1. © David N. Jamieson 1999 Divergence,  o Energy Spread,  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 Ion source Accelerator Magnet Ray used in maximum d c calculation Danger zone Conclusions: Not much beam at edge of phase space Chromatic aberration is not a severe problem *Thank you G.W. Grime

2. © 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) = B 0 0 < z < L B(z) = 0 z > L Results from this model agree with ray tracing codes that use B(r 0, z) measured at r = r 0 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 z 0 Coefficient RM TFFM (x/  2 ) -130 -130 (x/  2 ) -390 +10 (y/  3 ) -220 -190 (y/  2  ) -390 +2

3. © 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!

4. © 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  m 200  m 75  m 2 MeV He + Current (pA)

5. © David N. Jamieson 1999 shadow 130mm525mm 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!

6. © David N. Jamieson 1999 shadow 130mm525mm grid Deflect here 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?

7. © David N. Jamieson 1999 Stray AC Magnetic Fields: Beam spot jitter x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x B stray (t) object virtual object Stray AC field causes a shift in the virtual object position The beam spot is scanned by the stray field in a complex fashion image shift h Mh http://www.meda.com/fm3page.htm lens

8. © David N. Jamieson 1999 Stray AC fields cause virtual movement of the object collimator Used a 2-D scan with y-coils disconnected Gives position as a function of time in map of Cu x-rays B y (nT) Stray AC Magnetic Fields: Beam spot jitter 3  m

9. © 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 It is good to have: High demagnification systems Short systems On the Melbourne system it is required that: B stray < 20 nT for x i < 0.1  m

10. © 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