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Design and Operating Experience with SNS Superconducting Linac. FNAL September 30, 2010 Sang-ho Kim SCL Area Manager SNS/ORNL. Machine layout. Accumulator Ring: Compress 1 msec long pulse to 700 nsec. Chopper system makes gaps. 945 ns. mini-pulse. Current. Current. 1 ms macropulse. - PowerPoint PPT Presentation
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Design and Operating Experience withSNS Superconducting Linac
FNAL September 30, 2010
Sang-ho KimSCL Area ManagerSNS/ORNL
2 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
H- stripped to p
Machine layout
2.5
DTL
86.8
CCL
402.5 MHz 805 MHz
SRF, =0.61 SRF, =0.81
186 387 1000 MeV
Linac; 1 GeV acceleration
Front-End: Produce a 1-msec long, chopped, H-beam
PUP
945 ns
1 ms macropulse
Cur
rent
mini-pulse
Chopper system makes gaps
Accumulator Ring: Compress 1 msec
long pulse to 700 nsec
Liquid Hg Target
Cur
rent
1ms
259 m
3 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
SNS SCL History and initial design concerns
• SNS baseline change from NC to SC in 2000, relatively late in the project
• RF frequency; followed that of the NC CCL (from LANSCE)• SRF Cavity designs were mainly driven by two constraints
– Power coupler; maximum 350 kW (later increased to >550 kW)– Cavity peak surface field; 27.5 MV/m field emission concerns
• Later increase to 35 MV/m for HB cavities by adapting EP
• With one FPC to cavity; HB cavity 6 cell• Long. Phase slip at low energy; MB cavity 6 cell• And then usual optimization process
– TTF, peak surface field balancing, raise the resonant mechanical frequency, LFD, HOM, etc
4 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
SNS SCL Components
Cryomodule and all internal components developments
; done by JLAB including prototyping
• Power coupler; scaled from KEK 508 MHz coupler
• HOM coupler; scaled from TTF HOM coupler
• Mechanical tuner; adapted from Saclay-TTF design for TESLA cavities
• Piezo tuner; incorporated into the dead leg for possible big LFD (later on)
• Cryomodule; similar construction arrangement employed in CEBAF
• Nb material RRR>250 for cells and Reactor grade Nb for Cavity end- group
5 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
SNS Cavities and Cryomodules look;
Fundamental Power Coupler
HOM Coupler
HOM Coupler
Field Probe
=0.61 Specifications:Ea=10.1 MV/m, Qo> 5E9 at 2.1 K
Medium beta (=0.61) cavity High beta (=0.81) cavity
SlowTuner
Helium Vessel
FastTuner
=0.81 Specifications:Ea=15.8 MV/m, Qo> 5E9 at 2.1 K
11 CMs 12 CMs
6 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
SNS SCL, Operations and Performance• The first high-energy SC linac for protons, and the first pulsed operational
machine at a relatively high duty • We have learned a lot in the last 5 years about operation of pulsed SC linacs:
– Operating temperature, Heating by electron loadings (cavity, FPC, beam pipes), Multipacting & Turn-on difficulties, HOM coupler issues, RF Control, Tuner issues, Beam loss, interlocks/MPS, alarms, monitoring, …
• Current operating parameters are providing very stable and reliable SCL operation – Less than one trip of the SCL per day mainly by errant beam or control noise
• Proactive maintenance strategy (fix annoyances/problems before they limit performance)
• Beam energy (930 MeV) is lower than design (1000 MeV) due to high-beta cavity gradient limitations (mainly limited by field emission)
• No cavity performance degradation has occurred to Oct. 09– Field emission very stable – Recently Nov. 09; Two cavity has shown performance degradation
• Several cryomodules were successfully repaired without disassembly– Multiple beam-line repairs were successfully performed
7 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Parameters DesignIndividually
achieved
Highest production
beam
Beam Energy (GeV) 1.0 1.01 0.93
Peak Linac Beam current (mA) 38 42 42
Average Linac Beam Current (mA) 1.56 1.1 1.1
Beam Pulse Length (s) 1000 1000 825
Repetition Rate (Hz) 60 60 60
Beam Power on Target (kW) 1440 1100 1100
Linac Beam Duty Factor (%) 6.0 4.8 4.8
Beam intensity on Target (protons per pulse) 1.5 x 1014 1.55x 1014 1.1 x 1014
SCL Cavities in Service 81 80 80
Machine Performances
8 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Cavity Specifications
Frequency 805 MHz
N. of cells 6
Cell-to-cell k [%] >1.5
Geom. 0.61 0.81
Epk [MV/m] 27.5 35
Lorentz KL [Hz/(MV/m)2] < -2 ± 1 (static)
Q (2.1 K) 5 109
9 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Cavity Shape Design (scan parameter space)
2a
2b
Iris aspect ratio(a/b)
Slope angle
R Dome (Rc)
R E
quat
or (
Re
q)
R I
ris (
Ri)
()
For circular dome(Elliptical dome cases are same)
Rc, Ri, , one of (a/b, a, b) ; 4 controllable parametersReq (for tuning)
Geometry optimization;Pretty well understood and straightforward
0.90
0.95
1.00
1.05
1.10
1.15
1.20
1.25
1.30
1.35
0.0 0.2 0.4 0.6 0.8 1.0
Iris ellipse aspect ratio (a/b)
No
rma
lize
d r
ati
os
Ep/EoT(g)
k
Bp/EoT(g)
RsQ
ZTT
For fixed , Rc, Ri
Now, a/b isdependent parameter
Ex. =0.61, 805 MHz
10 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Cell Shape optimization-criteria dependent Scanning all geometry space (systematic approach)-Example
Bp/Ep=2.0 (mT/(MV/m))
Bp/Ep=2.2
Bp/Ep=2.4
k=2.5 %
k=2.0 %
k=1.5 %
KL=4
KL in Hz/(MV/m)2
KL=3 KL=2
30
Bore Radius=50 mmBore Radius=45 mmBore Radius=40 mm
4.0
3.6
3.2
2.8
2.4
2.0
Ep
/EoT
(g)
32 34 36 38 40
Dome Radius (mm)
Ex=0.61, 805 MHz at the slope Angle=7 degree
11 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
End Cell design and Qex
1E+05
1E+06
1E+07
1E+08
-25 -20 -15 -10 -5 0 5 10 15 20 25
Penetration [mm]
Qex
t
; Measured
; Calculated
Increase magnetic volume Qex estimation is quite
accurate even for the high Qex system (Also alignment error analysis is available)
12 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Beam, Qex, RF, margins (design)
0.0E+00
2.0E+05
4.0E+05
6.0E+05
8.0E+05
1.0E+06
1.2E+06
1.4E+06
1 21 41 61 81
Cavity Number
Qs
0.0E+00
1.0E+05
2.0E+05
3.0E+05
4.0E+05
5.0E+05
6.0E+05
0 20 40 60 80 100 120
cavity number
Po
we
r (W
)
P_Qref
P_Q-20%
P_Q+20%
Pb
Early stage of SNS; 36mA, Epk=27.5 for both betas with 11 MB CM + 14(15) HB CM
Qb
Qex +/- 20 %
Highly non-linear region
Control margin, dynamic detuning
0.0E+00
2.0E+05
4.0E+05
6.0E+05
8.0E+05
1.0E+06
1.2E+06
1.4E+06
1.6E+06
1.8E+06
1 21 41 61 81
Cavity Number
Qs
0.0E+00
1.0E+05
2.0E+05
3.0E+05
4.0E+05
5.0E+05
6.0E+05
0 20 40 60 80 100 120
cavity number
Po
we
r (W
)
P_Qref
P_Q-20%
P_Q+20%
Pb
Final SNS; 26mA, Epk=35 for high beta with 11MB CM +12 HB CM
Highly non-linear region
Control margin, dynamic detuning
13 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Dynamic Mechanical Behavior of Elliptical Cavities-in design stage
Many groups have done series of analysis with FEM codes.
Static properties; we can find pretty accurately
Mode, damping, modal mass findings; Strongly depends on boundary condition, especially finding damping degree for each mode
very difficult
Analysis before having experimental results statistical like any other resonance issues Relative comparisons
14 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Dynamic detuning
-200
0
200
400
600
800
1000
0 500 1000 1500
Time (us)
Dyn
am
ic d
etu
nin
g (H
z)
-200
-100
0
100
200
300
400
500
600
0 500 1000 1500
Time (us)
Dyn
am
ic d
etu
nin
g (H
z)
Medium beta cavity (installed cavity)KL: 3~4 Hz/(MV/m)2
17 MV/m
High beta cavity (installed cavity)KL: 1~2 Hz/(MV/m)2
16.5 MV/m
Observed detuning agrees with expectations
-200
0
200
400
600
800
1000
0 500 1000 1500
Time (us)
Dy
na
mic
De
tun
ing
(H
z)
15Hz
30Hz
60Hz
filling flattop
-400
-200
0
200
400
600
800
1000
0 300 600 900 1200 1500
Time (us)
Dyn
am
ic D
etu
nin
g (
Hz)
0.0E+00
2.0E+06
4.0E+06
6.0E+06
8.0E+06
1.0E+07
1.2E+07
1.4E+07
1.6E+07
1.8E+07
Ea
cc (
MV
/m)
Dynamic detuning
Eacc
In this example the accelerating gradient is 12.7 MV/m. (high beta cavity)
The 2 kHz components shows resonances at higher repetition rate
in some of medium beta cavities
But, a few cavities show bigger resonance phenomena as higher repetition rate
15 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
While learning
AFF learning
AFF fully learned At beginning
Some cavities need ~>25 % more RF at the beginning of AFF
16 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
• Overall concerns between peak field, operating gradient, inter-cell coupling, RF margin, detuning, Qex (fixed or variable), cost, and system stability
At the present operating condition
0.0E+00
2.0E+05
4.0E+05
6.0E+05
8.0E+05
1.0E+06
1.2E+06
1.4E+06
1.6E+06
1.8E+06
2.0E+06
1 21 41 61 81
Qs
Cavity Number
0.0E+00
1.0E+05
2.0E+05
3.0E+05
4.0E+05
5.0E+05
6.0E+05
1 21 41 61 81P
ow
er
(W)
cavity number
Qb
Qex
RF power at 26mA average currentIn steady state
17 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
0 10 20 30 40 50 60
Electron loading (EG heating, gas burst, quench)
Coupler Heating
HOM (1cavity disabled,
5 limited by large coupling)
Quench (hard, Eacc<10 MV/m)
Lorentz force detuning
No limits up to 22 MV/m
No. of cavities
Cavity Gradient Limiting Factors (60 Hz Operation)
-Dominated by Electron Loading (Field Emission & Multipacting)-~14 cavities are limited by coupler/end-group heating (MP),
but close to the limits by radiation heating-Operating gradients are around 85~95% of Elim
One does not reach steady state mechanicalvibration
1 cavity is disabledCM19 removed and repaired
CM12 removed and found vacuum leaks at 3 HOM feedthroughs (fixed)
18 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Interactions between systems/cavities (collective effects)• Cavity radiation/cold cathode gauge interaction
• Helium flow in one cavity creates “vapor lock” in another heating of coupler’s outer conductor
• Multipacting triggers radiations
• Electron activity in one cavity triggers cold cathode gauge in another
• Field emission in one cavity heat up beam pipe in another(s) depending on relative phase and amplitude– creates difficulty in finding proper op. gradient for all ranges of
phases– Main limiting factor in SNS– Higher duty more problematic
19 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
End group heating/beam pipe heating + quenching/gas burst
Electron Loading and Heating (Due to Field Emission and Multipacting)
Multipacting; secondary emission– resonant condition (geometry, RF field)– At sweeping region; many combinations
are possible for MP Temporally; filling, decay time Spatially; tapered region Non-resonant electrons accelerated
radiation/heating
– Mild contamination easily processible– But poor surface condition processing
is very difficult in an operating cryomodule
Source of electrons
Result
Easy to remove with DC biasing
● Field Emission due to high surface electric field
20 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
End Group Heating & Partial quench
Analysis for end group stability ; >4-5 W (overall) or ~1W local can induce quench
Electron activity (Field emission, non-procesible MP)-induced end group quench:
Large temperature rise (24 K) at beam pipe.
Quench leads to semi-stable intermediate state condition: Qo~ 2-3 x 105
FE at OC
Low RRR & long path to the thermal sinkThermal margin is relatively small, Results in thermal quench
At partial Quench (Measured data)
Cavity Field Forward P
21 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
SNS Cavity Operating Regime
Time
Measurements of Radiation during RF Pulse
Rad
iatio
n (a
rb.
Uni
t)
Rad
iatio
n (in
log,
arb
. U
nit)
Eacc
FE onset
Radiation onset
MP Surface condition
We don’t have MP induced radiation at op. gradient, if any, very small.Basically running in the field emission regime.
22 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Back to Cavity performances in VTA test
1.E+08
1.E+09
1.E+10
1.E+11
0 5 10 15 20 25
Gradient (MV/m)
Qo
1.E-02
1.E-01
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
0 5 10 15 20 25
Gradient (MV/m)R
ad
iatio
n (
arb
. u
nit)
MP
FE
Typical high beta cavity
More precisely this MP indication is MP induced radiation.We observed MP starting from 3 MV/m in both medium and high beta cavities.In general MP can be processed and does not hurt operation that much.A few cavities are showing a symptom of non-processible multipacting
23 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
HOM; in design stage
• No Beam dynamics issue
• Centroid error, f spread & location of cavities were in question
• When Q>105, 106, there’s a concern. – HOM power ~ fundamental power dissipation– but the probability is very low even under the conservative
assumptions
• Extra insurance– SNS is the first pulsed proton SC linac– Any issues were treated in a very conservative way
• Ex. Piezo tuner; we’ve never used them
24 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Problems while running RF only
Any electron activity (multipacting, burst of field emitter, etc)Destroy standing wave pattern (or notching characteristics)Large fundamental power couplingFeedthrough/transmission line damage (most of attenuators were blown up)Irreversible damages could happen statistically
Electric Field
Magnetic field
CCG
f or tau Eacc
Conditioning after removing feedthrough;Large electron activities around HOM couplers were observed ranging from ~3 MV/m up to 16 MV/m.
16Mv/m
25 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Fundamental mode thru HOM coupler
Normal waveform of fundamental mode from HOM ports (y-axis; log scale)
HOMA
HOMB
Fundamental mode couplingHigh 1010~ 1012
; much less than a few W during pulse
26 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Abnormal HOM coupler signals (RF only, no beam)
1~5 Hz 10 Hz 30 Hz
~’0’ coupling and rep. rate dependent signals
Electron activities (MP & discharge; observations under close attention)
27 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Leak, severe MP, contamination, large coupling, …
28 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
HOM in SNS
• Availability & Reliability; Most Important Issue– HOM couplers in SNS have been showing deterioration/failure as reported
– Reliability & availability of SNS SRF cavities will be much higher w/o HOM coupler
• More realistic analysis with actual frequency distributions measured.– Probabilities for hitting dangerous beam spectral lines are much less than expected.
– Beam amplitude fluctuation is also very small
• Future Plan– HOM feedthroughs will be taken out
as needed
– PUP cryomodule
• Will not have HOM couplers
SNS beam (FFT)
29 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Turn-on difficulties
Vacuum
Gradient
6 days
early 2006;After a long shut-down, some cavities showed turn-on difficulties.Gradients were lowered down or turned-off in order to reduce the down time.
Severe contaminations in coupler surfaces or cavity surfaces ?????
Erratic behavior due to the erosions of electrode; no responses or too much
Vacuum Interlock
30 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Turn-on and High power commissioning First turn on must be closely watched and controlled
(possible irreversible damage) Initial (the first) powering-up, pushing limits, increasing rep.
rate (extreme care, close attention) Aggressive MP, burst of FE possibly damage weak
components Similar situation after thermal cycle (and after long shut down
too) behavior of the same cavity can be considerably different from run to run
Subsequent turn-ons (after long shut-down) also need close attention: behavior of the same cavity can be considerably different from run to run gas re-distribution
Cryomodules/strings must be removed and rebuilt if vented/damaged
31 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Individual limits & collective limits
• Operating gradient setting in SNS are based on the limiting gradients achieved• Operational stability is the most important issue
0
5
10
15
20
25
30
1a 2a 3a 4a 5a 6a 7a 8a 9a 10a
11a
12a
12d
13c
14b
15a
15d
16c
17b
18a
18d
19c
20b
21a
21d
22c
23b
Cavity number
Ea
cc (
MV
/m)
10 Hz individual limits 60 Hz collective limits
Large fundamental power through HOM coupler
CM19; removed
Field probe and/or internal cable (control is difficult at rep. rate >30 Hz)
Design gradient
Average limiting gradient (individual)
Average limiting gradient (collective)
32 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Current Operating Condition• 1105 us RF (250 us filling + 855 us flattop) at 60 Hz
– Flattop duty; 5.1 %
• Eacc setpoints; about 85 % of collective limits in average– Average gradient; ~12.5 MV/m– 925 MeV + 10 MeV (energy reserve)
• Stable operation; < 1 trip/day (<5 min./day) mainly by errant beam, control noise
0
2
4
6
8
10
12
14
16
18
1a 1c 2b 3a 3c 4b 5a 5c 6b 7a 7c 8b 9a 9c 10b
11a
11c
12b
12d
13b
13d
14b
14d
15b
15d
16b
16d
17b
17d
18b
18d
19b
19d
20b
20d
21b
21d
22b
22d
23b
23d
Cavity number
Eac
c (M
V/m
)
33 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Stable Operation of SCL
• Better understanding of: – underlying physical phenomena (outgassing, arcs, discharges,
radiation, field emission, beam strike, dark current etc.)– components response (arc detectors, HOM couplers, Cold Cathode
Gauges, coupler cooling, end group heating) – controls (LLRF logic, programming, choice of limits and stability
parameters)
• Improve performances and ultimate beam power by:– Optimizing gradients, modulator voltages/configuration, matching of
klystrons to cavities, circulator settings, available forward power for beam loading, cryomodule repair, etc.
34 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Status of components and parts
• FPC; very stable/robust
• HOM coupler; vulnerable component especially during conditioning
• Cavity – MP; about 25 cavities show MP, not a showstopper– Field emission; very stable; not changed, main limiting factor– Errant beam could degrade cavity performance (had 2
events)
• Tuner; vulnerable component (both piezo and mech.)
35 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Performance degradation by errant beam• First time in 5-years operation + commissioning
• Limiting gradient of two cavities; 14.5 MV/m due to FE Partial quench at 9 MV/m
• Beam between MPS trigger and beam truncation off-energy beam much bigger beam loss at further down-stream gas burst redistribution of gas/particulate changes in performance/condition
• Random, statistical events; made HOM coupler around FPC worse
Partial quench
Cavity fieldForward power
36 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
At errant beam condition; MPS
• MPS– If RF field regulation becomes bad, RF/beam truncation– If BLM signal touches the threshold, beam truncation– MPS; supposed to be less than 20-30 us
• Had performance degradations with 2 cavities claimed that errant beam is too frequent and MPS delay looks long
• Measured all MPS delay in the linac; 50-300 us– Caps, some open collector circuit
37 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Errant beam from the sourceMPS truncation <30 us
Before improvements of MPS(50~300 us)
38 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Tuner• Pressure incidents, 2-4-2K transition, or just short life time
about 10 tuners are replaced.– Harmonic driver, piezo stack (and/or motor) failure
– Worn out in progress, loosen connection, slips; unstable mechanical boundary; irregular detuning
Piezo Actuator(2X) Flexure
Connection to Cavity
(2X) Flexure Connection to
Helium Vessel
Motor & Harmonic Drive
Connection to Helium Vessel
39 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Irregular dynamic detuning (9b)
Eacc
Tuner motion
40 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Cryogenic loads (I)Dynamic load estimation; provide constant load condition to cryogenic system for reliable 2K operation
Static loss (20~25 W/cryomodule) total ~500 W
Thermal radiation from fundamental power coupler static; without RFdynamic; with RFestimation 20~50 W to 2K circuit at 1MW beam operation
Cavity surface dynamic loss (design parameter Qo > 5e9 at 2.1 K)BCS resistance (~6.5 nOhm at 2.1K, 805 MHz)residual resistance (10 nOhm)Other heating; FE, MP, pure Q-deacy
Ex. at 6.5% duty at 60 Hz & at design gradientPbcs(6.4nOhm)+Pres(10nOhm)=130 W, Pother=210 W Qo=5e9,
Qo~1.1e10 (MB)Qo~1.3e10 (HB)
41 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Cryogenic loads at the present operating condition
• Overall Qo~4.5e9
Helium pressure; ~0.04 atm
Helium flow rate; ~105 g/s
Total heater power; 1490 W
Total heater power; 1750 W
Turned offall SRF cavities
RF on RF off
42 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Operation temperature
50
70
90
110
130
150
170
190
210
2.0 2.5 3.0 3.5 4.0 4.5 5.0
Tb (K)
Su
rfa
ce M
ag
ne
tic F
ield
(m
T)
SNS High Beta Cavity Nominal
20 30 50 70 100 200 300
500
700
RF surfacedissipation=1000 W
2000
Limits(CW operation)
Best SNS
with the existing SNS cryo-plant at the SNS SCL layout
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 0.02 0.04 0.06 0.08 0.1
Duty
No
rmal
ized
Ele
ctri
c P
ow
er a
t RT
4.2 K (450W/W)
2.1 K (1200W/W)
4.4 K4.6 K
Relatively low frequency, low field, high static loss, field emission; 2K is not optimum
43 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Operational efficiency
0.0
0.2
0.4
0.6
0.8
1.0
1.2
2 2.5 3 3.5 4 4.5
Operating Temperature (K)
No
rma
lize
d O
pe
rati
ng
Co
st
Duty=1 %
Duty=8 %
Duty=5 %
Duty=3 %
with a cryo-plant to be designedat the SNS SCL layout
Given the design of the cryogenic plant, the highest overall efficiency is not necessarily achieved when the nominally optimal thermodynamic conditions are reached. Since the cryogenic plant has to run at a fixed load no matter what the actual static and dynamic loads from the cryomodules, a more efficient use of the plant would be at temperatures different from the designed ones.
0.0E+00
1.0E+03
2.0E+03
3.0E+03
4.0E+03
5.0E+03
6.0E+03
2 2.5 3 3.5 4 4.5 5
Operating Temperature (K)
Cry
og
en
ic L
oa
d (
W)
0.02
0.04
0.06
Duty=0.08
Limitation of He flow rate Cold box
w/ existing SNS cryo-plant~30 Hz operation very marginal
44 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
SCL for the Design Goal
• 1 ms beam pulse– 1350 us HVCM 1270 us RF (300us filling + 30us FB
stabilization + 950us beam)
– Shorter filling time (need more RF) 950us 1000us
• 26-mA average current (or 38-mA midi-pulse current) at 1-GeV operation– Need more RF available for the design beam current
• 1-GeV energy + energy reserve (~40 MeV)– All cavities in the tunnel in service 940~950 MeV (no reserve)
– SCL HB cavity performances should be improved (+2.5~3 MV/m)
Additional HVCM/HPRFConfiguration; done
45 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
• Repaired ~12 cryomodules to regain operation of 80 out of 81 cavities– CM19 removed: had one inoperable cavity (excessive power
through HOM); removed both HOM feedthroughs – CM12 removed: removed 4 HOM feedthroughs on 2 cavities– Tuner repairs performed on ~9 CMs– We have warmed up, individually, ~12 CMs in the past 4 years– Individual cryomodules may be warmed up and accessed due to
cryogenic feed via transfer line.
• Installed an additional modulator and re-worked klystron topology in order to provide higher klystron voltage (for beam loading and faster cavity filling)
• Further increases in beam energy require increasing the installed cavity gradients to design values
Increasing the Beam Energy
46 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Efforts for SCL performance improvement
• Reworks; removing, disassembling, reprocessing, assembling not a realistic approach
• Attempted Helium processing did not work due to heavy MP around HOM coupler
• Plasma Processing the first attempt gives a promising result. R&D programs are on-going
• Spare cryomodule for major repair work of weak cryomodule. Fabrication is on-going
47 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
in-situ plasma processing; first attempt• In-situ plasma processing; First attempt with H01 showed very
promising results
• Set a systematic R&D program to find optimum processing conditions
• Hardware preparations are in progress
Ionization ChamberInternal Ionization ChamberPhosphor Screen, Camera, Faraday Cup
IC0
IC1
IC2IC3 IC4 IC5 IC6
IC7
IC-int
Phosphor Screen& Faraday Cup
Phosphor Screen& Faraday Cup
Cavity ACavity BCavity CCavity D
48 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Cavity D 12 MV/mCamera exposure; 30 ms
Cavity A 9.3MV/mCamera exposure; 30 ms
Phosphor screen images before processing
0.01
0.10
1.00
10.00
100.00
0 2 4 6 8 10 12
Eacc (MV/m)
Do
se
Ra
te (
BL
M7
)
baseline before processing
after processing
Processed at cold and warm upRGA analysisAll kinds of C-H-(O)-(N)
R&D for room temperature processingCould be a post additional processing(H2 removal, oxygen layer removal)
49 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Spare cryomodule
• Revisit SNS HB cavity processing– Vertical test data has traditionally not been a good indicator of
module performance due mainly to field emission limiting the collective gradients of all installed cavities
– Field emission on-set point is more relevant criteria– What else can enhance electron activity, especially FE
• Lots of processing/testing for 4 cavities – Additional BCP made performance worse in many cases– Random variations of performance/field emission after
processing cycle– Visual inspection tells that end group(reactor grade Nb)/first iris
is very rough– EP seems to be the best option for the exiting SNS cavities– HOMless cavity achieved highest VTA results; 23MV/m
50 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Endgroup Roughness
Rough Surface to the First IRIS
Cells have normal surface finish
51 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Cavity Number Emax (MV/m) Rad at Emax (mR/hr)
HB53 17.6 2.0
HB58 17.2 0.0
HB56 17.5 408
HB54 13.0 850
Summary of Cavity VTA Performance:
Tunnel Data RF Only
Field Emission
Multipacting Combination20 Seconds
-1
0
1
2
3
4
5
6
0 2000 4000 6000 8000 1 104
1.2 104
TDS_082809_140540 17MV/m
PincPtransPrefRad
Time
52 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Spare Cryomodulefield emission, pressure vessel, other minor improvement (HOM, cooling, etc)
53 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Power Upgrade Project
• Cavity– Field emission onset is more important– End group material; high RRR
• Coupler– Inner conductor; improve thermal conduction
• HOM coupler– Remove
• Pressure vessel
54 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Lessons learned, experiences on design vs. real world• Performances, Cost, vs. Stability/Availability• ‘Ideal’ vs. ‘Practically better’• General vs. Machine specific• Predictable vs. Unpredictable
– Unexpected problems will arise. More complex systems lead to more troubles.
• R&D devices vs. Devices for operational machine – To address a specific problem good chance to generate other problems– Typically designers are not operators and vice-versa
• Simpler is always better as long as the consequence is acceptable.
• Balanced performances lead to the most efficient system.– Overdesign for something while overlooking something else– Limited by the scarcest resource; Law of the minimum
• Identify what are the practically important parameters.
• Have rooms for failures, system degradation, and unknowns.
• Establish ‘reasonably’ conservative physics/engineering margin and avoid designing systems using overly optimistic/pessimistic assumptions.
55 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Thank you for your attention!
56 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
supplementary
57 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Presentation_name
Overall RF characteristic curves
21b
0
100
200
300
400
500
600
0.0E+00 2.0E+08 4.0E+08 6.0E+08 8.0E+08 1.0E+09 1.2E+09
FCM output^2
Kly
stro
n f
orw
ard
po
wer
HP
M r
ead
ing
(kW
)
KlyF 71kV
KlyF 73kV
KlyF 75kV
9b
0
100
200
300
400
500
600
0.0E+00 2.0E+08 4.0E+08 6.0E+08 8.0E+08 1.0E+09 1.2E+09 1.4E+09
FCM output^2
Kly
stro
n f
orw
ard
po
wer
HP
M r
ead
ing
(kW
) KlyF 69kV
KlyF 72kV
KlyF 75kV
8b
0
100
200
300
400
500
600
0.E+00 2.E+08 4.E+08 6.E+08 8.E+08 1.E+09
FCM output^2
Kly
stro
n f
orw
ard
po
wer
HP
M r
ead
ing
(kW
)
KlyF 69kV
KlyF 72 kV
KlyF 75 kV
12a
0
100
200
300
400
500
600
700
0.0E+00 1.0E+08 2.0E+08 3.0E+08 4.0E+08 5.0E+08 6.0E+08 7.0E+08 8.0E+08 9.0E+08
FCM output^2
Kly
stro
n f
orw
ard
po
wer
HP
M r
ead
ing
(kW
)
KlyF 72kV
KlyF 75kV
KlyF 69kV
58 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Klystron power (HPM readings) at saturationvs. HVCM voltage
22kW~25kW of RF at saturation/kV of HVCM
350
400
450
500
550
600
650
68 69 70 71 72 73 74 75 76
HVCM voltage (kV)
Kly
tro
n f
orw
ard
po
wer
HP
M r
ead
ing
s (k
W)
21b
22c
9b
12a
7c
8b
59 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Linac 08, Victoria Canada
Gradient Limitations from “Collective Effects”
a b c d
Beam pipe Temperature
individual limits; 19.5, 15, 17, 14.5 MV/m collective limits; 14.5, 15, 15, 10.5 MV/m
Flange T
Coupler or Outer T
• Electrons from Field Emission and Multipacting– Steady state electron activity (and sudden
bursts) affects other cavities
• Leads to gas activity and heating with subsequent end-group quench and/or reaches intermediate temperature region (5-20k); H2 evaporation and redistribution of gas which changes cavity and coupler conditions
• Example for CM13:
• Electron impact location depends on relative phase and amplitude of adjacent cavities
60 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Collective effectsFE; heating vs. relative phase and amplitude
-80
-70
-60
-50
-40
-30
-20
-10
0
05:17 06:43 08:10 09:36 11:02 12:29 13:55
Time [mm:ss]
13
b R
F P
ha
se
[d
eg
ree
]
8
8.5
9
9.5
10
10.5
11
11.5
13
a B
ea
m P
ipe
Te
mp
era
ture
[K
] o
r E
ac
c
[MV
/m]
13b Phase [degree]
13a BeamPipe [K]
13a Eacc [MV/m]
a b
b cavity phase a cavity beam pipe a cavity phase a cavity beam pipe
-20
0
20
40
60
80
100
120
44:10 44:27 44:44 45:01 45:19 45:36
Time [mm:ss]1
3a
RF
Ph
as
e [
de
gre
e]
5
7
9
11
13
15
17
13
a B
ea
m P
ipe
Te
mp
era
ture
[K
] o
r E
ac
c
[MV
/m]
13a phase [degree]
13a BeamPipe [K]
13a Eacc [MV/m]
61 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Radiation signals with RF only
15a; 19 MV/m15b; 17 MV/m15c; 21.5 MV/m
13a; 14.5 MV/m13b; 15 MV/m13c; 15 MV/m13d; 10.5 MV/m
(1 unit=10 us)
Rad
iatio
n (a
rb.
unit)
Radiation Signals
62 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Accelerating gradients distributions
0
5
10
15
20
25
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Ea (MV/m)
no
. of
cavi
ties
Collective Limits at 60 Hz
Individual Limits
0
5
10
15
20
25
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Ea (MV/m)
no
. o
f c
av
itie
s
Collective Limits at 60 Hz
Operating setpoints at 60 Hz
63 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
0
2
4
6
8
10
12
14
16
18
20
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Eacc (MV/m)
No
. of
ca
vit
ies
Set 1
Set 2
Set 3
Set 1; Below FE threshold average~9 MV/m
Set 2; 80 % of individual limitsaverate~13.8 MV/m
Set 3; 88 % of collective limitsaverage~12.8 MV/m Total dynamic heat loads due
to different sources
0
50
100
150
200
250
300
350
Dyn
amic
Cry
og
enic
Lo
ad (
W)
Coupler
Other heating
Residual
BCS
Set 11300us
(300+1000)30 Hz
Set 21300us
(300+1000)15 Hz
Set 21300us
(300+1000)30 Hz
Set 31300us
(300+1000)30 Hz
Set 3900us
(300+600)30 Hz
Qo~2.5e9
Qo~4e9
Qo~1e10
Cryogenic loads (II)
0
5
10
15
20
25
1a 1c 2b 3a 3c 4b 5a 5c 6b 7a 7c 8b 9a 9c 10b
11a
11c
12b
12d
13b
13d
14b
14d
15b
15d
16b
16d
17b
17d
18b
18d
19b
19d
20b
20d
21b
21d
22b
22d
23b
23d
Cavity number
Eac
c (M
V/m
)
setpoint at 2K, June_07_30Hz April 07 30 Hz 2K
64 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
0.00
0.05
0.10
0.15
0.20
0.25
0.E+00 5.E-03 1.E-02 2.E-02 2.E-02
Time (sec)
Po
wer
Dis
siap
tio
n o
r H
eat
Flu
x (W
/cm
2)
130 mT(Ti=4.36 K)
150 mT(Ti=4.42 K)
RF Power dissipation
Heat flux to helium
130 mT(Ts=4.32 K)
150 mT(Ts=4.36 K)
Tb=4.2 K, f=805 MHz, niobium thickness=4 mm, pulse length=2 mm
Pulsed operation at 60 Hz, 4.2 K, 805 MHzNo limitation up to or close to the critical field (Rs enhancement at close to critical field is not concerned) (due to relative low operating frequency, pulsed nature)
CW, Pulse (BCS and cooling)Tb=4.2 K, f=805 MHz, niobium thickness=4 mm
0.00
0.05
0.10
0.15
0.20
0.25
0.E+00 1.E-02 2.E-02 3.E-02 4.E-02
Time (sec)
Po
wer
Dis
siap
tio
n o
r H
eat
Flu
x (W
/cm
2)
150 mT
Reaches film boiling regime (Ts=4.53 K)
130 mT (Ti=4.56 K, Ts=4.5 K)
(Ti=4.6 K)
CW operation at 4.2 K, 805 MHzMeet film boiling regime (quench)
We tested all cavities at both 2 and 4.5 KThe performances are exactly same
65 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Plasma cleaning
• Ablation– Soft– Etching
• Activation
• Crosslinking
• Deposition
Base material
contaminants
Ion, molecule (radical), electron
before
afterwettability
66 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
R&D tools
3.4 GHz, TM020 modeEp/Bp=1.12 (MV/m)/mTEx. Ep=50 MV/m, Bp=56 mTPdiss=36 W at 4.2 K
OD; 150 mm
-Cold testw/ dual mode (CW or pulse) -Plasma processing
3-cell cavity
TM020 Test cavity
Cavity (3.4GHz, TM020 mode) Assembly Schematics
Witness Sample for Chemistry
Demountable witness plate
SRF cavityFPC Flange
Surface analysis
Microwave Plasma processor
FPC Flange
67 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Beam induced trip
Beam trucation
1. Errant Beam and RF truncation at upstream
2. Dark current
At normal beam At errant beam
Beam pipe temperaturew/o beam
68 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
SCL tuning; beam operation
•Beam Energy
−Have operated with output energies of
1010, 952, 930, 880, 860, 850, 550 MeV.
−Routine operation has been near 860-930 MeV
•Tune-up:
−It is faster to establish 81 phase/amplitude setpoints in SCL
than for the 10 normal conducting setpoints
•Flexibility
−One of the main benefits of a superconducting linac
for proton beams is operational flexibility
−We have taken advantage of the flexibility of individually powered
superconducting cavities to “tune around” cavities
with reduced gradients, etc.
−Have operated with as many as 20 cavities turned off
in initial tune-up.
69 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Beam loss
• Still not fully understood– Halo, Intra beam scattering, etc (multiple sources)– Normal production; mismatched tuning gives less beam loss
• Operation expert’s touch after initial physics tuning
• quads ~40 % lower than design
Warm Linac vacuum improvements
70 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
Activation decay
We finished 1 MW production run 6/29/10. Residual activations in SCL2 days after 25-30 mrem/hr6 days after 10 mrem/hr
Decay around SCL; very fast
71 Managed by UT-Battellefor the U.S. Department of Energy FNAL Visit, September 30, 2010
SCL Activation HistoryNOT Loss limited
• Over the last year the SCL activation is not increasing, even though the accelerated charge increased– Reduced beam loss helps