Transcript New RF Systems for the Super-ISR and Super-SPS Joachim Tuckmantel, CERN Thanks to Trevor Linnecar and Eric Montesinos 11/6/2015 CARE-HHH LHC-LUMI 05 Arcidosso.

New RF Systems for the
Super-ISR and Super-SPS
Joachim Tuckmantel, CERN
Thanks to Trevor Linnecar and Eric Montesinos
11/6/2015
CARE-HHH LHC-LUMI 05 Arcidosso
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Contents
· Hardware Considerations, an Overview
· cavities,power transmitter, transmission lines
· What exists around LHC: where we stand
· f-swing, loop gain and RF power basics
· Study of Options
· Input from Elena Shaposhnikova
· Conclusion
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CW1 Traveling Wave Cavities
· Wide f-range without tuner: follow speed modulation
at lower particle energy (SPS 10 GeV to ∞)
· ‘Fast’, allow e.g. fixed f acceleration (ions)
· Good RF power efficiency to beam
· Long (large ‘centralized’ power); not cheap
· Low gradient -> large parasitic impedance per MV
· HOM damping not straight-forward
· Impossible (and useless) superconducting version
|1: In contrast to pulsed TW as in 2-mile SLAC or CLIC
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Standing Wave Cavities
· High gradient: less parasitic impedance per MV
· Superconducting OK (with different shape)
· Sharp resonance: Wide range tuner following
speed modulation at lower particle energy
· ‘Slow’ (no ‘tricks’)
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Multicell SW Cavities
· Less ancillaries (couplers, control-units,…)
· Pass-band modes with closely grouped frequencies
Field polarity between power coupler and reference
antenna (compared to accelerating mode) opposite for
at least one mode.
With RF vector feedback (necessary) have to
prevent auto-oscillation: add sharp filter(s) ->
control-loop delay, gain limitation, stability …
Avoid if possible
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Superconducting Cavities (1)
· Less wall plug energy per MV: key importance LEP,
TESLA, … LHC no real importance (small system)
· Much Higher gradient: much less parasitic impedance
per MV
· Shape (fabrication reasons) lower R/Q also HOMs
· HOM couplers in any case: build max. damping
· Due to low R/Q and high voltage: less reactive beam
loading detuning -> the ‘tilt’ for sc. cav. in LHC
· Larger QL -> larger critical loop gain
· More sensitive to any ‘dirt’ in the vacuum chamber
· Need cryogenic installation, He recovery, storage
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Superconducting Cavities (2):
Tuning Range
· Plunger tuner forbidden (multipacting)
· Tuning done by elastic homogeneous longitudinal shape
deformation, today only ‘distinction’ between labs: how
produce the deformation (gear, piezo, magnetostrictive)
· Limited tuning range due elastic limit: LHC (stiff)
200 kHz/400 MHz, LEP (‘softer’) 300 kHz/352 MHz
(50 kHz exploited by ‘drive’, largely sufficient in LEP)
· Material fatigue for (rapid) cycling machines. No free
choice of material: high RRR Nb (or Cu for Nb/Cu)
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Superconducting Cavities (3):
Design Frequency Range
· Today major sc. cav. accelerators 352 MHz … 3 GHz:
solid experience in technology
· Cavity shape = ‘spherical’ : size scales with inverse
frequency (i.e. 200 MHz a ‘big beast’, ≈1.5 m diam.)
· Prototype 200 MHz single cell Nb/Cu (µ acceleration):
worked (not immediately) but below expectations,
development suspended (done at CERN, ‘sponsored’
by Cornell University)
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Superconducting Cavities (4):
“Field Limitations”
· Sc. cavities are limited by real-life defects and
fundamental physics properties of superconductivity
· Today’s record 45 MV/m (laboratory, th. lim.)
· LHC 400 MHz cavities: 5.5 MV/m (2 MV/cavity) ???
· RF power proportional Voltage*beam-current !!!
· LHC cavity field (voluntarily)
limited by Power Coupler Capability
· (matches well with single cell cavities)
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Superconducting Cavities
at CERN
· After development, installation, operation and
dismantling for LEP2 (1979 - 2000):
· All ‘first generation’ people are retired or work
on (completely) different problems
· There is a minimum team for the LHC
400 MHz modules (there is more to it …)
· Today till luminosity in LHC, no serious R&D
for (sc.) cavities and ancillaries possible at
CERN except by hiring (and teaching ?) staff.
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High Power Transmitters (1)
(Wall plug power -> (RF) power)
· Historically used: gridded tubes
· Stray capacitances more important for higher f
-> limit tube size -> limit power
· Way out for larger power: Combine output of
many tubes, e.g. in SPS system of 32 tubes of
35 kW (nom.) = 800 kW (true): expensive,
very complex in set up and maintenance
· (‘usine a gaz’, come to SPS-BA3 to have a look …)
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200 MHz Power system (Philips) at BA3/SPS: Each of the 4
cupboards delivers 2x35 kW … 1 MV/cavity
200 MHz Power system (Philips) at BA3/SPS: Hybrids to
‘join’ power flux from different cupboards
200 MHz Power system (Siemens) at BA3/SPS: The man-high
tube amplifier (right) contributes 135 kW …. 1 MW/cavity
High Power Transmitters (2)
· Large size of high power grid tube installations: House
them on ground level, very long transmission lines into
tunnel
· Gridded tubes have low gain at high frequency ->
· Need chain of amplifiers with increasing output power
(e.g. 5 levels)
· Significant increase of loop delay -> reduction of loop
gain if RF vector feedback (necessary for high beam
current application)
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High Power Transmitters (3)
· e-speed modulation tubes e.g. klystrons
· High gain: small solid state driver OK
· Scale (about) with wavelength: get very big for lower
frequencies
· Need circulator/insulator against reflected wave
· No principal obstacle to extend ranges but today:
· Down from 200 MHz: gridded tubes
· Up from 400 MHz: klystrons, ..
· Important: Industry ‘catalogues’ reflects this, each
‘transgression’ will be very expensive
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High Power Transmitters (4)
· Some recent developments:
· 1) Prototype of ‘diacrode® {Thales}’: 1 MW
@ 200 MHz, BUT price = 32 ‘classical’ tubes
(1000 h test only, suspended).
· 2) Prototype combination of several hundred
solid state amplifiers of 300 W for 30 kW @
353 MHz (klystron too expensive) -> 190 kW
in total envisaged {‘Soleil’}
· 3) IOT = merge tetrode and klystron (tests
‘low’ power only)
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Transmission Lines
· Wave guides or coaxial lines ?
· Wave guides have lower losses, less expensive
(for LEP: ‘precision ventilation air ducts’)
· Wave guides scale with wavelength
· Coaxial lines are ‘laborious’ to use (2 separate
conductors, bends), expensive
· No holy grail but
· 200 MHz: Coax, 400 MHz: waveguide
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Real Life: High Power RF (1)
· Today no more high power TV emitter (main
market) (cable TV or satellite)
· 1) High power RF equipment is getting more
and more expensive (small series)
· 2) Spare parts get sparse (we stockpile ...);
adapt hardware for different parts : Work
· 3) Industry is less ready for R&D (except $$)
· 4) Future engineers/technicians prefer ‘to flip
bits’ instead of learning high power RF
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Real Life: High Power RF (2)
· It is very difficult to find qualified staff
· -> Before throwing away ‘old’ hardware think
twice (not only $$):
· New hardware needs manpower + time
· 12.5 ns bunch distance in LHC ->
· Death of any 200 MHz (incl. capt. RF in
LHC !!) + PS, booster, ...
· -> use 10 ns or 15 ns (check PS, …)
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What exists (1)…
… shows the ballpark
· In SPS:
200 MHz TW cavities (2 x 2 cavities)
· total RF voltage ≈ 7 MV;
· RF power factory: 4 x 800 kW (1.1 MW nom.)
· ( 2 x 2 different designs: 32 or 8 tubes in end-stage)
· on the surface (due to size of installation)
· (Long) coaxial lines form surface down and along the
tunnel to cavities
· No fast RF vector feedback: ramping of beam induced
voltage at head of batch causes phase position slip ->
1-turn-delay feedback, feed forward: not perfect …
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200 MHz TW cavity (1 of 4 cav.)
What exists (2)…
… shows the ballpark
· For LHC:
· 400 MHz: superconducting cavities 2 MV (5.5. MV/m ) per
cavity; 1 set of 8 per beam; R/Q=45W
(grouped 4 cavities per cryo-module)
· HOM impedance a few kW (‘strong’ HOM couplers);
· Power coupler rated 300 kW (tested > 500 kW: reliability);
· RF Power : one klystron 330 kW per cavity ->
allows RF vector feedback without RF summing
· all under ground (‘Aleph-cavern’): ‘short’ delay (600ns)
· (Qext = 75,000 sc. cav. in coast)
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ACS 400 MHz superconducting
4-cavity module
What exists (3)…
… shows the ballpark
200 MHz capture system: 3 (short time 4) MV:
· 2 x 4 Cu cavities|1 0.75 (1) MV; R/Q=192W (built)
· Staged (data estimated) to be designed/adapted/built
· Main HOM impedance estimates (couplers !)
Rsh=23 (247) / 15 (487) / 5 (714) kW (MHz)
· Power coupler 300 kW
· gridded tube RF power chain (hybrids) 4x60kW –>
240 kW CW (300 kW short time)
· Position: under ground (LEP klystron. Gallery)
· (‘short’ delay for RF vector feedback) not more space
|1 design constraint: 2nd beam at 42 cm distance has to pass outside …
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ACN 200 MHz capture cavity
Frequency swing in SPS ring
p
p/200 MHz p/400 MHz Pb/200 MHzPb/400 MHz
f-f•
f-f•
f-f•
f-f•
[GeV/c] [kHz]
[kHz]
[kHz]
[kHz]
26
130.0984 260.1969 832.6792 1665.3585
150
3.9124
7.8249
25.1697
50.3395
450
0.4347
0.8695
2.7971
5.5942
1000
0.0880
0.1761
0.5664
1.1328
7000
0.0018
0.0036
0.0116
0.0231
???? SW
hardly possible SW
For ions read p: same momentum/charge (or B-field) as proton with …
Tuner range must also cover fabrication tolerances, changes
in cooling water temp., atm. pressure, power level (T), ….
‘Classical’ tuner : slowly a little bit from time to time ..
Now: “full stroke” for each acceleration cycle (bellows, …)
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Loop gain and loop delay (1)
Critical gain (when it really starts to auto-oscillate) depends
on ‘round-trip’ loop delay T (as short as possible !!!)
gcrit


QL
Qext

; 
for sc. cav.
2 f RF T  2 f RF T

Example for 200 MHz SW (ACN) capture cavities:
for nominal LHC beam current: Qsyst = 6000.
If ‘Power factory’ on surface: T[300 m] + (ampli.)≈2·10-6 s
gcrit << 7 : g = 3 usable; Rsh,0=192W·6000≈1.2MW -> 400kW
(per installed cavity of 0.75 MV)
(1-turn-delay feedback (filter!), feed forward: SPS close to limits)
Higher beam current -> set lower Qsyst-> lower gcrit
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Loop gain and loop delay (2)
RF vector feedback to reduce fundamental mode impedance:
RF power factory has to be under ground close to cavities
In SPS tunnel only possibility (except new excavations):
ex-UA-1 or ex-UA-2
caverns
Infrastructure ? High Power Transformer + AC input feeds,
cooling water, ventilation; if superconducting: He recovery
(refrigerator can be partly on surface)
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RF Power Basics (1)
· Major ‘problem’ with strong RF vector feedback (as
necessary in LHC / new injectors) and a beam with gaps:
· Not a ‘smooth’ beam but reactive beam loading to be
compensated for two ‘states’ - beam ‘on’ (train) and ‘off’
(gap / rest of turn) - while tuner too slow to adapt. Optimize
a static tuner setting but remainder to be done by
brute force RF power
· In LHC only weak acceleration (0.5 MV/8), but in fast
ramping injector several MV for acceleration
supply beam power = RF power
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RF Power Basics (2)
complex (model) generator current
 V 1


1 
V  
Ig  
 
  I b, DC fb sin   i   I b, DC fb cos  

 (R /Q) 

 2(R /Q)  Q0 Qext 

Real world generator RF power
Pg 
1
2
(R /Q) Qext Ig
2
V = accelerating voltage to be obtained (defines real axis)
(R/Q), Q0, = cavity constants; Qext = power coupling constant
I b,DC = instant. DC beam current;
 fb = normalized bunch form factor (| fb |≤1)
 = synchr. phase angle (‘proton convention’: zero for rising RF zero crossing)
 = (static) cavity detuning -> ‘half-detuning’ in coast with cos(1
Half detuning is not optimum under acceleration with ≠0º,180º
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RF Power Basics (3)
LHC nominal beam current on train
I b,DC = instantaneous DC beam current on train
nb = 1.1·1011 p -> qb = 1.6·10-19 C ·nb = 1.76·10-8 C
Tinterbunch = 25 ns -> I b,DC ≈ 0.7 A what cavity ‘sees on train ’(nominal)
(<I> = 1.76·10-8 C ·Nbun · frev -> <I> ≈ 0.55 A (average nominal in LHC)
fb = normalized bunch form factor = ‘ 1 ‘ for point bunches
Bunch-length changes but sufficiently short compared to l (especially at
200 MHz)
fb = 1 good enough
(we look for estimate, not 5th digit)
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SPS 25 -> 150 GeV (1)
· Elena |1: 400 MHz: l (bucket) too ‘short’ for
capture from PS -> need 200 MHz system
· Existing 200 MHz TW: Good for frequency
swing, 7 MV deliverable, but no ‘tight’ RF
vector feedback (long delay)
· Elena’s scenario(s), most demanding
situation V=9.2 MV (to contain a bunch of
0.5 eVs) while accelerating in 1s with 160
GeV/c/s @ 200 MHz
|1 previous talk
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SPS 25 -> 150 GeV (2)
· 1) Exploit the existing 200 MHz TW system and add 5th
cavity to reach 9.2 MV; cavity has to be produced as copy of
the existing 4; a 5th ‘RF power factory’ has to be established
(diacrode® ?) with coaxial lines down into the tunnel (space
for lines, factory in BA3 ?) ->
· Impedance in the SPS 25% higher
· no fast RF vector feedback (for future higher currents)
· RF power per section is limited to present value
Trevor L.: for higher current, rearrange cavity sections ->
5 shorter cavities, but still new power factory needed
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SPS 25 -> 150 GeV (3)
· 2) Add 3 SW cavities (3*0.75=2.2 MV) to existing TW
system: technically no improvement, still TW system
without RF vector feedback is dominant; rapid frequency
swing SW cavities
· 3) Remove whole 200 MHz TW system, replace by 12 SW
cavities (12*0.75=9 MV); technical advantage: full RF
system under RF vector feedback, less apparent fund. mode
impedance; rapid frequency swing SW cavities
· ( The 800 MHz Landau system would also need upgrade
for higher beam current, not treated here )
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SPS 25 -> 150 GeV (4)
3) -> Accelerate in SPS in 1 s -> 160 GeV/s |1, frev = 43 kHz ->
3.7 MV/turn or 310 kV/cav (12 @ 0.75) -> =180-24.5º
At 0.75 MV, Q0=31000 : 45 kW Cu heating to keep field up
For nominal train current 0.7 A: 220 kW power for acceleration
Qext = 6000 (as designed for the ACN cavities)
·
 V 1


1 
 V 
Ig  
 
  Ib,DC f b sin   i   Ib,DC f b cos  

 (R /Q) 

 2(R /Q)  Q0 Qext 

(High V/(R/Q) -> low  -> favors superconducting cavities)
Half-detuning -> 75% detuning : 270 kW / cav (12) (Qext=5000)
About ACN design + setting errors, …. need more in reality
|1 E≈c·p here for protons
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SPS 25 -> 150 GeV (5)
· Accelerate in 2 s … Elena -> 6 MV to keep bucket size for
0.5 eVs bunch: (existing TW for nominal Ib : no feedback)
· -> 8 cavities of 0.75 MV can do the voltage-job
· SPS 80 GeV/s, frev = 43 kHz ->
1.86 MV/turn or 230 kV/cav (8 @ 0.75) -> =180-18.1º
· Qext = 5000 (Qext = 6100 foreseen for the ACN cavities)
230 kW / cav (8)
––––––––––––––––––––––––––––––––––––––––––––––––
2x nominal current (same e): -> lower Qext = 3000 (? ! ?)
420 kW / cav (8)
Preferred -> more cavities with less V
( but higher impedance)
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SPS 25 -> 150 GeV (6)
· (Accelerate in 2 s … 6 MV to keep bucket size, contd.)
· -> 12 cavities of 0.5 MV can do the voltage-job
· SPS 80 GeV/s -> 155 kV/cav -> =180-18.1º
2x nominal current: -> lower Qext = 3000 (? ! ?)
300 kW / cav (12) (OK)
( = design limit ACN ; use again later)
3x nominal current: -> lower Qext = 1500 (? ! ? ! ? ! ?)
410 kW / cav (12)
Preferred -> more cavities with less V
( but higher impedance)
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Scaled 200 MHz superconducting
4-cavity module (‘virtual reality’)
SPS 25 -> 150 GeV (7)
‘Virtual option’: superconducting (1)
Scale 400 MHz modules to 200 MHz (cryost.≈2m diameter):
R/Q=45W; 5.5 MV/m -> V=4 MV (2x cavity length !)
· Power coupler might be 2x larger -> assume 1200 kW rated
· (very optimistic … never built ‘cold’)
· Acc. in 2 s -> 80 GeV/c/s -> 1.9 MV / turn -> =180-18.5º
2x nominal current: -> 2.7 MW beam power to be supplied
with some reserve (beam loading compens., setting error)
need 3 sc. cavities @ 2 MV/cav.
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SPS 25 -> 150 GeV (8)
‘Virtual option’: superconducting (2)
2x nominal current: -> Qext = 30000 (no problem sc.)
1000 kW / cav (3 sc.)
Problem: 3 x 1 MW plants under ground … ??? …
(same ’factories’ as SPS 200 MHz TW in BA3)
-> diacrode (life time not really tested !! … $$$ …)
–––––––––––––––––––––
Less daring option: Couplers of 450 kW -> need ≥ 8 cavities
(for which 0.75 MV are sufficient): sc. option not worthwhile
sc. option @ 200 MHz:
only if high power couplers OK
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SPS 25 -> 150 GeV (9)
‘Virtual option’: superconducting (3)
• Total RF power about same in nc. and sc. : 3-3.5 MW
( 2x nominal current)
• But: parasitic impedances lower for sc. (H. pow. couplers !)
3 sc. ‘low Rsh,HOM’ cav. < 12 nc. ‘high Rsh,HOM’ cav.
Rsh = 4 MW (3 sc.) ≈ 6.9 MW (12 nc.), no f.b.
(provided Qext=3000 possible nc.)
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SPS-HPS transfer (1)
Cavities do not ‘see’ particle energy (speed ≈ c, correct for frev)
Same current SPS and HPS (I hope at least …):
for same acceleration -> identical RF system will do
Beam transfer is done at zero-acceleration -> power reserve for
reactive beam loading compensation at slightly higher voltage
(ACN 200 MHz cavities can withstand 1 MV for a short time !!)
Elena: 2 paths for transfer and further acceleration in HPS
a) accelerate with 400 MHz in HPS ( 200 -> 400 MHz into HPS)
b) accelerate with 200 MHz in HPS ( 200 -> 400 MHz into LHC)
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SPS-HPS transfer (2)
Elena:
a)
Transfer/capture at 150 GeV -> 400 MHz in HPS needs
• voltage to make bunch short enough in SPS or
• capture long bunch @ 200 MHz + transfer in HPS
• SPS +9 MV @ 200 MHz (6-7 MV done) (+150%!!!)
HPS ‘for free’ = less MV(10) @400 MHz than needed for acc. (23)
• SPS +10 MV @ 400 MHz (6-7 MV @ 200 MHz done) (1/3 LHC)
HPS ‘for free’ = less MV (10) @400 MHz than needed for acc. (23)
• SPS ‘for free’ (4), HPS +8 MV @ 200 MHz (+130% of SPS 200 MHz)
b) Stay @ 200 MHz SPS -> HPS: 4 MV -> 4 MV:
SPS&HPS ‘for free’ : less MV than needed for acceleration
{ need b) 13 MV @200 MHz instead a) 23 MV @ 400 MHz -> 1 TeV }
->
Only reasonable solution: Continue @ 200 MHz Cu
(sc. cavities cannot use full possible field: power limitations)
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HPS acceleration 200 MHz
(ramp 3 s)
Keep bucket in HPS: need 13 MV @200 MHz during acceleration
-> 18 ACN cavities (0.75 MV) can de the voltage-job
150 GeV -> 1000 GeV in 3s, frev=43 kHz : 6.6 MeV/turn
-> for 18 cavities 370 kV/cav
-> 500 kW beam power/cav (2x nominal Ib)
Play numbers: 0.375 MV & 0.19 MV(acc) -> 300 kW/cav
35 cavities @ 200 MHz @300 kW RF power Qext=2000
(11 MW RF power under ground -> 20 MW wall plug, ‘pulsed’)
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HPS acceleration 200 MHz
(ramp 6 s)
Keep bucket in HPS: need 7 MV during acceleration
-> 10 ACN cavities (0.75 MV) can de the voltage-job
150 GeV -> 1000 GeV in 6s, frev=43 kHz : 3.3 MeV/turn
-> for 10 cavities 330 kV/cav
-> 460 kW only beam power/cav (2x nominal Ib)
Play numbers: 0.375 MV & 0.19 MV(acc) -> 300 kW/cav
18 cavities @200 MHz @300 kW RF power Qext=2000
(5.5 MW RF power under ground -> 10 MW wall plug, ‘pulsed’)
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HPS acceleration 400 MHz
(ramp 3s (6s), academic exercise)
Keep bucket in HPS: need 23 MV during acceleration
-> 12 sc. cavities (2 MV) can de the voltage-job
6.6 MeV/turn -> for 12 cavities 550 kV/cav
-> 770 kW only beam power/cav (2x nominal Ib)
lower voltage -> lager detuning for bl. comp. -> relat. more power
3s ramp: 40 sc. cav. @ 400 MHz (0.6/0.17 MV !) -> 300KW
6s ramp: 32 sc. cav. @ 400 MHz (0.75/0.11 MV !) -> 300 kW
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Conclusions (1)
Evident fact: Only few cavities, copper or superconducting, can
easily supply the desired voltage.
Gradients have to be lowered voluntarily since the power
coupler cannot transmit the corresponding RF power to
accelerate high beam currents and compensate react. beaml.
• Power coupler capabilities have to be
increased considerably, Qext ??
• for sc. cav. couplers: RF losses into liquid
He, “deconditioning”
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Conclusions (2)
For a 200 MHz system the existing ‘RF power factories’ for
large power are very space consuming -> problem to house
them close to cavities under ground (loop delay !!)
• Study compact RF power transmitter at
200 MHz
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Conclusions (3)
To keep the superconducting cavity option open - except copy
the existing 400 MHz system as is –
• Re-launch superconducting cavity research
activity at CERN
(The sputter activity Nb on Cu is not yet ‘dead’, possible study
for LHC crab cavities )
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