possible proposed designs part i : general considerations spl power coupler march 16, 2010eric...
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Eric Montesinos / CERN-BE-RF-SR
Possible proposed designsPart I : general considerations
SPL power coupler
March 16, 2010
2
Eric Montesinos / CERN-BE-RF-SR
Summary
Preliminary remarks
Set of questions and preliminary answers:
A single or a double window coupler ?
Adjustable or fixed ?
Why a double walled tube ?
What about cryomodule integration requirements ?
Vertically above or below the cavity ?
DC biasing ?
Air or water cooled ?
Conclusion
March 16, 2010
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Eric Montesinos / CERN-BE-RF-SR
Preliminary remarks: Tight planning
We wanted eight couplers fully operational, ready to be mounted on the cryomodule by end 2011 ! → very tight planning
Crucial component of a power coupler is the main ceramic window
Due to that very tight planning, we do NOT have time available for a new ceramic window design, so we must use :
An existing design A new design using a very well
known brazing process
March 16, 2010
Coupler design reviewMarch 2010
8 couplers fully RF conditioned
for beginning 2012
Interfaces definitionfor end 2009
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Eric Montesinos / CERN-BE-RF-SR
Preliminary remarks: Chamonix
Chamonix workshop: New R&D program
LP-SPL parameters not required anymore
From the coupler point of view we would prefer to stick to the initial planning (which is optimistic)
Still have NO time for a new ceramic window design
f0 704.4 MHz
Low Power SPL
2.5 kW average600 kW pulsed0.4 + 1.2 + 0.4 = 2.0 ms2 Hz (500 ms)
High Power SPL
1000 kW pulsed0.4 + 1.2 + 0.4 = 2.0 ms50 Hz (20 ms)100 kW average
Cavity design gradient
19-25 MV/m
Qext of input coupler
1.2 x 106 for LP-SPL and HP-SPL(to be redefined ?)
Input line Ø
100 / 43.5 mm = 50 Ω(from the cavity design)
Waveguides WR 1150 March 16, 2010
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Eric Montesinos / CERN-BE-RF-SR
Preliminary remarks:Proposed designs November last year (2009)
This R&D program could (will?) be followed with a whole project requiring more than 250 couplers
So, with the design of these SPL couplers, we would like to always keep in mind some key parameters:
As reliable as possible Easy for operation and maintenance Avoid mass production difficulties As simple as possible Reducing the costs
This raises several questions…
March 16, 2010
CEA Saclay HIPPICoaxial disk
820 kCHFLHC cylindrical560 kCHF
SPS coaxial disk640 kCHF
SPS waveguide disk560 kCHF
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Eric Montesinos / CERN-BE-RF-SR
Double window couplers
Double window coupler is for: Clean beam vacuum Safe operation
Typical example of double window couplers is the TTF – XFEL family couplers (1.3GHz, 1MW pulsed peak , 5kW average)
“Cold Window” for: Beam vacuum / line vacuum insulation Closing the beam vacuum
No coolant (water, gas) in contact with the “cold window”
“Warm Window” for: line vacuum / air side Closing the line vacuum
March 16, 2010
A single or a double window coupler ?Adjustable or fixed ?Why a double walled tube ?What about cryomodule integration requirements ?Vertically above or below the cavity ?DC biasing ?Air or water cooled ?
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Eric Montesinos / CERN-BE-RF-SR
Double window couplers
Unfortunately, with more average power it is difficult to use such a design
Maximum power reached with the Cornell ERL power coupler was 61kW cw (at 1.3GHz)
(2007: HIGH POWER TESTS OF INPUT COUPLERS FOR CORNELL ERL INJECTOR*, V. Veshcherevich†, S. Belomestnykh, P. Quigley, J. Reilly, and J. Sears (Cornell University, Ithaca, NY, USA))
Ceramics have losses The cold window is difficult to cool
down because fully vacuum insulated (both sides)
More thermal intercept points Increase the sizes Use a coolant but this increases the
line vacuum leak risk
March 16, 2010
Cold window: difficult to cool down
Cornell ERL coupler with bigger cold ceramic and air cooling of a radiator for the cold ceramic
XFEL coupler
A single or a double window coupler ?Adjustable or fixed ?Why a double walled tube ?What about cryomodule integration requirements ?Vertically above or below the cavity ?DC biasing ?Air or water cooled ?
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Eric Montesinos / CERN-BE-RF-SR
Double window couplers
Another possibility is a double window coupler with a coolant in contact with the two ceramics to allow high average power
The APT coupler is a possible example (APT: Accelerator Production of Tritium, Los Alamos National Laboratory)
Very interesting possibility, but requires a full development, → impossible to be ready with eight couplers conditioned beginning 2012
March 16, 2010
APT coupler: two disk ceramic windows coupler
A single or a double window coupler ?Adjustable or fixed ?Why a double walled tube ?What about cryomodule integration requirements ?Vertically above or below the cavity ?DC biasing ?Air or water cooled ?
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Eric Montesinos / CERN-BE-RF-SR
Single window couplers
We have at CERN a long experience with single window couplers
SPS, 16 couplers in operation: From 1975 to 2000 (~25 years) with a
few couplers broken, mainly during tests procedures, when they were not properly conditioned
Since 2000, new coupler design (for more power, 550 kW cw) without any problem (already 10 years)
LEP I couplers at ESRF: 20 years of operation with NO fault Still in operation even if some couplers
have been faulty after 20 years with small pinehole in the equator of the ceramic
We are building a new coupler based on LHC ceramic (collaboration with ESRF and SOLEIL)
March 16, 2010
New SPS single coaxial window coupler 550 kW cw
in operation since 2000 (10 years)
SPS single cylindrical window coupler 375 kW cw in operation from 1975 to 2000
A single or a double window coupler ?Adjustable or fixed ?Why a double walled tube ?What about cryomodule integration requirements ?Vertically above or below the cavity ?DC biasing ?Air or water cooled ?
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Eric Montesinos / CERN-BE-RF-SR
Single window couplers
More than 250 couplers in operation with LEP II, even if we had only five years of operation: No vacuum leaks
16 couplers in operation with LHC, operation just started, but all couplers were already built since 2005, and tested up to 550 kW cw full reflection: No vacuum leaks
Oak Ridge SNS linac also has 81 single window couplers in operation since 2005, no major problem reported
We believe that single window couplers are increasingly becoming more robust and reliable
March 16, 2010
LEP II single cylindrical window couplerin operation from 1995 to 2000
New LHC single cylindrical window coupler
A single or a double window coupler ?Adjustable or fixed ?Why a double walled tube ?What about cryomodule integration requirements ?Vertically above or below the cavity ?DC biasing ?Air or water cooled ?
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Eric Montesinos / CERN-BE-RF-SR
Single window couplers
Nevertheless, in situ repair will be absolutely NOT an option:
The LHC test cavity presented a very small leak (2 x 10-9 mbar l/s) at a coupler second ceramic
We tried a vacuum liquid sealant repair (as often successfully done with SPS warm cavities)
The leak was repaired It was absolutely catastrophic:
We could not reach more than a few MV/m
We had to completely dismantle everything
All the components were polluted It took a year to repair it
We re-learnt in-situ repair with high gradient cavities is forbidden
Must have spare modules for exchange
March 16, 2010
LHC test cavity #21
Leaky biasing ceramicVacuum sealant
A single or a double window coupler ?Adjustable or fixed ?Why a double walled tube ?What about cryomodule integration requirements ?Vertically above or below the cavity ?DC biasing ?Air or water cooled ?
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Eric Montesinos / CERN-BE-RF-SR
Single window couplers: conclusion
Based on past experiences, we believe a single ceramic coupler is now reliable enough for ~ 20 years of operation
A double window coupler would be difficult to design for 100 kW average and would need to be carefully studied:
It is much more complex than a single window coupler
Unfortunately, we have NOT enough time available
This could be a very interesting parallel study program
But if we believe on a single window coupler, do we really need it ?
In case of breakage, we should be aware that:
NO in-situ repair is possible
We must be able to exchange a whole module
This is a huge job This has to be taken
into account for sectorization issues
March 16, 2010
A single or a double window coupler ?Adjustable or fixed ?Why a double walled tube ?What about cryomodule integration requirements ?Vertically above or below the cavity ?DC biasing ?Air or water cooled ?
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Eric Montesinos / CERN-BE-RF-SR
Adjustable coupler or not ?
Advantages: Better for Qext matching Better for power distribution
Drawbacks: More complex to design Need a moving system not stressing
the ceramic Need bellows somewhere:
More EB welding Increases the vacuum leak risk Increases mutipacting difficulties
Need an alignment system to keep the antenna at the right position
Increases the number of mechanical operations
Subsequently increases the total price
March 16, 2010
Disk window - fixed coupler
Disk window - adjustable coupler
A single or a double window coupler ?Adjustable or fixed ?Why a double walled tube ?What about cryomodule integration requirements ?Vertically above or below the cavity ?DC biasing ?Air or water cooled ?
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Eric Montesinos / CERN-BE-RF-SR
Adjustable coupler or not ?
Do we really need it ?
For the HP-SPL we will (probably) have one power source per cavity
The coupler will probably be machined accurately enough with an antenna position within +/- 0.3 mm, this should guarantee a Qext spread within 1.18 to 1.23 x 106 (~ +/- 2% Qext, hopefully acceptable)
The cavity will be previously measured, so we can choose which coupler to combine with which cavity to reduce the total error
March 16, 2010
40 45 50 55 60 65 701E+05
1E+06
1E+07 distance iris/coupler 35 mmdistance iris/coupler 40 mmtarget : Qext = 1,2e6
distance axis/antenna (mm)
Qe
xt
Juliette PlouinCEA-Saclay
25th February 2010
60 mm +/- 0.3 mmQext = 1.18 to 1.23 x 106
55.5 mm +/- 0.3 mmQext = 1.18 to 1.23 x 106
35
Ø 100
Ø 43.5
60
A single or a double window coupler ?Adjustable or fixed ?Why a double walled tube ?What about cryomodule integration requirements ?Vertically above or below the cavity ?DC biasing ?Air or water cooled ?
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Eric Montesinos / CERN-BE-RF-SR
Why a double walled tube ?
Due to the high average power, thermal screens are not enough to insulate the coupler from cold to warm
A double walled tube is needed
It has several features: It is a part of the coupler, because it
is the outer line of the antenna, i.e. outer part of a 50 Ω coaxial line
From an RF point a view, it is a simple outer conductor tube with a guaranteed minimum of 3 µm of copper (+ margin)
It is a cryogenic pipe which ensures the thermal interface between 2K to ambient air temperature
March 16, 2010
δ : skin depth [m]f : frequency [Hz]μ0 : permeability of free space = 4π x 10-7
[H/m]µr : relative permeability of the conductor
for copper = 1ρ : resistivity of the conductor [Ω.m]
for copper = 1.68×10-8 [Ω·m]
For copper at 704.4 MHz : δ = 2.45 [µm]Inner gas ≈5K
Outer gas abovedew point≈300K
Helium vessel ≈2K
A single or a double window coupler ?Adjustable or fixed ?Why a double walled tube ?What about cryomodule integration requirements ?Vertically above or below the cavity ?DC biasing ?Air or water cooled ?
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Eric Montesinos / CERN-BE-RF-SR
Why a double walled tube ?
All the important parameters will be taken into account in Ofelia’s, Vittorio’s and Sergio’s calculations:
Length Input gas temperature Gas flow Stainless steel thickness Copper plating issues and high pressure
water tests …
Its contraction must be perfectly pre-calculated, because this will give the coupling value (Qext)
It may be used as support for the cavity, independently of the chosen coupler designs
We will have on both sides the same “all in one RF and vacuum” seal as with LEP and LHC double walled tube
March 16, 2010
A single or a double window coupler ?Adjustable or fixed ?Why a double walled tube ?What about cryomodule integration requirements ?Vertically above or below the cavity ?DC biasing ?Air or water cooled ?
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Eric Montesinos / CERN-BE-RF-SR
Specific Double walled tube “RF and vacuum” seal
Specific shape designed for RF perfect continuity and vacuum sealing
It has been designed for LEP couplers, then re-used with LHC couplers
For bake out compatibility: MATERIAL : Cu OFS 0.1% Ag SURFACE STATE : Rolled bright, free of
scratches and burrs STANDARD : ASTM F68-82 ALLOY SYMBOL : Cu -OF + Ag ASTM C
10700 STATE : HA (half-hard) CHEMICAL COMPOSITION : Cu (+Ag) =
99.95% min. Ag = 0.1 + 0.02%
We have asked a specialized company (Garlock Cefilac) to build a series for the prototypes
March 16, 2010
A single or a double window coupler ?Adjustable or fixed ?Why a double walled tube ?What about cryomodule integration requirements ?Vertically above or below the cavity ?DC biasing ?Air or water cooled ?
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Eric Montesinos / CERN-BE-RF-SR
What about cryomodule integration requirements ?
As already presented in November, the RF conditioning of the couplers will be done prior to joining to the cavity
A pair of couplers will be brought into the clean room with two cavities
The couplers will be mounted onto the cavity horizontally with its double walled tube (at the time as HOM, antenna, …) to not pollute the cavity: XFEL “cold window” process
March 16, 2010
CavityTrolley
CouplerTrolley
Non perturbed clean room laminar air flow
A single or a double window coupler ?Adjustable or fixed ?Why a double walled tube ?What about cryomodule integration requirements ?Vertically above or below the cavity ?DC biasing ?Air or water cooled ?
Eric Montesinos / CERN-BE-RF-SR
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What about cryomodule integration requirements ?
March 16, 2010
The eight cavities will be connected together
After that, the beam vacuum should remain closed until installation into the machine
Very important decision for the design of the coupler
→ imposes a short distance from the ceramic to the beam axis …
Non perturbed clean room laminar air flow
A single or a double window coupler ?Adjustable or fixed ?Why a double walled tube ?What about cryomodule integration requirements ?Vertically above or below the cavity ?DC biasing ?Air or water cooled ?
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Eric Montesinos / CERN-BE-RF-SR
What about integration into the cryomodule requirements ?
… because this defines the croymodule internal size
As we would like to test several coupler designs, the “worst” sized one will define the size of the cryomodule
With the next Talk, Vittorio will present us all these issues
March 16, 2010
A single or a double window coupler ?Adjustable or fixed ?Why a double walled tube ?What about cryomodule integration requirements ?Vertically above or below the cavity ?DC biasing ?Air or water cooled ?
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Eric Montesinos / CERN-BE-RF-SR
What about integration into the cryomodule requirements ?
We tried to make the different coupler designs compatible: Same double walled tube Same “integrated” length
Only 61 mm difference but different ways to connect
the waveguides
This has naturally eliminated the waveguide window coupler solution: Impossible to insert it inside
the cryomodule if already mounted onto the cavity
March 16, 2010
A single or a double window coupler ?Adjustable or fixed ?Why a double walled tube ?What about cryomodule integration requirements ?Vertically above or below the cavity ?DC biasing ?Air or water cooled ?
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Eric Montesinos / CERN-BE-RF-SR
Why Vertically above/below the cavity ?
Within the last SPL workshop, we received the following comments:
Particles will fall down forever from the coupler and will slowly pollute the cavity degrading its performances
Nevertheless, effect of pollution to the cavity is very difficult to predict
Before vertically, why not horizontally ? Because, an horizontal position will:
Not be better than below the cavity from the cavity pollution point of view
Not help for integration Makes the coupler vulnerable to shocks (it
would be in, or near the transport area, not enough space at the opposite wall side with the foreseen tunnel)
Give more stress to the coupler (effect of the weight of the “pretty long” antenna over 25 years ?, mechanical vibrations, …)
No clear advantages for an horizontal positioning of such a coupler
March 16, 2010
A single or a double window coupler ?Adjustable or fixed ?Why a double walled tube ?What about cryomodule integration requirements ?Vertically above or below the cavity ?DC biasing ?Air or water cooled ?
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Eric Montesinos / CERN-BE-RF-SR
Why Vertically above/below the cavity ?
Our experience with LEP and LHC couplers:
We always had couplers above the cavity
With various angle, 30°and 45° with LEP couplers, vertically with LHC couplers
We always had a “dust bin” in front of the couplers, at the bottom side of the cavity
While repairing a LEP cavity which had been in operation we were very surprised when finding “big” dust particles in the “dust bin”
SNS cavities have their couplers below the cavity, no problems were reported due to this position
The two positions seem to be acceptable
March 16, 2010
SNS cryomodulewith couplers below the
cavities
A single or a double window coupler ?Adjustable or fixed ?Why a double walled tube ?What about cryomodule integration requirements ?Vertically above or below the cavity ?DC biasing ?Air or water cooled ?
LHC cryomodulewith couplers above the cavities
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Eric Montesinos / CERN-BE-RF-SR
Why Vertically above/below the cavity ?
March 16, 2010
Below the cavity Above the cavity
the coupler will be the “dust bin”
No dust to the coupler
No dust to the cavity
Difficult to quantify how the cavity will be polluted by
the couplersand if it will really be ?
Could be a problem for the coating of the ceramic ?
No effect of dust onto the coating
Less convenient for wave guide distribution
More convenient for wave guide distribution
More protected from shocks Less protected from shocks
Same level of difficulty for cryomodule integration
In case of catastrophic event (as LHC 2008), same consequences, the cavity AND the coupler will be affected. Both will have to be taken out and fully reprocessed
Choice for the cavity Choice for the coupler
A single or a double window coupler ?Adjustable or fixed ?Why a double walled tube ?What about cryomodule integration requirements ?Vertically above or below the cavity ?DC biasing ?Air or water cooled ?
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Eric Montesinos / CERN-BE-RF-SR
Multipacting
Because we want to close the beam vacuum only once, the distance between the beam axis and the ceramic is given by :
1. The helium vessel, which gives the coupler port flange position, 130 mm from beam axis
2. The double walled tube length (See Ofelia’s talk)
3. The cryomodule size (see Vittorio’s talk)
Within all the proposed designs, the position of the ceramic will be given by these parameters
We will have to handle multipacting regardless of how it will appear
Amos’ and Rama’s talks this afternoon
March 16, 2010
1
2
Beam axis
3
A single or a double window coupler ?Adjustable or fixed ?Why a double walled tube ?What about cryomodule integration requirements ?Vertically above or below the cavity ?DC biasing ?Air or water cooled ?
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Eric Montesinos / CERN-BE-RF-SR
Multipacting and DC biasing
For long term operation it will be extremely useful to have DC biasing, because it suppresses multipacting
After a careful conditioning of the coupler without DC biasing, applying it will protect us from operational mistakes (remember SPS breakage during “wrong” tests)
The DC biasing has proven to be very efficient even if other methods could be used such as second frequency, magnetic surface roughness,…
It can be easily implemented with an air cooled antenna and a “doorknob free” waveguide (see talk part II)
March 16, 2010
A single or a double window coupler ?Adjustable or fixed ?Why a double walled tube ?What about cryomodule integration requirements ?Vertically above or below the cavity ?DC biasing ?Air or water cooled ?
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Eric Montesinos / CERN-BE-RF-SR
Air or water cooled ?
Mandatory from vacuum considerations:
NO water directly in contact with brazing making vacuum/water insulation
(except… exceptions…)
Water cooling of the antenna: Machine access in case of fault ? Risk of freezing water ? Time to warm-up a cavity ? More complex to apply the DC
biasing, need “insulating” pipes
No major difficulties to run air cooled coupler with LHC and SPS SC cavities couplers
March 16, 2010
Brazing making
insulation
between water
circuit and
beam vacuum
A single or a double window coupler ?Adjustable or fixed ?Why a double walled tube ?What about cryomodule integration requirements ?Vertically above or below the cavity ?DC biasing ?Air or water cooled ?
Eric Montesinos / CERN-BE-RF-SR
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Conclusion: the SPL coupler should be…
March 16, 2010
Set of questions Preliminary answers
A single or a double window coupler ? A single window coupler
Adjustable or fixed ? A fixed coupler
Why a double walled tube ? With a Double Walled Tube
What about cryomodule integration requirements ?
Mounted in clean room with its double walled tube in only one operation
Vertically above or below the cavity ?
With its final position vertically above the cavity
DC biasing ? With a HV DC biasing capacitor
Air or water cooled ? Air cooled
Eric Montesinos / CERN-BE-RF-SR
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March 16, 2010
End of part I, to be continued… 11h30
Many thanks for your attention
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