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Minutes of the ABP-RLC team meeting of 10.02.2006
present: AG, WH, EM, TP, FR, GR, RT, FZ
web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/
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(1) Minutes of last meeting, pending actions, report from
meetings (FR et al)
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A draft EDMS functional specification for the TCDQ exists,
in which the impedance calculations and contributions from
the RLC team are not mentioned. FR asked whether action
should be taken, to include these considerations.
There was no progress yet on the question of the 2-beam
resistive wall wake field. FZ will try to develop a theory
a.s.a.p.
Concerning the comparison of two SPS working points,
a simulation result exists from Giuliano Franchetti.
He ran simulations with xi_y=0 and xi_y=-1.25. In neither
case did he observe a significant difference between
the two working points. The real vertical chromaticity
was xi_y~0.5, FZ will ask him for simulations of this
case as well. In parallel, EB has re-launched HEADTAIL
simulations for both SPS working points. Results are
not yet available.
EB is also performing HEADTAIL simulations of emittance
growth as a function of e-cloud density for the LHC and SPS
with a dipole field. A result is expected towards
the end of next week, as these simulations will
become part of her thesis.
FR has arranged for a meeting with L. Tavian
on Friday 17.02. at 14:15. The exact questions for
L. Tavian still need to be defined.
T. Linnecar and M. Tigner have discussed possible
Cornell contributions to the LHC upgrade, in particular
concerning RF couplers and reduction of HOM heat load
in 1.2 GHz cavities for bunch shortening.
The transverse collimator impedance was extracted from
FC&TK's bench measurements including detuning and driving
wake fields.
FZ reported from the second meeting of LHC commissioners.
Different phases of the commissioning will be detailed.
GA and FZ were asked to collect arguments for maintaining
a stage of 75-ns spacing 'in the absence of electron cloud'.
FR announced that the AB committees will soon be restructured and
that the new APC will be merged with the LTC led by Steve Myers.
(2) Transverse impedance of the TCTV and possible improvements (AG)
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There are TCLIA and TCTV collimators. The second are made from
tungsten and have a gap of 3 mm, the former consist of graphite
with a gap of 12 mm. AG performed calculations for the TCTV type,
since its gap is smaller. However, the TCLIA is used at injection
while TCTV at collision. Therefore, TLCIA could also prove
important and it needs to be checked afterwards.
AG conducted simulations with HFSS for the two beams passing
inside and outside the jaws. For the outside beam the impedance
is 3 orders of magnitude smaller. The cross coupling
impedance (2nd beam on 1st beam) was not computed, but it
can roughly be estimated as the square root of the product
of the two single-beam impedances.
With a 15-degree taper and 3 mm gap the broadband impedance is
about 300 kOhm per meter, which amounts to 15% of the total LHC
impedance for one of the TCTV collimators. In addition there
are various trapped modes, which could drive the beam unstable.
AG estimated the magnitude of the CB growth rate for one of
these trapped modes at 2e-4 divided by the revolution period.
FZ asked how many of these devices exist in the LHC. The
answer was found in the following presentation by EM, where
two low-gap TCTV's per beam could be identified.
AG then studied possible means to reduce the impedance.
A 10 degree nonlinear taper (circular arc segment) may reduce
the BB impedance, but has little effect on the trapped modes.
therefore, damping with ferrite 4S60 is recommended. This
requires removal of the rf contact, in order to be able to
place to the ferrite outside of the vacuum chamber proper
(ferrites cannot be put inside the beam pipe).
FR suggested a mixed solution where graphite is added to the
metal.
Next week the possible remedies will be discussed with
the collimation team.
FR commented that we are taking a high risk with the LHC, and
that different safety margins are applied for different objects.
FZ met with Elena Shaposhnikova, who provided old impedance data
for the SPS 200 and 800-MHz rf systems. The corresponding
effective impedance can be included in fitting current-dependent
phase advance measurements for SPS impedance localization.
Unfortunately, the only data available are the shunt impedance
and Q value for the lowest frequency mode for either rf system.
FZ suspected that from this one cannot estimate the broadband
impedance of the rf cavities, which would be needed for fitting
the single-bunch measurement. His suspicion was confirmed
by the RLC team.
(3) Coupled-bunch instabilities with the TCTV (EM)
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EM independently evaluated the effect of the TCTV
trapped modes on beam stability, on the request of AG.
Considering only two modes and pessimistically assuming
that the resonant frequency of the second mode falls
onto a betatron line, EM found that the beam stability
is lost, when taking into account that there are two
objects of this type in the ring. (A table shown by
EM listed the number and gap of all collimators
foreseen.) The broadband impedance was not included
in this estimate.
FR stressed that the impedance of TCTV is unacceptable
at the moment, and that this is a high priority issue.
(4) Follow-up of the SPS MKE kicker impedance (EM)
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In 2006 9 kickers will be installed (4 more than before).
The impedance was measured by FC with 1 wire and with
2 wires.
The 1-wire method shows an imaginary transverse impedance
of about 2 Mohm / m for 1 kicker at low frequencies,
which increases towards high frequencies.
The transverse impedance is obtained from a 2nd order
fit of the measured longitudinal impedance as a function
of transverse position. The fit quality is not good.
A 6th order fit improves it.
The 2-wire method more directly gives the transverse
impedance. For this method, it is found to be close
to the prediction by Bruno Zotter's theory up to the
maximum measured frequencies of ~1 GHz, whereas
the Burov-Lebedev theory deviates.
The result 2-wire method is more believable since
the result is close to expectation and not compromised
by poor fits.
EM pointed out, however, that the 1-wire method
indicates a resonance at 600 MHz, similar to a frequency
observed in the SPS instability measurements.
EM recommended to extend the 2-wire measurement to
2 GHz.
The measurement should also be redone with ceramic
insert modifications.
(5) Follow-up of the impedance design for the Totem Roman Pots (FR)
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The green light has been sent. A final test of the Roman-Pot
prototype is planned for June.
(6) Follow Up Transverse Collimator Impedance with Driving and
Detuning Wake (EM, FZ)
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EM compared the transverse impedance of the LHC collimator
prototype deduced from 1-wire measurements of the prototype
with the theoretical prediction, taking into account that
the measurement includes contributions from both detuning
and driving wake fields. The minima of the measured curves
nearly coincide with the theory. FZ made a similar comparison
and obtained roughly the same result, though it is not
identical.
FR expressed bewilderment why the 1-wire measurement seems
to work for the LHC collimator, but is highly unreliable
for the SPS MKE kickers.
(7) Vertical trapped modes of SPS BPMs (EM)
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There are 108 horizontal and vertical BPMs in the SPS,
called BPH and BPV. Bruno Spataro has calculated their
impedance with MAFIA. The total shunt impedance amounts to
0.84 MOhm/m. For this impedance alone, EM calculated the
threshold of mode coupling at a bunch population of
3.5e14 (more than three orders of magnitude above operating
value). Therefore, trapped modes cannot explain this type
of instability. Rather, a broadband impedance would be
needed.
FR suggested that a spread of resonance frequencies
between BPMs could give rise to a lower value of the
effective Q (the value computed and used was 16000).
(8) Rating of different LHC IR upgrade options: first ideas (RT)
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FR recommended that RdM joins the RLC meetings in which IR
upgrades are discussed.
RT presented first ideas and the preliminary web site,
which presently contains 4-5 different optics, mostly with
dipole first.
Aperture requirements and limits on beta* will be set by
collision debris, chromaticity, magnet, technology, etc.
FR asked to take the same free length to the IP
of 19 m for all the optics designs, as had been decided
in Arcidosso.
FZ pointed out that the optics by Stephane Fartoukh
refers to the nominal LHC and not to the upgrade.
He also proposed to include two conventional quadrupole
first options, one with a common bore, the other
with separated magnetic channels.
FR pointed out that the aperture constraints need to
be clarified, as JPK makes different assumptions, which
lead to different conclusions. In principle the
transverse position of each triplet quadrupole can be
optimized so as to maximize the physical aperture.
Parasitic dispersion gives another contribution
to the needed aperture.
FR and RT plan to launch a discussion on rating with
other interested parties.
(9) EPAC06 participation and AOB
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FR reported that three young members of the RLC team will attend
EPAC'06, namely Andrea Latina, Rogelio Tomas, and Giovanni Rumolo.
Peter Urschuetz has a question mark, as he may attend LINAC'06.
(10) Items for the next meeting
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As agenda items for the next meeting are foreseen:
- FC reporting on reflectometer measurements of the beam-screen
ring periodicity
- also FC on measuring the impedance of LHC wire scanners
- AG with an update on TCTV
- UD on beam-beam and compensation simulations
Posted on the web: Slides by AG, EM and RT
Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/