Possible proposed designs Part I : general considerations SPL power coupler March 16, 2010Eric...

Eric Montesinos / CERN-BE-RF-SR

Possible proposed designsPart I : general considerations

SPL power coupler

March 16, 2010

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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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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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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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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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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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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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



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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


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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


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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


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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


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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


LHC cryomodulewith couplers above the cavities

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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


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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


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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


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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



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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


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


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March 16, 2010

End of part I, to be continued… 11h30

Many thanks for your attention

Possible proposed designs Part I : general considerations SPL power coupler March 16, 2010Eric...

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