Aperture Considerations in the FEL Upgrade
• Accepted design process
– generate design  known
– set aperture = N + W
• N typically 4 to 6
• W is “beam handling allowance”
example: IR Demo has A = 6+ 4 cm
– “Other restrictions may apply”
• constraints imposed by FEL - optical mode size
• Here, programmatic considerations force deviation
from accepted practice  risk escalates
• Can reduce risk by using all available information
– previous design studies
– experience with IR Demo
What Do We Know?
• No design  unknown
• Injector not quantitatively understood N135 pC
unknown
 unknown
• FEL optical mode larger  3” aperture needed
unless we can compress e- beam transport
What Can We Reasonably Surmise?
• N135 pC > N60 pC
• upgrade > demo
– larger machine  larger and/or more quads
• more quads undesirable
– higher cost
– increased chromatic aberration (in turn a limit on larger
required momentum acceptance)
2
• 1st iteration linac optics (actually, 2nd - 1st was UV Demo
design study) has larger beam envelopes
– ’s “same” in modules  2” may be okay for modules provided
emittance does not increase too much
– ’s 2 x larger in warm regions
» triplet focussing needed to handle longer linac, higher RF
focussing from increased module gradient
 for same emittance, need 2 bigger aperture
50
40
30
20
10
0
0
-10
-20
10
20
30
r
e
c
i
r
c
u
l
a
t
o
r
beta x
10*eta x
beta y
40
50
60
70
Geometric Emittance Comparison to Demo
accelerate
150 MeV
100 MeV
50 MeV
10 MeV
10 MeV
50 MeV
100 MeV
150 MeV
energy recover
• upgrade > demo with upgradegeometric > demogeometric  larger spots
•
upgradegeometric > demogeometric with upgrade ~ demo  larger spots
• N135 pC > N60 pC likely, upgrade > demo certain
• Injector setup required for high FEL gain (tapered
wiggler tests) limited to 1.5 mA by BLM hits  2”
aperture inadequate even at 60 pC when high gain
configuration required?
Conclusion #1
Though 2” possibly (probably?) adequate in modules,
peak ’s in upgrade are in warm regions and will
drive increase in aperture there
Recommendation(s) #1
• Make effort to understand injector quantitatively and run 5 mA CW at 135 pC
– helps define if 2” injector chamber allows reliable
operation
– characterized normalized emittance at elevated charge
•
3” warm region in linac
Linac-to-FEL Transport at 100-200 MeV
• It is possible geometricupgrade < geometricdemo in the
module to FEL transport even with space-charge
driven degradation (higher energy)
–  upgrade > demo is washed out in spot size in full energy
transport
– note that at same energy (mid linac in upgrade, end of
linac in demo) spot sizes are larger in upgrade
– at low end of energy range (~100 MeV) spots may be same
or larger in upgrade due to increased normalized
emittance and larger beam envelopes
Conclusion #2
2” tube may be adequate for full energy beam from
end of linac to start of FEL insertion
Recommendation(s) #2
100-200 MeV beam
start 2”
10 MeV beam
optical cavity chicane
end 2”
to wiggler
Component Reuse
Larger aperture requirements limit component reuse
to regions such as linac-to-FEL transport
• Diagnostics reusable without modification
• QB quads probably reusable without modification
– 48 MeV IR Demo QB maximum current ~2 A
– QBs spec’d to 10 A with LCW
 can get to ~200 MeV with 20% headroom for matching
• Correctors may prove useful under similar analysis
FEL Insertion Region
• Optical mode significantly larger than in IR Demo:
– either use 3” aperture (including dipoles)
– or restrict matching regions to ~ 5 m length
• Current “existence proof” uses ~10 m match
– manages aberrations at 5% momentum offsets by
adjusting phase advances amongst telescopes/arc
components
• causes destructive interference of chromatic effects
– y~ ds/ if L reduced,  must reduce
• good for small apertures, but,
  smaller  quads stronger
• stronger quads  aberrations larger
– higher order chromatics ~quadratic in quad strength, 
halving lengths doubles quads, quadruples aberrations
Conclusion #3
10 m match “meets spec”5 m match “4 x out of spec”
- go with 3”
Recommendation(s) #3
• FEL insertion region:
optical cavity chicane
3”
wiggler
end 2”
– basic optimization for matching telescope length must
balance keeping  small - for good performance and
acceptance while keeping L large - to limit quad strength
– ~10 m match in this machine
• Choose magnet families to keep construction
simple
– fringe models developed for spectrometer magnets; 3” is
not “large” so predictive capability likely okay
– match magnet gaps in “similar” families
– p-bends probably tolerate 2” because , (and h) “smaller”
– power requirements dominated by p-bends (180o out of
300o bending per end loop, so draw most of power)
– IR Demo successful matching magnets within and across
families; should anticipate similar results in upgrade
Conclusion(s) #4
To avoid undue risk must make FEL insertion 3”
“Little” additional cost in making all reverse bends 3”
• moderate additional DC power (most in p-bends)
• no overhead in “lost” magnets
– no dipoles “lost” as none upgrade
– need new trim quads, 6-poles, 8-poles due to horizontal
aperture increase necessary to accommodate 10% dp/p
• significant risk reduction, especially for lower
energy operation at higher space charge (can
tolerate ~2x larger emittance)
Injection/Reinjection Region - 2” or 3”?
upgrade ~2 or 3 x demo at reinjection
Nupgrade > Ndemo (space charge)
geo.upgrade~ 1/2 to 1/3 geo.demo (adiabatic damping)
 it will not get better
How good is it now?
• Cavity 8 tunes a fair bit ( losses)
• ILM0F062 hits have been limitation
• ILM0F06 hits are a limit when running injector for
high wiggler gain
Conclusion #5
3” prudent risk reduction at modest incremental cost
• new injection/extraction dipoles needed to increase
available dynamic range of injection/final energy
– “small” magnets (~DU/DV)  minor power impact
• QJ quads/associated correctors support 3”
• need additional quads for recirculator
– not enough QBs to populate reinjection region
– at very least, need to re-coil some QGs (~4 for linac to
FEL transport, this region would require an additional 6
or 7)
– could build an additional half-dozen 3” quads