accelerator physics progress and challenges

Managed by UT-Battellefor the Department of Energy

Accelerator Physics – Progress and

Challenges

J. Galambos

SNS AAC Meeting

Jan. 22-24, 2008

On behalf of the AP Team

2 Managed by UT-Battellefor the Department of Energy Presentation_name

Accelerator Physics Group Activities

Reduce Beam Loss

Reduce Beam Loss

Reduce Beam Loss

Reduce Beam Loss

Perform beam studies

– Same team that commissioned the machine

– Devise measurements to understand and correct causes of beam loss

Request new modified beamline equipment

Develop and maintain the high level software

Perform simulations and beam modeling– ORBIT code for Ring

– Parmila, IMPACT models for linac

Keep an eye on the future– high intensity effects

– laser stripping


SNS Time Structure Nomenclature

2.4845 ns (1/402.5 MHz)

260 micro-pulses

645 ns 300 ns

945 ns (1/1.059 MHz)

1ms

16.7ms (1/60 Hz)

15.7ms

Macro-pulse

Structure

(made by the

High power

RF)

Mini-pulse

Structure

(made by the

choppers)

Micro-pulse

structure

(made by the

RFQ)


Accelerator Physics Beam Study Rhythm

Red –

extended

outage

Yellow =

AP

Green =

neutron

Production

•Inter production periods: incremental power ramp up, beam

studies aimed at loss mitigation, understanding beam

behavior.•End of run studies: beam loss mitigation, high intensity studies

(Danilov talk)

Post extended maintenance: linac setup, beam RF, diagnostic studies, equipment shakedown


SRF, ß=0.61, 33

cavities

1

from

CCL

186 MeV

805 MHz, 0.55 MW klystron

805 MHz, 5 MW klystron

402.5 MHz, 2.5 MW klystron

86.8 MeV2.5 MeV

RFQ

(1)

DTL

(6)

CCL

(4)

Layout of Linac RF with Warm and SCL Modules

SRF, ß=0.81, 48 cavities1000 MeV

(81 total powered)

379 MeV

Warm

Linac

SCL

Linac

•SCL has 81 independently powered cavities

Many values to set w.r.t. the beam

6 cells/ cavity, b changes only a small amount

Many acceptable amplitude / phase setups

•Warm linac has 10 independently powered RF

structures

Typically each structure has 50-100 gaps

b change is substantial

One correct amplitude and phase setting


Normal Conducting LINAC Tuneup Procedures

Warm Linac RF Amplitude and phase setpoints determined with a phase scan method– Accurate to ~ 1%, 2 degrees

Use design quadrupole strengths and RF settings

First machine to routinely use this method

BP

M P

has

e D

iffe

ren

ce

RF Phase

•Scan RF phase for multiple

amplitude settings

•Solve for input beam energy, RF

amplitude calibration, RF phase

offset


SCL Cavity Tuneup

Use highest available SCL gradients – far from design

Set the SCL cavity phases using phase scan technique

Scale design quadrupoles with beam energy

RF Cavity Phase

BP

M P

ha

se

Diffe

ren

ce

Example SCL Phase Scan

Black line = measurement fit

Dot = model

Red = cosine fit

RF Cavity Phase

BP

M P

ha

se

Diffe

ren

ce


RF Cavity Phase

BP

M P

ha

se

Diffe

ren

ce

RF Cavity Phase

BP

M P

ha

se

Diffe

ren

ce


Black line = measurement fit

Dot = model

Red = cosine fit


SCL Cavity Amplitudes

Strategy is to run cavities at their maximum safe amplitude limit (S. Kim’s talk)

Need to be flexible – SRF capabilities change, not near the design

Linac output energy is a moving target

Cavity Design

0

5

10

15

20

25

30

1 5 9

13

17

21

25

29

33

37

41

45

49

53

57

61

65

69

73

77

81

cavity

E0

(M

V/m

)

First Run

0

5

10

15

20

25

30

35

1 5 9

13

17

21

25

29

33

37

41

45

49

53

57

61

65

69

73

77

81

cavity

E0

(M

V/m

)

Second Run

0

5

10

15

20

25

30

35

1 5 9

13

17

21

25

29

33

37

41

45

49

53

57

61

65

69

73

77

81

cavity

E0

(M

V/m

)

Ring Commissioning Run

0

5

10

15

20

25

30

35

1 5 9

13

17

21

25

29

33

37

41

45

49

53

57

61

65

69

73

77

81

cavity

E0

(M

V/m

)


SCL Setup Times are Decreasing

The procedures used to setup the superconducting linac have matured, and the setup time is now minimal

Still exists a need for fast recovery from changes in the SCL setup

Representative SCL RF Phase Setup

Times

0

20

40

60

80

100

120

Aug-0

5

Nov

-05

Feb-0

6

May

-06

Aug-0

6

Nov

-06

Feb-0

7

May

-07

Ho

urs

Power cavities

on sequentially

Use Beam

Blanking


SCL Cavity Fault Recovery Scheme

1 GeV is not ultra-relativistic – change in upstream cavity has a large imapct on downstream cavity phase settings

Use a model to predict change in measured downstream arrival times from a change in an upstream cavity

In April 2007 the SCL was lowered from 4.2K to 2 K to facilitate 30 Hz operation, 20 cavity amplitudes changed

The fault recovery scheme restored beam to the previous loss state

-2500

-2000

-1500

-1000

-500

0

1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81

cavity

D P

ha

se

(d

eg

)

-30

-20

-10

0

10

20

30

D A

mp

litu

de

(M

V/m

)

Phase

Change

Amplitude

Change


Linac RMS Beam Size (Nov. 2007)

Lines are model predictions with design Twiss parameters, and dots are wire profile measurements

Warm linac beam size is in good agreement with design values

S (m)

RM

S (

mm

)

CCL1 & 2 Beam Size

DTL Beam Size


Linac Beam Profiles

Profiles at the start of the HEBT 12/10/2007

Beam profiles are close to Gaussian at the end of the linac than previously observed– Ignore startup portion of the beam

– Quadrupole settings are closer to design values

– Source dependent


Linac concern: Chopper Quality

(S. Aleksandrov’s Talk)

The MEBT chopper has never been used during neutron production

Sometimes have leading/trailing satellites

Protection measures introduced in the LEBT system have slowed the rise / fall times

Improperly centered beam through the LEBT can cause mini-pulse to mini-pulse position jitter - effectively increasing the beam size.

The chopper system provides clean gaps between mini-pulses to provide a gap to fire the extraction kicker in the Ring

It is a 2 stage system designed to clear the gap to 1 part in 104

– LEBT chopper at 65 keV

– MEBT chopper at 2.5 MeV

March 6,

2007

Satellites

Mini-

pulse

Cu

rren

t

Time


Beam Loss / Residual Activation

SNS is designed to be a hand’s on maintainable accelerator

100 mRem/hr at 1 foot is considered the limit for relatively easy hands on maintenance– Corresponds to ~ 1 W/m beam loss

BLMs give a measure of beam loss– (~ 400 BLMs throughout the machine)

Residual activation measured after every production run

Use the relationship between BLM readings/ measured activation to predict activation during production setups and prioritize areas for beam study

BLM

DisplayCCL SCL

Radiation survey (mRem/hr at 30 cm after ~ 24 hrs)


CCL Residual Activation

Hot spots:10-30 mRem/hr

Scaled to 1.4MW: 90-250 mRem/hr

Context: similar residual activation as Dec. 2006 at ~ 30 kW– Better trajectory control (S. Aleksandrov’s talk)

– Additional BLMs

Keys to further improvement:– Longitudinal RF setup refinement

– Transverse matching

– MEBT chopping

1/7/2008 Measurements


SCL Residual Activation Status

Average warm section residual levels are 10-20 mrem/hr

Context

– These activation levels correspond to 1-2 x 10-4

fractional beam loss

Scaled to 1.4 MW: 90-250 mRem/hr

1/7/2008 Measurements

05

10152025303540

2/2

0/2

00

7

4/2

0/2

00

7

6/2

0/2

00

7

8/2

0/2

00

7

10

/20

/20

07

12

/20

/20

07

MW

-hrs

, m

Rem

/hr

Delivered

Beam

warm section

15 Radiation"

warm section

32 Radiation"


SCL Beam Loss / Mitigation Efforts

Believe the SCL losses are longitudinal (S.

Aleksandrov’s talk)

– Loss magnitude is not sensitive to matching quadrupole settings

– Loss magnitude and distribution is very sensitive to linac RF phase setpoints

– Developing methods for measuring and increasing the SCL longitudinal acceptance

Measured SCL acceptance

(courtesy Zhang)

-4

-3

-2

-1

0

1

2

3

4

-60 -40 -20 0 20 40 60 80phase (deg)

dE

(M

eV

)

Simulated SCL acceptance


HEBT Transport Line

Not much beam loss / activation

Upstream transverse + momentum collimation has been tested but is not used

– Causes more beam loss in the arc than halo reduction benefit

Off Energy Beam


Ring

Primary functions:– Injection

– Fast Extraction

– Collimation

– RF


Ring Setup Recovery / Repeatability is Good

~ 1 shift to recover a Ring / Transport line setup after an extended maintenance period

Magnet cycling application for hysterisis effects– Determine which magnets require cycling and the minimal cycling

periods

Sophisticated save/compare/restore application

Magnet cycling Save / Compare / Restore


Ring Injection Area (M. Plum’s talk)

“As delivered” Ring could not transport beam to the injection dump and circulate beam in the Ring (M. Plum’s talk)– Inconsistency in chicane values for Ring and dumpline designs

– Did not fully appreciate the influence of 3-D magnetic effects

Remedial actions– Moved chicane

– Use oversize injection foil (reduce fractional beam to the Injection dump)

– Added additional diagnostics and magnet to the dumpline to understand waste beam transport

– Further upgrades are planned

H- beam from Linac

ThinStripping Foil

To InjectionDump

ThickSecondary Foil

p

H0

H-Dipole magnets

H- beam from Linac

ThinStripping Foil

To InjectionDump

ThickSecondary Foil

p

H0

H-Dipole magnets


Injection Dump-line Beam Loss

One hot spot from secondary foil scattering

Rest of the Injection dump line ~ 10 mRem/hr residual activation

Future upgrades:

– New, larger aperture septum to be installed in Feb. 08 outage

– Additional quadrupole in the injection line

80

mRem/hr

IDmp Loss

0.00E+00

1.00E+03

2.00E+03

3.00E+03

4.00E+03

5.00E+03

1 2 3 4 5 6 7 8 9 10

BLM

Ra

d/C

8/2006 - 10 kW

8/3 - 150 kW


Ring Extraction / Collimation beam loss

Extraction area is sensitive to beam in gap

Collimation works close to expectations (J. Holmes talk)

– Presently we are using short pulse beams (small beam size) and to not employ collimation

-5.00E-01

0.00E+00

5.00E-01

1.00E+00

1.50E+00

2.00E+00

2.50E+00

3.00E+00

3.50E+00

40 45 50 55 60 65 70 75 80 85 90

Position (m)

En

erg

y D

ep

ositio

n (Jo

ules)

ORBIT

Experimental

1st collimator

2nd collimator

3rd collimator

Q

V

11 Drift

QV01Dipole

scraper


Ring RF

Use 2 fundamental cavities, 1 2nd harmonic cavity– Purpose of the dual harmonic is to reduce the bunch factor

to minimize space charge

– 2nd harmonic useful for gap cleaning

– Can further clean the gap with RF manipulations, but time scale is long (100’s of turns) – injection losses increase.

Bunch Shape at

Extraction


Ring Beam Loss Progress

• In general we are making progress

• Ring injection is the toughest area in the accelerator

• Most of the Ring is loss free

Ring Beam Loss / Activation

0

100

200

300

400

500

600

1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65

BLM

Rad

/C

6/20 - 60 kW - 9 MW-hrs

7/19 - 120 kW - 22 MW-hrs

12/27 - 160 kW - 36 MW-hrs

Injection

50

90

120 Collimation

15

20

20

Extraction

70

45

20

RF


Foil

location

Ring Injection Straight Prediction –

Residual activation from foil scattering

5000 hrs operation @ 1.4 MW, 3 hrs after shutdown

> 1000 mRem/hr downstream from the foil –we are on track

Keys to improvement is reducing foil traversals with:

– better injection painting

– Reduced linac beam tails

– Smaller / thinner foil


Beam Size Control On Target

Use wire profiles and harp to predict the beam size and beam density on the Target (T. McManamy’s talk)

Difficulties understanding transport in the RTBT (M. Plum’s talk)– Swapped plane in harp, coupled H/V beam in the RTBT

Reluctance to vary RTBT quads from values used with view-screen during commissioning

With the power density on Target at the upper limit– Painting a larger beam in the Ring is the only option, but this causes

excessive beam loss at the end of the RTBT

RM

S S

ize m

m)

S (m)

wires

HarpTarget

1/9/2007 Tuneup


RTBT Radiation

Hot spot from extraction loss – reduced with improved chopping

End of RTBT losses reduced with updated lattice to keep the beam small there.

Beam in gap

induced hot spot

(38 mRem/hr)

Large beam at the end of the

RTBT,

10-40 mRem//hr spots


Equipment Requirements for 1.4 MW Capability

Ion source current, pulse length and repetition rate requirements to meet the power ramp-up– These requirements assume a 1 GeV beam

Presently at 60 Hz we are limited to:– ~ 850 MeV beam energy

– ~ 880 ms flattop pulse length

– ~ 30 mA current at ~700 ms

0

200

400

600

800

1000

1200

1400

1600

10/1

/2006

12/1

/2006

2/1

/2007

4/1

/2007

6/1

/2007

8/1

/2007

10/1

/2007

12/1

/2007

2/1

/2008

4/1

/2008

6/1

/2008

8/1

/2008

10/1

/2008

12/1

/2008

2/1

/2009

4/1

/2009

6/1

/2009

Po

we

r (k

W)

Pu

lse

Le

ng

th (m

s)

0

10

20

30

40

50

60

70

Re

p R

ate

(H

z)

Cu

rre

nt

(mA

)

Power

Pulse Length

Peak Current

Rep Rate

Today


Equipment Concerns for Power Ramp Up

Ion source needs to provide 38mA at 1 mS/60 Hz.

– M Stockli’s talk

SCL needs to provide ~20% more accelerating gradient with an additional installed cryomodule + enhanced high beta cavity gradients through cavity reworking and surface processing.

– S. Kim’s + J. Mammosser’s talks

Starting the SCL RF fill during the HVCM ramp-up will provide ~ 70 mS longer flattop .

– S. Kim’s talk

Increase the (medium b / high b) HVCM operating voltages from the present (69/71) kV settings to 73/75 kV to provide an additional 50 ms flattop capability, support the increased cavity gradients, and support beam loading associated with 38 mA.

– D. Anderson’s and T. Hardek’s talks


AP Concerns

Linac

– Quality of beam chopping (A. Aleksandrov’s talk)

– Understanding and controlling the source of beam loss in the SCL (A. Aleksandrov’s talk)

– Controlling the transverse beam size and halo delivered to the foil

Ring

– Injection area

– Clean transport of waste beams to the Injection Dump (M. Plum’s talk)

– Good understanding and control of the beam distribution delivered to the Target (M. Plum’s talk)

– Foil scattering losses

– Foil survivability (M. Plum’s talk)

– High intensity effects (V. Danilov’s talk)


Summary

We have increased beam powers from a few kW to > 200 kW.

– Large reductions in normalized beam loss through the ramp-up

Have been equipment issues, but none are show-stoppers.

Now we are dealing with low loss fractions, and are continuing to develop strategies to understand them and further reduce them.

accelerator physics progress and challenges

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