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

• N135 pC > N

60 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)

• 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

– ’s2 x larger in warm regions» triplet focussing needed to handle longer linac, higher

RF focussing from increased module gradient

for same emittance, need bigger aperture

What Can We Reasonably Surmise?

2

2

-20

-10

0

10

20

30

40

50

0 10 20 30 40 50 60 70

beta x

10*eta x

beta y

recirculator

• upgrade > demo with upgrade

geometric > demogeometric larger spots

• upgradegeometric > demo

geometric with upgrade ~ demo larger spots

• N135 pC > N

60 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?

Geometric Emittance Comparison to Demo

accelerate

energy recover

150 MeV 100 MeV 50 MeV 10 MeV

10 MeV 50 MeV 100 MeV 150 MeV

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 < geometric

demo 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

10 MeV beam

start 2”

to wigglerend 2”

optical cavity chicane

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

– ~ 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

10 m match “meets spec”5 m match “4 x out of spec”

- go with 3”

Recommendation(s) #3• FEL insertion region:

– 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

Conclusion #3

wiggler

end 2”

optical cavity chicane

3”

• 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

– -bends probably tolerate 2” because , (and ) “smaller”– power requirements dominated by -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

To avoid undue risk must make FEL insertion 3”

“Little” additional cost in making all reverse bends 3”

• moderate additional DC power (most in -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% p/p

• significant risk reduction, especially for lower energy operation at higher space charge (can tolerate ~2x larger emittance)

Conclusion(s) #4

upgrade ~2 or 3 x demo at reinjection

Nupgrade > N

demo (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

Injection/Reinjection Region - 2” or 3”?

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

Conclusion #5