could be anodized to avoid any
sparking in the gap (S. Belomestnykh, H. Padamsee).
The flange cannot be placed at 230mm because the upper flange would
interfere with the first rod inductor, as shown below, and there would be
some issues for welding and cooling this high magnetic field region. So the
filter could be moved away from the hook, but this solution would lead to a
quite long tube that does interfere with other elements in the cryomodule.
The second option is to use the 90 degrees filter that B. Xiao is designing to
provide a lateral output to avoid clearance issues with the cryomodule. S.
Verdu-Andres expects that this kind of filter, shorter in length, would allow
increasing the coaxial line between filter and hook, so the gasket could be
placed further away from the cavity (as shown below). In addition, the first
inductive rod (a sink for the intensity currents) is after the tube elbow for the
90 degrees filter, much further away from the cavity, so fields will damp
strongly between the cavity and this rod. B. P. Xiao also proposes a tri-axial
structure for the 90 degrees filter to shield the flange from high magnetic
field region.
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A third option suggested by I. Ben-Zvi consists in building the hook and first
inductive rod as part of the cavity. The flange could then be placed between
the first and second inductive rods, where the fields are not so high. This
solution, however, poses some difficulties for cleaning the cavity.
Finally it was agreed that the position of the flange for the HOM port tubes
will stay at 217mm until the 90 degrees filter design is further advanced.
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The latest FPC hook presented by BNL is an optimized version of previous
designs, leading to about 106 W of dissipated power on the FPC hook for an
external Q-value of 4.87e5 in the FPC port. These values are fine for the
technical review meeting. However, CERN pursues power dissipation in the
FPC hook to be about 60 W (cooled down with an active circuit of water,
radiative losses of about 0.4 W – see F. Carra’s slide; John Skaritka says that
radiative losses in the range of 0.2W are acceptable) for an external Q-value
of 5e5 (this might be a limitation for LHC – S. Verdu-Andres: I assume that
this is in case they want to push the deflecting voltage beyond the nominal
value… – not for the testing in SPS; could this value be negotiated?, BNL
requests). BNL will try to decrease the power dissipation on the FPC hook.
The latest FPC hook may have some multipacting issues because of the
proximity of the hook (previously with circular section; now with ellipsoidal
section and longer section going parallel to the tube wall – see below) to the
tube walls. The hook model should be provided to Zenghai Li (SLAC) to study
the multipacting in this region. S. Verdu-Andres thinks that an elliptical
section hook will have less multipacting than a rectangular section hook: to
be checked.
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Q. Wu requests who is going to deliver the cavity model to NioWave. R.
Calaga says that the models will be delivered after the technical review
meeting.
BNL reminds to CERN that the “scissor” of the tuning system must be heat
stationed to the helium vessel.
S. Belomestnykh says that typically the thickness of metal for magnetic
shielding is about 1 to few millimetres.
CERN explains that some clearance issues where found for the tuning system
inside the cryomodule (see below). The frame allowing for tuning actuation
is not yet fully integrated. Design effort will be put into optimizing the
dimensions of components in order to fit all systems in the available space at
the SPS. The vacuum vessel will become larger (in line with the other
cryomodules), and heat stationed and magnetic shielding sheets will be
added.
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Tasks / actions:
First priority
1. (S. Verdu-Andres) Expected frequency shift due to Lorentz detuning. CERN
offers technical assistance to set the simulation.
2. (S. Verdu Andres) Complete the calculation to determine the losses in the
FPC port connection, taking into account incoming power and leaking out
from the stored energy in the cavity.
3. (S. Verdu-Andres) Share “Tolerance Studies” table with Terry Grimm (CC
Tom Nicol).
Soon
1. (S. Verdu-Andres, F. Carra) Thermal simulations for new FPC hook.
2. (S. Verdu-Andres) Provide cavity model with new FPC hook design to Z. Li for
multipacting studies with ACE3P.
3. (S. Verdu-Andres, B. P. Xiao, R. Leuxe) Update the latest FPC hook design and
HOM filter with lateral feedthrough output in the CATIA model.
4. (BNL) Provide an outline of the talks for the technical review meeting and
identify the material that has to be provided by CERN.
5. (B. Xiao, S. Verdu-Andres) Evaluate RF performances and power losses for 90
degrees filter option.
6. (S. Verdu-Andres) Modify all the hooks from elliptical section to rectangular
section with rounded edges to simplify fabrication.
Material to be exchanged between BNL and CERN teams:
BNL to CERN:
- Upload to EDMS the documentation on the parameterization of the cavity
geometry and ancillary (S. Verdu-Andres, R. Leuxe).
Tentative date for next videoconference:
April 28, 2014. Scope of the meeting: review the talks prepared for the Crab Cavity
Technical Review meeting of May 2014 at BNL.
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