Design and Operating Experience with SNS Superconducting Linac

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


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


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


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


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


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


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


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


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


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)


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


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


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


While learning

AFF learning

AFF fully learned At beginning

Some cavities need ~>25 % more RF at the beginning of AFF


• 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


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)


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


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


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


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.


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


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


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


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


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)


Leak, severe MP, contamination, large coupling, …


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)


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


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


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)


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

)


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.


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.)


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


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


Errant beam from the sourceMPS truncation <30 us

Before improvements of MPS(50~300 us)


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


Irregular dynamic detuning (9b)

Eacc

Tuner motion


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)


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


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


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


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


• 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


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


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


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)


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


Endgroup Roughness

Rough Surface to the First IRIS

Cells have normal surface finish


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


Spare Cryomodulefield emission, pressure vessel, other minor improvement (HOM, cooling, etc)


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


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.


Thank you for your attention!


supplementary


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


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


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


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]


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


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


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


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


Plasma cleaning

• Ablation– Soft– Etching

• Activation

• Crosslinking

• Deposition

Base material

contaminants

Ion, molecule (radical), electron

before

afterwettability


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


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


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.


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


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


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

Design and Operating Experience with SNS Superconducting Linac

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Transcript of Design and Operating Experience with SNS Superconducting Linac