Transcript No Slide Title

NLC - The Next Linear Collider Project
Three Important Questions Studied for
a Linear Collider
&
Putting in Exergue the Importance of
NEG
Frédéric Le Pimpec
xxx – xxx
November, 2003
NLC - The Next Linear Collider Project
Outlook
1.
Vibrations caused by Near Field Sources inside the LINAC
Components  Deteriorate the Luminosity
2. Acoustic Localization of RF-Breakdown and Autopsies of RF
Structures  Length of the LINAC (Energy of particles)
3. Measures of the Secondary Electron Coefficient of Technical
Surfaces (TiN, TiZrV) – Electron Cloud  Deteriorate the
Luminosity
4. Molecular Desorption Induced by Electronic Transitions in
Technical Surfaces – NEG St707  Background noise inside
detectors
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NLC - The Next Linear Collider Project
Next Linear Collider
Final focus : beam size of the
beam spot ~ sx/sy :250nm* 3nm
Vibrations must be minimalized over
the 30km – Beam size ~mm
Dissipated Heat by the RF power
(4kW/m): => Need of large water
cooling flow (1 l/s at 70 MV/m)
Tolerances on vibrating structures
is rather loose (mm)
More Troublesome: The coupling,
even small, between structures and the
quadrupole (14 nm tolerance)
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Structure vibration tests at NLCTA
Sensors
• 1.8m Long RF structure DDS1 (~100 Kg)
• Installed on Hollow Aluminum girder
• Girder is connected to concrete block
• The block is installed on rubber balls (~14Hz
resonance) to isolate from noisy NLCTA floor
• Nominal total flow is 16GPM (~1 liter/s)
• Sensors: four piezo-accelerometers;
one piezo-transducer to measure water
pressure fluctuations (not shown)
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Vibration
setup test in SLD
Water vessel
• Study vibration transmission to
quadrupole in a structure-quad
assembly
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SLD pit. Gravity fed experiment
• Study vibration of the Structure girder due to internal turbulence
using gravity-fed water
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NLC - The Next Linear Collider Project
Amplitude of vibration in function of
the water flow inside a structure
Result with a Gravity
fed system
nominal
Conventional cooling System :
Result
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Vibration at FNAL : Primary vacuum
Structure (60cm)-support
FNAL geoph & piezo
FNAL STS-2
SLAC piezo
SLAC Piezo
Water IN
Strongback
FNAL Geophone
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NLC - The Next Linear Collider Project
In Summary (1/2):
• Vibration of RF structure-girder with NLCTA water
supply
– And found that external turbulence is important
• Vibration of RF structure-girder cooled with gravity
fed water
– And found that internal turbulence gives only 1/3 of the
total effect
• Vibration coupling from RF structure to Linac
quadrupoles
– And found that coupling is small – on a percent level (~2%)
• Primary vacuum in the system
– Does not affect the vibration level
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In Summary (2/2):
• Vibration of a structure-support set in Aluminium
and SS, water cooled by FermiLab water system
– Al support is noisier than a Stainless Steel support
• Made ANSYS simulations
– Further optimization of girder likely possible
• Klystrons RF Generated pulse in X-band (11.424
GHz) accelerating structure and coupling to the floor
– Effect due to RF pulses is negligible compared to induced
vibrations by cooling water
– Vibration transfer, by the floor, between a Klystron gallery
(8 pack type) and the LINAC is negligible (noise)
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Acoustic Localization of RF
Breakdown
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H90VG5R : Historic of RF arcing and
localization
+
Al
e-
1 mm
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Input Couplers - “pulse heating”
local
1
2
4
3
50 mm
Horn at the base of the waveguide side.
Cracks observed solely on RA;
Stress? voids inside the lattice?
Typical for edges on the
cavity side: T53VG3R & RA
Sharp edges can be back bombarded by ions ?
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Examples of particles
distribution inside cavities
60% of particles < 1 mm of diameter
50000
Metals (not S, Mn, or Al)
Mn-S
45000
4000
T53VG3RA (Interior Cells)
Particles per Square Centimeter
Al compounds
Metals (not S, Mn, or Al)
Mn-S
Al compounds
Mn-other
S compounds
Carbon
3500
40000
Mn-other
3000
S compounds
35000
2500
Carbon
2000
30000
1500
25000
1000
20000
500
D
57
57
D
U
57
57
U
D
44
44
D
U
44
D
30
U
30
16
U
16
D
0
15000
Cell Number
10000
5000
44
U
30
D
30
U
16
D
16
U
2D
2U
1D
0
1U
Particles per Square Centimeter
4500
Cell Number
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In summary
• No proven link between particles contamination and
breakdowns.
- Results are different for superconducting RF, DC breakdown at
room temperature cf. Cornell & Windowtron experiment(SLAC). However
particles are still players !
• H90VG5R & H90VG3N : contamination by a foreign
element is responsible of quasi-systematic breakdown.
- Areas of low E field (7 MV/m – 65MV/m) but high H (0.6MA/m –
0.03MA/m) . Arcing depends : RF Pulse length ? Energy level ? E or H
field ?… Combination ?
• New design for the input couplers; rounded edges
design
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Secondary electron yield (SEY):
• Thin film TiN – TiZrV (NEG) measure SEY,
reproducible results. Test of different surface treatment : Ionic
conditioning, electronic, bakeout, air baking (CERN), Free
radical exposure Freon11 (CERN), O, H, N …
• Measure of the energy spectrum of the SEY at low energy <
40eV
• Results will be used to improve electron clouds simulation
models
• NEG SEY results after 43 days of vacuum exposure does not
seem to be in agreement with CERN ones (d=1.35)
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My Three most Important Questions for a
Linear Collider : Conclusions
- Support design for RF structures or quadrupoles is important, but also
the design of the cooling system and its components. RF generating
components are not problematic (vibration wise) as long as they are
installed on an adequate anti-vibratory structure. An independent Klystron
gallery does not guarantee sufficient vibrations attenuation.
- The technique of acoustic localization for breakdown is complementary,
and could be supplementary, of the RF technique.
- Autopsies have revealed that the level of vacuum cleanliness for the
structures must be comparable to the one in superconducting RF, even
perhaps at the level of semi-conductor industry. Surprising ?
- Surface treatment to deal with the electron cloud issue, LHC, NLC, KEK
B, PEP II… Not New ! But new ways, NEG films.
As we talked of NEG…
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NEG : Advantages – Drawbacks
non-exhaustive List for an accelerator
Distributed Pumping (S(l/s) increase with the Roughness)
Low activation Temperature, TiZrV - 180°C
Lump pumping
Low initial ESD / PSD compared to standard technical
surfaces– ISD it depends (Light or heavy ions and their energies)
Low d (SEY)– Electron cloud solution
Inserted “linear” pump
Life Cycle : Number of possible activation
Inserted “total” pump
Future
(TiZrV) Surface pump
/ diffusion barrier
Cf. Benvenutti
Pumping Capacity : depends of the thickness
Wakefield
Film stability in time on the substrate
Sputtering - flaking
Require activation
Does not pump CH4, unless hot 110°C - St707 or ionized
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Dynamic Vacuum
(300K)
Desorption Coefficient h
Electrodesorption Multipacting Fct
Molecular density
(SEY…)
Is i
n
 Is i



V
= h  + hi
n + he 
n 1   2  f(I)
t
e
 e

2
 n z 
Photodésorption
 Aq -  S n  u
 z2
Ionodesorption
h
Thermal
Outgassing
Num ber of desorbed m olecules
Num ber of irradiating particles
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Linear Pumping
NEG
Molecular density coming
From the sides
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Photodesorption at EPA
(1997-1999)
c = 45 eV and 194 eV
Roughness NEG St 707 [Zr (70%) V (24.6%) Fe (5.4%)] ~ 60 x RSS
Exposed Surface 119 cm2 ; directly irradiated ~5 cm2
.
= 3.7 1016 photons.s-1
.
17
-1
Ee- = 308 MeV
Ee- = 500 MeV
 = 1.08 10 photons.s
.
NEG St 707 ????
No access to TiZr (1998)
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PSD and/or ESD
Measurement Limitation
• Parasitic molecular desorption - RGA resolution limit
Molecular desorption induced by reflected photons from the target
- Important for an activated NEG Qparas ~ 0.5 QNEG100%
Outgassing flux of a baked RGA (80% H2) Q ~ 10-9 Torr.l.s-1
• Electronic Target Current Limitation, Thermal outgassing H2
• Correction for an activated NEG
Activation  Target is pumping  Need to correct the measured h
Coefficient h under-estimated
Coefficient h over-estimated, OK for ESD
Photodesorption coefficient are corrected by a Monte Carlo which take care of
the system geometry (from the target to the conductance).
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NLC - The Next Linear Collider Project
h CO at c = 194 eV
1.E-03
NEG 0%
Sat
(13C18O) 13C18O
CO
ETA (molec/photon)
1.E-04
1.E-05
SS
1.E-06
Sat (13C18O) CO
1.E-07
1.E+18
1.E+19
1.E+20
1.E+21
NEG 100 %
1.E+22
1.E+23
Dose photons
An activated NEG desorbs less H2 CO CH4 CO2 than a baked Stainless Steel .
A saturated NEG desorbs more CO than a baked Stainless Steel
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Electrodesorption
Ee- = 300 eV
Roughness NEG St 707 [Zr (70%) V (24.6%) Fe (5.4%)] ~ 60 x RSS
Irradiated NEG Surface ~15 cm2
<Ie- > = 0.25 mA
.
 = 1.6 1015 élect.s-1
Tmax = 52 °C
NEG St 707 !!!!
No Access to TiZr or TiZrV
(1998-2000)
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h CO at Ee- = 300 eV
1.E-01
CO
NEG Sat (13C18O) 13C18O
1.E-02
Eta (molec/elect)
NEG Saturated by
CO
1.E-03
Cu
1.E-04
1.E-05
NEG 100 %
1.E-06
1.E+16
1.E+17
NEG Sat (13C18O) CO
1.E+18
1.E+19
1.E+20
1.E+21
Dose Electrons
An activated NEG desorbs less H2 CO CH4 CO2 than a 120 °C baked OFHC
Cu surface.
A saturated NEG desorbs less *C*O than a 120 °C baked OFHC Cu surface
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NLC - The Next Linear Collider Project
Conclusion : NEG from none to
everywhere…
NEG St 707 – TiZr and TiZrV activated at 100% photodesorbs and
electrodesorbs less gas than a baked standard technical surface.
Molecular desorption in 13C18O or CO is relatively low despite the quantity
of gas deposited onto the surface !! (Identical observation with photons)
• Roughness and porosity of St707 !
• Diffusion under photon or electron bombardment of surface atoms !
• 2 hypothesis combination – Smooth thin film can be of used to test those
hypothesis
NEG study in the vacuum group – Competition with the Material group
 Better NEG  LHC (TiZrV)… Experimental areas
NEG MATERIAL of the FUTUR
With advantages which of
today are undeniable …
LHC – NLC – RHIC (60m) – SOLEIL …
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Extra Transparents
NLC - The Next Linear Collider Project
Raison pour autopsier les
structures RF?!
T53VG3R : Autopsie du coupleur d’entrée (IC- horns) .
Contamination par des particules de MnS (ou métalsoufre)
T53VG3RA : Problème accru au niveau de l’IC (horns).
Contamination par des particules de MnS
H90VG5R : Une cavité RF, sur l’ensemble de la structure montre
un niveau de claquage anormal – Débris d’Al
H90VG3N : Une cavité RF, sur l’ensemble de la structure montre
un niveau de claquage anormal – Particule d’acier
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NLC - The Next Linear Collider Project
Présentation des résultats
Photodesorption at EPA : 11 mrad incidence c = 194 eV
An activated NEG desorbs less H2 CO CH4 CO2 than a baked Stainless Steel
SS
Cu
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NLC - The Next Linear Collider Project
Preliminary evaluation for
cooling induced vibration (nominal flow
•
RF structure vibration:
– About 110nm if fed with quiet water
– About 350nm mostly due to turbulence in supplying
pipes for water system similar as in NLCTA
– About 150nm if fed with SLD water
SLD water system seems similar as NLCTA
1l/s)
(Slide 8)
(Slide 4)
(Slide 8)
•
EM quad receive
– About 13nm (182-122)0.5 due to coupling to structure if structure
fed with SLD water; (2.4nm with quiet water; >30Hz) (Slide 8-9)
24nm > 4Hz
•
Total vibration (if all sources are independent) for EM quad with
NLCTA-like water system (cf Nanobeam talk): 8.3nm

all “if” , all simplifications
and difference from the real system
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