Chromatic Aberration, A closer look

1
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

• Conclusions
• Not much beam at edge of phase space
• Chromatic aberration is not a severe problem

Thank you G.W. Grime
2
Spherical Aberration, A closer look

• Traditionally, spherical aberration is computed
  from the rectangular model (RM)
• Rectangular model
• B(z) 0 z lt 0
• B(z) B0 0 lt z lt L
• B(z) 0 z gt 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!

z
L
0
Coefficient RM TFFM (x/? 2)
-130 -130 (x/?? 2) -390
10 (y/? 3) -220 -190 (y/? 2?)
-390 2
3
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!

4
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

For 5 nA divergence is 2.5 times less than
uniform model so spherical aberration is reduced
by a factor of 16
2 MeV He
Current (pA)
5
Stray DC Magnetic Fields Parasitic aberration
Without magnet
With Magnet

• 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!

6
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?

7
Stray AC Magnetic Fields Beam spot jitter

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

object
lens
http//www.meda.com/fm3page.htm
8
Stray AC Magnetic Fields Beam spot jitter

• 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

3 mm
9
Stray AC Magnetic Fields

• It is good to have
• High demagnification systems
• Short systems
• On the Melbourne system it is required that
• Bstray lt 20 nT for xi lt 0.1 mm

• Where
• M Magnification 1/Demagnification
• q beam particle charge
• L Length of beam line
• E beam energy
• m beam particle mass

10
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