Cooling of Hadrons at Relativistic Energies: Performance of FNAL’s Recycler Electron Cooler

Cooling of Hadrons at Relativistic Cooling of Hadrons at Relativistic Energies:Energies:

Performance of FNAL’s Recycler Performance of FNAL’s Recycler Electron CoolerElectron Cooler

BNL – Collider-Accelerator DepartmentAccelerator Physics Seminar

November 13th, 2008L. Prost, Recycler Dpt. personnel

Fermi National Accelerator LaboratoryFermi National Accelerator Laboratoryf

BNL – Accelerator Physics Seminar November 2008 L. PROST, et al. 2

OutlineOutline

Fermilab brief overview Luminosity history

Antiproton production and storage at FNAL Role & Description of the Recycler ring Current mode of operation

• Electron cooling in operation

Electron cooling performance characterization Performance

Conclusion


Fermilab complexFermilab complex

The Fermilab Collider is an Antiproton-Proton Collider operating at 980 GeV

Tevatron

Main Injector\Recycler

Antiprotonsource

Proton source

D0

CDF


Luminosity performanceLuminosity performance

Luminosity increase due to:Luminosity increase due to: Antiproton ProductionAntiproton Production

• Injector Chain: more beam on targetInjector Chain: more beam on target• Pbar source improvementsPbar source improvements

Integration of Recycler into operationsIntegration of Recycler into operations• Electron CoolingElectron Cooling

Tevatron improvements for higher beam intensityTevatron improvements for higher beam intensity

Ecool installationEcool installation


AntiprotonsAntiprotons and Luminosity and Luminosity

Integration of the Recycler into Collider operations Final storage ring for antiprotons Improve average accumulation rate Implementation of electron cooling

• Remove 1/N cooling rate limitation of stochastic cooling

Double Antiprotons available to the collider

Accumulator Only

Accumulator+

Recycler

Recycler Only

Pbars are also used more efficiently in the Tevatron


Fermilab’s antiproton production chainFermilab’s antiproton production chain

Nickel target

Proton beam 71012 protons every 2 sec120 GeV

Debuncher (stochastic cooling)

Accumulator (stochastic cooling)

8 GeV 8 GeV

1 TeV

Tevatron

150 GeV

Main injector

8 GeV

Recycler (stochastic and electron cooling)

61011 = 2108

p

41012 = 4109

p

1108 = 1

p1108 = 300

p

until 2005

Electron coolingElectron cooling in the Recycler Ring eliminates one of the bottlenecks in the Recycler Ring eliminates one of the bottlenecks in the long chain of the antiproton productionin the long chain of the antiproton production


Antiprotons Antiprotons production and storageproduction and storage

Every 2.2 seconds: 1-2 x 108 pbars are transferred from the Debuncher

to the Accumulator 5-8 x 1012 120 GeV protons strike an Inconel target 8 GeV pbars are focused with a lithium lens 1-2 x 108 pbars are collected in the Debuncher

Every N hours: transfer pbars from Accumulator to Recycler

• N to maximize operational performance

Every M hours: Transfer pbars from Recycler to Tevatron

• M to maximize operational performance


Antiprotons Antiprotons flow flow (Recycler only shot) - Illustration(Recycler only shot) - Illustration

Recycler

Tevatron

Accumulator

Transfers from Accumulator to Recycler Shot to TeV

3000 e9

400 e10

100 mA

35 mA

17 hours


Recycler – Main ParametersRecycler – Main Parameters

Recycler: Fixed energy storage ring (uses strontium ferrite

permanent magnet)

Goal of cooling in the Recycler Increase longitudinal (and transverse) phase space

density of the antiproton beam in preparation for• Additional transfers from the Accumulator• Extraction to the Tevatron

Circumference, m 3310.400

Momentum, GeV/c 8.889

Maximum Beta, m 100/55

Maximum Dispersion, m 2.0

Nominal Tune Horizontal 25.457

Nominal Tune Vertical 24.464

Nominal Chromaticity -2Main Injector

Recycler


Recycler stochastic cooling system – Main Recycler stochastic cooling system – Main featuresfeatures

Longitudinal: 0.5 - 1 GHz and 1 - 2 GHz

• 1-2 GHz now a horizontal system Notch Filter Cooling Planar loop pickups and kickers

Transverse (H & V): 2-4 GHz Planar loop pickups and kickers

1/6th of ring from Pickup to Kicker Signals travel on laser light link


Recycler Electron Cooler (REC) – Main featuresRecycler Electron Cooler (REC) – Main features

Electrostatic accelerator (Pelletron) working in the energy recovery mode

DC electron beam 100 G longitudinal magnetic field in the cooling section Lumped focusing outside the cooling section

Electron energy MeV 4.338

Beam current used f or cooling

A 0.05 - 0.5

Magnetic field in CS G 105 Beam radius in the cooling section

mm 2.5 - 5

Pressure nTorr 0.2 - 1 Length of the cooling section

m 20


Electron cooling system setup at MI-30/31Electron cooling system setup at MI-30/31

Pelletron(MI-31 building)

Cooling section solenoids

(MI-30 straight section)


Electron cooling status: From installation to operation Electron cooling status: From installation to operation

Bringing electron cooling into operations consisted of three distinct parts Commissioning of the electron beam line

• Troubleshoot beam line components• Check safety systems

– Ensure the integrity of the Recycler beam line at all times• Establish recirculation of an electron beam

Cooling demonstration• Energy alignment• Interaction of the electron beam with anti protons• Cooling demonstration

– Reduction of the longitudinal phase space

Cooling optimization• Optimization of the electron beam quality

– Stability over long period of times– Minimize electron beam transverse angles

• Define best procedure for cooling anti protons– Maximize anti protons lifetime

• Understand and model the cooling force


Electron cooling in operationElectron cooling in operation

Electron cooling is used when needed Electron cooling used for 6D cooling (i.e. both

longitudinally and transversely)• Transverse stochastic cooling most efficient just after

transfers into the Recycler (large transverse emittance), but limited effectiveness when the stack is large and the antiproton beam is compressed

Electron beam adjusted to provide stronger cooling as needed (progressively)

This procedure is intended to maximize This procedure is intended to maximize lifetimelifetime

Similarly to low energy coolers, cooling seem to induce secondary beam-beam effects. In our case, it affects the lifetime of the cooled beam


Adjusting the cooling rateAdjusting the cooling rate

Change electron beam position (vertical shift) Adjustments to the cooling rate are obtained by bringing

the pbar bunch in an area of the beam where the angles are low and electron beam current density the highest

5 mm offset 2 mm offset

Area of good cooling

pbars

electrons electrons

pbars

This procedure can be regarded as ‘painting’ and, in fact, is almost equivalent to the ‘hollow beam’ concept for low energy coolers.


Cooling sequence (15-20 hours)Cooling sequence (15-20 hours)

17 hours

Transverse emittance (95%, n)

(1.5 mm mrad/div)

Electron beam position

(2 mm/div)

Longitudinal emittance

(15 eV s/div)

Number of antiprotons

(90×1010/div)

Transverse emittance (95%, n)

(1 mm mrad/div)

Electron beam position

(1 mm/div)

Longitudinal emittance

(10 eV s/div)

Number of antiprotons

(1.5×1010/div)

When strong cooling is applied, the antiproton distribution becomes peaky (i.e. high density of the core) and the lifetime deteriorates

1.5 hours


Accumulation performance Accumulation performance

Use of electron cooling allowed the storage and extraction of more than 450 × 1010 antiprotons > ~3 times what would be possible with stochastic cooling

alone• Much faster too

Delivers very consistent bunches (i.e. same emittances shot to shot)

Plays a major role in increasing the initial and integrated luminosities in the Tevatron

• Record initial luminosity > 300 × 1030 cm-2 s-1

-96

-88

-80

-72

-64

-56

-48

-24 -20 -16 -12 -8 -4 0 4 8 12 16 20 24

Momentum offset [MeV/c]

Am

plit

ud

e [d

Bm

]

Longitudinal Schottky distribution

Before extraction to the TeV:

N = 375 × 1010

L (95%) = 68 eV s

t (95%, n) = 3.1 mm mrad (Schottky)

08/22/08


Electron cooling performanceElectron cooling performance

Evaluated two ways: Drag rate measurements

• As a function of various parameters• Characterizes the intrinsic performance of the electron

beam

‘Standard’ cooling rate measurements• Operation driven measurement

– i.e. how fast are we really cooling the antiprotons ?

-100

-95

-90

-85

-80

-75

-70

-65

-60

-55

-6.0 -4.0 -2.0 0.0 2.0 4.0 6.0

Antiproton momentum offset [MeV/c]

Am

pli

tud

e [d

Bm

]

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

0 20 40 60 80 100 120 140 160 180 200

Time [min]

Dis

trib

uti

on

mea

n [

MeV

/c]

0.0

0.2

0.4

0.6

0.8

1.0

1.2

s [M

eV/c

]


Pencil-like antiproton beam with small momentum spread Drag rate equal to cooling force

Used same assumption in our case and fitted drag rate data to non-magnetized force model to estimate electron beam properties

Drag rate and cooling force – First approachDrag rate and cooling force – First approach

)( 0pppFp

e

pe

peeee fcrmn v

vv

vvvF 3

3

22 d4

2||

2||

2

2

||22/3 22

exp2

1)(

ssssee

e

vvf v

ces

mc

W

s //

Lab frame quantities

ce

jn e

e

e ≡ electron beam anglesW ≡ electron beam energy spreadje ≡ electron beam current density


0

10

20

30

40

50

60

0 5 10 15 20

Momentum offset, MeV/c

Dra

g r

ate

, M

eV

/c/h

r

Fit 1 Fit 2 Fit 3 Fit 4

Data 1 Data 2 Data 3 Data 4

Drag Rate as a function of the antiproton momentum Drag Rate as a function of the antiproton momentum deviationdeviation

100 mA, nominal cooling settings 100 mA, nominal cooling settings ((before October 2007before October 2007))

Drag rate very

sensitive to antiprotons transverse emittance

Fits: Non-magnetized force model

Transverse stochastic cooling applied at all times for sets 2-4


Given some simplified assumptions, drag rate for finite emittance beams can be written as:

Cooling force dependence on radius comes from• Radial distribution of the current density

• Radial distribution of electron beam angles

Interpretation of the dependence of drag rate Interpretation of the dependence of drag rate measurements on the transverse emittance (I)measurements on the transverse emittance (I)

dpdrrrpfpjWFp ee 2),(),,,(

2

2

0 1)(a

rj

B

B

B

Brjrj

cath

cs

cath

cscathe

220

220

2 11b

r

r

r

bo

be

from gun simulations

Linear dependence from envelope scalloping

Thermal velocities and dipole perturbations from magnetic field


For the distributions given previously, the first terms of the Taylor expansion of the cooling force on axis are

with

Interpretation of the dependence of drag rate Interpretation of the dependence of drag rate measurements on the transverse emittance (II)measurements on the transverse emittance (II)

2

)0,(

2

)0,()0,(

2

2

22

2

2rp

r

pF

p

pFpFp

ss

dpdrrrpfrr s 2),(22;2),()( 22 dpdrrrpfppp s

For typical parameters, we recoverif sp < 0.4 MeV/c and b > 2 mm (from angle distribution)

)0,( pFp

Easy to fulfill during drag rate measurements Estimate from

the radial dependence of the drag rate


For the present parameters of the antiproton and electron beams, the drag rate significantly differs from the cooling force experienced by an antiproton on axis. The area of the electron beam where cooling is effective is significantly smaller than the physical size of the electron beam, b 1 mm.

Drag rate as a function of a parallel offset of the Drag rate as a function of a parallel offset of the electron beam w.r.t. the pbars beamelectron beam w.r.t. the pbars beam

Voltage jump was 2 kV, Ie = 0.1 A, Np = 4×1010.Set 1: the antiproton beam was scraped to the radius in the cooling section of 1.1 mm, 25 min prior to the measurement.Set 2: negative offsets measured the same day 2 hours after the scrape. During both measurements, FW = 0.3-0.7 (sr ~ 0.5 mm).Set 3: data of Feb. 2006, taken several hours after scraping. Sch = 1.5-3. Point 4: drag measurement immediately after scraping to 1.1 mm. FW ~ 0.1-0.2 (sr ~ 0.3 mm)


0

2

4

6

8

10

12

14

16

18

20

0 1 2 3 4 5 6

Emittance (95%, n) [ mm mrad]

Lo

ng

itu

din

al c

oo

ling

ra

te [

MeV

/c p

er

ho

ur]

Sensitivity of drag rate to antiprotons transverse Sensitivity of drag rate to antiprotons transverse emittance is carried over to cooling ratesemittance is carried over to cooling rates

Red data: Bunched antiproton beam (with arbitrary fit)

Green data: Un-bunched antiproton beam

Emittance from flying wire measurements

Rates ‘normalized’ to sp = 3.6 MeV/c


Preliminary measurements with scintillator Preliminary measurements with scintillator

The electron beam appears to be very elliptical at the exit of the cooling section Indicative of quadrupole

envelope oscillations Also presence of a large halo

Could explain various observations & inconsistencies

Beam Image from YAG at cooling section exit (~100 mA)

Large beam size measured with scrapers

Small effective electron beam radius Shallow sensitivity of the drag

force/cooling rates on the matching solenoids settings

Large sensitivity of the drag force/cooling rates on antiprotons transverse emittance

YAG screen


Beam rounding procedure (A. Burov)Beam rounding procedure (A. Burov)

For two distinct values of SPQ01I Record initial image Change upstream quads successively

(6 quads)• Record associated images

Calculate ellipticities Fit ellipse to threshold (binary) image Extract semi-major and semi-minor e = 2 (a –b)/(a+b) Include effect of camera angle (i.e. distortion k)

Compute MULT (i.e. transfer matrix) SVD algorithm

Use MULT to make the beam more round Repeat…

• If e = 0 for two values of SPQ01I, then the beam is perfectly round

This was done automatically with a Java application written by T. Boshakov

YAG screen


Results (Results (after multiple iterationsafter multiple iterations))

First visible image + 1.5 s

118 119 120

121122123

Nominal

New nominal

Quads off Quads on SPB01I SPB02I SPQ01I Area Major Minor Angle Ellipticity118 X 19.1 8.7 0 5812 116.067 63.757 2.155 0.37119 X 19.1 8.7 10.4 1655 63.519 33.174 21.537 0.52120 X 13.5 12.1 10.4 913 42.654 27.253 32.664 0.40121 X 13.5 12.1 10.4 1000 38.682 32.916 144.048 0.20122 X 19.1 8.7 10.4 1941 52.483 47.089 163.214 0.10123 X 19.1 8.7 0 6871 101.519 86.176 37.57 0.19


Envelope analysis (Envelope analysis (from A. Burovfrom A. Burov))

Fit of SPQ01A scan for the ‘new’ nominal file: Give r = 2.4 mm, = 0.01, = 2.0 m for initial conditions

at the entrance of the cooling section Show that 10-15% envelope oscillation remain

22.28380

Wed Oct 17 11:40:10 2007 OptiM - MAIN: - Y:\MI-31\OptiM Files\BackFromQ1ToB1.opt

0.3

0

90

-90

Be

tatr

on

siz

e X

&Y

[cm

]

An

gle

[de

g][

-90

,+9

0]

a b Angle[deg]

5 10 15 20

0.125

0.15

0.175

0.2

0.225

0.25

0.275

SPQ01I [A]

Bea

m r

adiu

s [c

m]

SPQ01 scanw/ fit

Corresponding envelope in cooling section

~0.6 mmr = 2.4 mm


Drag rate performance after roundingDrag rate performance after rounding

0

5

10

15

20

25

30

0 0.5 1 1.5 2 2.5 3 3.5

Vertical offset [mm]

Dra

g r

ate

[MeV

/c p

er

ho

ur]

After beam 'rounding' Before beam 'rounding'

Expected improvements from beam rounding did not materialize

Effective beam size after rounding is unchanged


Interpretation and ConsequencesInterpretation and Consequences

Rounding of the beam has failed Possible reason: Pulse beam measurements (for

YAG) vs DC beam measurements (for drag and cooling rates)

• Ions capture (neutralization) effect

But YAG measurements proved the need for some quadrupole correction Empirical optimization based on drag rate

measurements with the electron beam offset while changing quadrupole settings


Current cooling performanceCurrent cooling performance

Red data: Bunched antiproton beam (with arbitrary fit)

Green data: Un-bunched antiproton beam

Emittance from flying wire measurements

Rates ‘normalized’ to sp = 3.6 MeV/c


Strong cooling is applied before extractionStrong cooling is applied before extraction

Beam is brought on axis just before the final manipulations before extraction and stays on axis throughout the extraction process Reduces longitudinal emittance of individual bunches

15 min.

Initial

30 min.

STUDY

Scope traces of the resistive wall monitor

6.1 s


77% beam up time since February 07 Several interruption/day Conditioning of the accelerating/decelerating structures

~once every 2 months• Following series of full discharges (2-3)• Conditioning only takes several hours

– Typically done when the electron beam is not absolutely needed

Routine maintenance every 5-6 months (opening of the Pelletron)

Pelletron reliabilityPelletron reliability

• Longest running time between openings: 3509 hours

• Clean/refurbish charging circuitry

CHAIN

Black deposit from the chain slipping on the pulleys


Cooling performance stabilityCooling performance stability

We have 3 major performance-limiting stability issues Degradation of the cooling section magnetic field (more

in L. Prost & A. Shemyakin, Poster Session COOL’07)• Likely due to ground motion in the tunnel• Beam based alignment procedure was developed and

tested– Large uncertainties but it worked

Drift of the antiproton trajectories• May be due to ground motion too• Needs to be re-align (3-bump) every 1-2 months

High voltage stability (and calibration)• Next slides

± 0.5 mm

± 0.5 mm


High voltage/Electron beam energy stabilityHigh voltage/Electron beam energy stability

Average energy drifts by up to 1-2 keV (over several weeks) Cooling efficiency is greatly reduced

Temperature dependent• ~ -300 V/C

– Mostly an issue at turn on (~6-10 hours to reach equilibrium)

– Equilibrium temperature stable to within 1C But not only…

Longitudinal Schottky profiles after cooling with the electron beam on axis for ~2h at 100 mA

Np = 200 × 1010 Np = 280 × 1010

Flatness of the distribution attributed to the electron beam energy to be offset


Beam position as an energy monitorBeam position as an energy monitor

Use the 180 bend magnet and beam position monitor downstream as an energy analyzer Absolute energy calibration done with antiprotons

(debunched)• Defines an absolute position

– Needs to be recalibrated ~once a month• Defines a relative position

– Very stable

4.3300

4.3305

4.3310

4.3315

4.3320

4.3325

4.3330

4.3335

4.3340

4.3345

4.3350

4.3355

200 300 400 500 600 700 800

Time [s]P

elle

tro

n v

olt

age

[MV

]

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Ver

tica

l (y)

bea

m p

osi

tio

n a

t R

01 [

mm

]

Calibration: 0.31 mm/kV

HV monitor

Beam position

Last CS solenoid

Bx/y ≡ BPM x- / y- direction DY ≡ Dipole y-plane QN ≡ Quadrupole (normal) BV ≡ Beam valve


Issues related to electron cooling and large Issues related to electron cooling and large stacksstacks

Since started to use the electron beam for cooling, we have dealt with three main problems Transverse emittance growth

• During miningmining Lifetime degradation

• Under strong electron cooling Fast beam loss

• Instabilities caused by high phase density and/or high peak current

MININGMINING


Cure to emittance growth (and lifetime Cure to emittance growth (and lifetime degradation)degradation)

Changed working point from 0.414/0.418 (H/V) to 0.451/0.468 (H/V) Increase tune separation to reduce coupling More room at higher tunes

• Recycler sensitive to 0.41 and 0.428

Although it worked… a coherent electron-antiproton instability is not the primary cause for the emittance growth during mining It was thought to be the mechanism by which the

emittance grew but experimental measurements where coupling was large showed that it was not the case


0

2

4

6

8

10

12

1 2 3 4 5 6 7 8 9

Parcel #

Ho

r. E

mit

. (9

5%, n

) [

mm

mra

d]

0

1

2

3

4

5

6

Par

cel i

nte

nsi

ty [

e11]

Emittance Low tunes Emittance High tunesIntensity Low tunes Intensity High tunes

Emittances of individual parcels during extractionEmittances of individual parcels during extraction

300×1010 in both cases

Max emittance growth

Max emittance growthFlying wire data


1

10

100

1000

10000

0 50 100 150 200 250 300 350 400 450

Number of pbars [e10]

On

e h

ou

r ru

nn

ing

av

era

ge

life

tim

e [

h]

Oct-06 High tunes Sep-06 Hight tunes Aug-06 High tunes

Feb-06 High tunes Aug-06 Low tunes Jul-06 Low tunes

Jun-06 Low tunes Jan-06 Low tunes Dec-05 Low tunes

Lifetime before miningLifetime before mining


Avoiding fast beam loss (i.e. instabilities)Avoiding fast beam loss (i.e. instabilities)

Dampers to increase the threshold to resistive wall instabilities Working on increasing the bandwidth

• High frequency lines may cause problems as the number of antiprotons increases

Changed mining RF waveform Wider buckets to decrease the peak current seen by

the dampers (so-called ‘soft’ mining)• Avoid saturating dampers electronics

100

150

200

250

300

350

400

450

500

-3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12

Time [min]

Pea

k cu

rren

t [

Arb

. Un

its] ×2

MINING

Np ~ 350 × 1010

‘Hard’ mining

‘Soft’ mining

‘Hard’ mining

‘Soft’ mining


0

10

20

30

40

50

60

70

80

0 5 10 15 20 25 30 35 40 45 50 55 60 65

Time with electron beam on axis [min]

L (

95%

) [e

V s

]

0

200

400

600

800

1000

1200

1400

1600

Lif

etim

e [h

]

Typical longitudinal cooling time (100 mA, on-Typical longitudinal cooling time (100 mA, on-axis)axis)

e-folding cooling time: 20 minutes

111×1010 pbars

5.2 s bunch


1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 10 20 30 40 50

Time with electron beam on axis [min]

6rm

s (n

orm

aliz

ed)

[ m

m m

rad

]Hor. Emit. (Schottky) Ver. Emit. (Schottky)

Hor. Emit. (FW) Ver. Emit. (FW)

Strong transverse cooling is now routinely Strong transverse cooling is now routinely observedobserved

e-folding cooling time (FW): 25 minutes

100 mA, on axisStochastic cooling off

135×1010 pbars

6.5 s bunch


0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

-12 -10 -8 -6 -4 -2 0 2

Position [mm]

Sig

nal

am

plit

ud

e [V

]Transverse (horizontal) profile evolution under electron Transverse (horizontal) profile evolution under electron

coolingcooling

100 mA, on axis for 60 min

Deviation from Gaussian

Flying wire data


Beam quality: Electron angles in the cooling Beam quality: Electron angles in the cooling section section

*Angles are added in quadrature

New measurements and refined analysis indicate that we had over estimated the quality of the electron beam


Conclusion - RecyclerConclusion - Recycler

Recycler is an essential component of Fermilab Tevatron Collider Complex significant contributions to the doubling of the peak and

integrated luminosity over the last two years taking advantage of

• improved performance of the Antiproton source stacking• Tevatron ability to handle higher intensities

Through mix of stochastic and electron cooling Prepare intense, bright antiproton beams doubled peak intensities while maintaining emittance

properties

Future: Collider program through 2009: Antiproton storage ring

Neutrino program: Proton stacker for Main Injector• single turn fill to maximize proton flux


Conclusion – Electron coolingConclusion – Electron cooling

The electron cooler reliability has been exceptional under an increased demand for electron cooling and is adequate for the remaining of the collider operation Full discharges are sparse and conditioning is only required

~2-3 months General maintenance of the Pelletron required ~5-6 months

Electron cooling rates are sufficient for the present mode of operation of the accelerator complex We found them (and drag rates too) to depend greatly on the

antiprotons transverse emittance Preliminary YAG measurements indicate that the electron

beam distribution may be the culprit• Good possibility that we can improve the cooling rates by fixing

the electron beam distribution• Perhaps will improve lifetime too

– It is the remaining most challenging issue related to electron cooling from an operational point of view

Documents

Cooling of Hadrons at Relativistic Energies: Performance of FNAL’s Recycler Electron Cooler