Accelerator PhysicsOak Ridge National Laboratory Jeff Holmes, Slava Danilov, and Sarah Cousineau...

Accelerator Physics Oak Ridge National Laboratory

Jeff Holmes, Slava Danilov,and Sarah Cousineau

SNS/ORNL

October, 2004

ICFA Advanced Beam Dynamics WorkshopBensheim, Germany

Painting Self-Consistent Beam Painting Self-Consistent Beam Distributions in RingsDistributions in Rings

Accelerator Physics Oak Ridge National Laboratory2

Brief Problem Description

• SNS example: beam distribution after MEBT

1) S-shape was formed;

2) Halo strings grew to 10 rms in y-direction.

• Reasons: tails and core have different frequencies, tails not properly populated. This causes fast dilution of the phase space. Fast dilution and/or core oscillations cause resonances; resonant particles are subject to amp. growth and losses


Self-Consistent Space Charge Distributions

1) To solve beam degradation problems related to space charge, we would like to create self-consistent distributions.

2) By self-consistent, we mean beam distributions that a) are time-independent or periodic in time, b) support linear space charge forces, and c) maintain their shape and density under all linear

transformations.

3) Ellipsoidal distributions of uniform density will satisfy self-consistency. A distribution which is self-consistent, or nearly so, will produce no loss and preserve rms emittance in the course of accumulation (acceleration).


How Many Analytic Solutions Have Been Found?

1) Time-independent up to 3D (Batygin, Gluckstern…). Their use is limited, because of the fact that the

conventional focusing uses alternating gradient.

2) Time-dependent with nonlinear force – none.

3) Time-dependent with linear force – up to 2D (Kapchinsky-Vladimirsky distribution).

4) New classes of self-consistent distributions have been described in Danilov, et al, PRST-AB 6 (2003) .

• This presentation will highlight progress in ongoing studies with these newly described distributions.


Basic Math of Self-Consistent Distributions

equationVlasovtosolutiontheis

nHamiltoniaoffunctionanyHf

UyxkmppH scyx

)(

2/)(2/)( 2222

ii

iirr

iiisc YdXdf

YYXX

dYdXRRrUYXF

))()((

)(2),(

22230

F(X,Y) is the space charge force, f is the distribution function; 2D case is taken just for example.

The first example: linear 1D case – the beam density is constant.

,2/

)()(

2/2

2

bH

kx

xkxH

dHHfconstdpHf

where the distribution function f doesn’t depend on phase.The integral equation is called Abel’s Integral Equation.

x0


Math for Self-Consistent Elliptical Distributions

222/)2/(,2

222222 )2/(22/)(

xxpPXxSubst

nxnxxnpxxTransf

x

PX

xpxppxpx

nxxn

xn

solutionsnoD

ondistributiKVHHC

HfCdpdpHfD

HH

CHfC

kxH

dHHfdpHfD

byx

b

H

x

x

b

3

);1959()()(,)(2

;)(,2/

)()(1

2/2

2

Outstanding fact – any linear transformation of the phase space preserves the elliptical shape. Valid for all-D cases. 1D sample drift transform proof:

After linear drift transformation there exist substitution of variables such that the density integral in new variables is exactly same. It means constant density again

x

y

constdPPXfNewconstdppxfOld xxxx )(2,)( 2222


Envelope Equations

Linear motion – quadratic invariant- solution to Vlasov equation for constant density as a function of this invariant- linear force-linear motion. We get closed loop of self-consistency. One final step –

boundaries of the beam determine the force, force determines the particle dynamics, including dynamics of the boundary particles.

Boundaries (or envelopes) must obey dynamic equations. In 2D case :

General Result – if distribution depends on quadratic form of coordinates and momenta and initially

produces constant density in coordinate space, this density remains constant under all linear

transformations.

./4

),/(/)(

),/(/)(

320

3

3

rwhere

babbsKb

baaasKa

yy

xx

a

b


New Solutions – 2D set

caseotherwiseRRXYYXCf b }2,2{)0(),()( 0000

The difference with previous cases- the distribution depends on other invariants, not only on Hamiltonian.

RbRotating disk – arrows show the velocities. In all xy, pxpy, pxx,pyy projections this figure givesa disk – different topology then one of the KV distribution x

y

Any linear transformation preserves elliptical shape. The proof: 4D boundary elliptical line remains always elliptical, the projection of elliptical line onto any plane is an ellipse, the density remains

constant under any linear transformation

.}0,2{,}1,2{);(

...)(.dim.},{

00 caseKVcaseYXHH

Cf

timesmHgfformtheinfSeekncasemn

b

The principle – delta-functions reduce the dimension in the density integral. Remaining eqn. for g(H) is same as in time-independent case


New Solutions – 3D set

.}0,3{

;}1,3{)0(...),()(

;}2,3{)0(...),(...)(

;}3,3{)0(...),(...)(...)(

00

0000

000000

caseinsolutionno

caseotherwiseHHaXXHHCf

caseotherwiseHHbXYaXXHH

Cf

caseotherwiseHHcXZbXYaXXCf

bb

b

b

b

3 new 3D cases found. All have ellipsoidal shape in xyz projection. The density inside is always constant.


SNS Injection – How the Distribution is Formed


Self-Consistency:No-Chromaticity Low-Dispersion Case

• SNS case – chromaticity can be eliminated by sextupoles, dispersion exists only in the arcs (no dispersion in the drifts, etc.), but there is strong double harmonic - 2D case is enough (neglect longitudinal space charge force.

1) Present injection – movingthe closed orbit from the foil, no injected particle angles

foil

Growing tails because of space charge-is a typical dilution effect

2) Modified injection – needsparticle angle on the foil, roundbetatron modes (e.g. equal betatron tunes).

This {2,2} distribution has outstanding property – it retains its self-consistent shape at any moment of injection


No-Chromaticity Low-Dispersion Case cont.

Painting without angle Painting with angle

Recall sharply peaked longitudinal density profile...

4.371013 protons

Finally, self consistent distributionis one having transverse elliptical shape, and its size should correspond to envelope parameters along longitudinalcoordinate – the last condition can be met whenwe introduce injected beam beta function variation along the beam. It needs an additionalfast kicker or RF quadrupole.

tail

Tr. size:center


Paint {2,0} Distribution in SNS:Nonlinear Effects Destroy Self-Consistency

Linear Fringe Fields

Sextupoles Fringe Fields + Sextupoles


Solution: Include Solenoids to Split Tunesand Paint to an Eigenfunction

• For the same SNS case as the previous slides, include two solenoids in straight section to split tunes and paint {2,0} distribution to one of the eigenfunctions.

• Results show painted beam in phase space and tune separation and spread of eigenvectors.

x-x'

y-y'Tunes


{2,0} Distribution: Effects ofSolenoids and Space Charge

Fringe Fields and Sextupoles Included Unless Noted

AddSolenoids

AddSolenoids

Add Space Charge Add Space Charge


Ring High-Dispersion Case

• Dispersion effect is stabilizing•Not every energy distribution provides linearity of the transverse force•The condition for self-consistency – any transformation x -> x + D p/p should preserve the linearity of the force •Newly found 3D distributions satisfy the conditions because the transformation x -> x + D p/p is linear in 6D phase space.•The longitudinal force, which is proportional to the derivative of the linear density is also linear!!! fully self-consistent

ring distribution is a uniformellipsoid in projection to3D real space

htoth thangreatertlysignificanp

pD

Chromaticity is equal to zero. Chromatic cases not investigated


{3,2} Distribution With Linear TransportIncluding 3D Uniform Focusing (Shown After 100 Turns)

x-y Φ-y

Φ-dE

x-y'

y-x'


{2,2} or {3,2} rotating disk case

ff XYYX /,/ ff XYYX

RYX

/,/

,222

The conclusion: the rotating disk self-consistent distribution can be transformed by skew quad into distribution with one (e.g. vertical) zero

emittance. This is the ideal choice for collider with flat beams

Rb

x

y

skew quad0,/2

,222

YYX

RYX

f

before

x

x’

y

y’

after

x

x’

y

y’

Amazing detail: regularly looking beam (in x and y phase spaces)transformed into zero emittance beam. Reason – xy correlations.


{3,2} Distribution: Transforming to Flat BeamWith Skew Quad Followed by Transport

• Take the {3,2} distribution shown in the previously:

– x-y‘ shown

– y-y‘ shown

• Pass through skew quad of appropriate strength:

– y-y‘ shown

• Transport through appropriate length:

– y-y‘ shown

• Top figure is without space charge. Bottom figure includes space charge, which opposes compression.

Add Space Charge


{3,2} Distribution: Create for SNS Lattice (Linear Transport) and Manipulate to Make Flat Beam

Uniform Focusing SNS Lattice

x-y x-y

x-y x-y

Add Space Charge Add Space Charge


Summary

• This presentation builds on previous work (V. Danilov et al, PRST-AB 6, 094202 (2003)) to explore some of the newly found 2D {2,2} and 3D {3,2} distributions. Specifically, it is shown that:

– In 2 dimensions: Although we can paint a {2,2} distribution into SNS, nonlinear

effects such as fringe fields or sextupoles destroy self-consistency.

This can be rectified by adding solenoids to split the tunes, and then painting to one of the resulting eigenfunctions.

With or without solenoids, the inclusion of space charge gives round beams.

– In 3 dimensions: We demonstrated the self-consistency of {3,2} distributions

under linear transport over 100 turns both for uniform focusing and for the SNS lattice.

We examined the process to create flat beams from the {3,2} distribution with the use of a skew quad followed by transport.

We found that this method works with linear transport, but that space charge limits the compression of the beam.


Next Steps

• There are several immediate steps to be taken:

– We need to work on matching with space charge. Although the inclusion of space charge results in round beams, they are not matched and the densities are not precisely linear.

– We need to study nonlinear effects on self-consistent 3D ({3,2} distribution) beams.

– We need to determine injection schemes to create self-consistent 3D beams. The 3D distributions studied so far were created externally and injected at once.

– We need to study space charge limits with self-consistent beams. One of the motivations for such beams is the smaller tune shifts provided by uniform space charge distributions.

Accelerator PhysicsOak Ridge National Laboratory Jeff Holmes, Slava Danilov, and Sarah Cousineau...

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