© David N. Jamieson 1999 Divergence, o

En

erg

y S

pre

ad,

o

+

high

low

Chromatic Aberration, A closer look

Are and correlated? Use MULE* to find out. Here is a slice of object plane phase space taken along and System was the HIAF accelerator in Sydney (From the work of Chris Ryan)

Not much beam in the danger zone Beam intensity is peaked in the paraxial zone

Ionsource

Acc

eler

ator

Magnet Ray used in maximum dc calculation

Danger zone

Conclusions: Not much beam at

edge of phase space

Chromatic aberration is not a severe problem*Thank you G.W. Grime

© David N. Jamieson 1999

Spherical Aberration, A closer look

Traditionally, spherical aberration is computed from the rectangular model (RM)

Rectangular model:

B(z) = 0 z < 0

B(z) = B0 0 < z < L

B(z) = 0 z > L Results from this model agree with ray tracing

codes that use B(r0 , z) measured at r = r0

Detailed studies have been done by Glenn Moloney

– Measured field profiles B(r , z) at several r– Provides 3-D profile of True Fringe Field (TFF)

Numerical raytracing from measured B(r , z) reveals different spherical aberration coefficients!

L z0

Coefficient RM TFFM

(x/ 2) -130 -130

(x/ 2) -390 +10

(y/ 3) -220 -190

(y/ 2) -390 +2

© David N. Jamieson 1999

Spherical Aberration, A closer look

Coefficients calculated from the TFF model give aberration figures of different shapes compared to the rectangular model

The figure is more intense in the paraxial region - good!

© David N. Jamieson 1999

Ion Source Brightness: Flux Peaking

Legge et al (1993) showed a 1 order of magnitude decrease in probe size required a 5 orders of magnitude increase in brightness for uniform model

True situation more complicated: 1 order of magnitude decrease in probe size requires 2 orders of magnitude increase in brightness

Uniform phase space

Set 5 nA

For 5 nA divergence is 2.5 times less than uniform model so spherical aberration is reduced by a factor of 16

100 m200 m

75 m

2 MeV He+

Cu

rre

nt (

pA

)

© David N. Jamieson 1999

shadow

130mm 525mm

grid

Without magnet

With Magnet

Stray DC Magnetic Fields: Parasitic aberration

Non-uniform stray DC fields are a problem

Shadows of a line focus on a fine grid should be straight line

Small bar magnet has severe effect See large sextupole field

component aberrations Sources of stray DC fields in the

MARC laboratory:– Iron gantry and stairway over

the beam line– Steel equipment racks– Gas bottles– Stainless steel beam tube itself!

© David N. Jamieson 1999

shadow

130mm 525mm

gridDeflect here

beam

beam

beam

beam

BEAM

PIPE

Stray DC Magnetic Fields: Aberrations of a beam pipe

Type 316 stainless steel beam pipe through quadrupole lenses

10 mm internal diameter Beam diameter 6 mm Grid shadow pattern reveals

aberrations See strong effect from different

deflections of the beam pipe! Effect here produced by a few cm

length What effect does 8 m have?

© David N. Jamieson 1999

Stray AC Magnetic Fields: Beam spot jitter

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Bstray(t)object

virtualobject

Stray AC field causes a shift in the virtual object position The beam spot is scanned by the stray field in a complex

fashion

imageshift

hMh

http://www.meda.com/fm3page.htm

lens

© David N. Jamieson 1999

Stray AC fields cause virtual movement of the object collimator

Used a 2-D scanwith y-coilsdisconnected

Gives position asa function of timein map of Cu x-rays

-2000-1000

01000

0 50 100

Time (s)

By (

nT

)

By

(nT

)

Stray AC Magnetic Fields: Beam spot jitter

3 m

© David N. Jamieson 1999

Stray AC Magnetic Fields

Where: M = Magnification =

1/Demagnification q = beam particle charge L = Length of beam line E = beam energy m = beam particle mass

Em

LMqBx stray

i22

2

It is good to have: High demagnification systems Short systems

On the Melbourne system it is required that:

Bstray < 20 nT for xi < 0.1 m

© David N. Jamieson 1999

Stray AC fields in MARC laboratory: Where from?

Field as a function of time tells the story Start: 6pm April 18 2000 Place: MP2 beam line, MARC laboratory

To MARC lab 50 m