RF‐Gun Workshop
Commi2ee Feedback
T. Shintake (OIST)Ma2hew Poelker (JLab)
Sasha Gilevich (SLAC)
Yosuke Honda (KEK)

RF gun review @ KEK, February 19 and 20, 2015

The Review Commi2ee “Philosophy”• Any component integrated into theaccelerator system must be Reliable,
Maintainable, Reproducible• Start with a simple, robust system. Improve itand add features step by step aXerwards• Cost is not irrelevant

The Commi2ee Knows…
• SuperKEKB requires state‐of‐the‐art RFguns and drive lasers• New hardware, must be designed andbuilt, some items without precedent• Resourceful group, modest size, must now
ﬁnish the R&D phase, and move to“produc`on” phase• SuperKEKB Phase 1 opera`ons begin
January 2016, 10 months from now.
Colleagues expect reliable beam soonThis came as a surprise … a new beamline using the oldthermionic gun and now two quasi‐travelling wave guns

How to Interpret?
• Management is gedng worried• Too many ideas, at this stage of theproject• The gun/laser group s`ll faces big
technical challenges, and the clock is`cking

What are the laser challenges?
• Timing synchroniza`on• Amplitude noise• Laser temporal pulse shaping• Ampliﬁed spontaneous emission• Two bunch genera`on• Reliable delivery of light to photocathode• The Commi2ee can't tell how many rf gun
problems are due to drive laser issues

Recommenda`ons• Develop a “simple” laser system without temporalpulse shaping: a simpliﬁed master oscillator, andmore rugged Nd:host free‐space ampliﬁers
Yes, it’s somewhat
forforCommi2eeto• Consider
diode lasercontradictory
gain‐switching
your mastersay “you have
too nevermany losesideas”lock.
and Jlabthencansuggestoscillator.
The MOhelp.
newthings
for you
to try!the origin of• Add diagnos`ctools
to help
pinpoint`ming and amplitude instability. Determine where to
devote your eﬀort for the biggest return• Purchase commercial components when possible.
Oscillators and ﬁber amps, free‐space amps?
• Do not modify the thermionic gun

Energy spread reduc`on using temporal manipula`on
Energy spread of 0.1% is required for SuperKEKB synchrotron injec`on.
35nC 10nC

2.5t15nC
20nC

GaussianCan you achieve your goals by shaping the electronbunch, rather than the laser pulse?
σE/E (%)

21.5

1t

Square 5nC
10nC

0.515nC
20nC

00510155nC electron

FWHM (ps)203015nC 25
Primary beam for positron produc`on• When the charge distribu`on is uniform in a cylinder, the energy spread of thebunch is lower than the result of Gaussian charge distribu`on.
• For the laser source, the pulse width should be reshaped to rectangle structure.

Two bunch genera`on• By elimina`ng pulse shaping task from lasersystem, does it become easier to generate
two bunches?
• Replace Yb:disk with rugged Nd:host ampliﬁermaterial: less thermal lensing, more uniform
gain applied to both pulses?

Preparing for Produc`on Running• Improve the environment for laser systems:
temperature, vibra`on, “wind”• Consider normal incidence laser illumina`on of thecathode. Or, eﬀec`ve normal incidence, using gra`ng orprism• Implement a laser imaging system to minimize poin`ng
instability, learn from LCLS
• Shu2er in front of gun + virtual cathode diagnos`cs to set
laser spot size at the photocathode, to avoid accidental`ght focus and damage• Harmonic crystals need to be carefully installed:
temperature, vibra`on. Transpor`ng UV light not trivial

How to avoid damaging ﬁbers?

Can your colleagues at KEK cERL help you with thisproblem?

IMPROVEMENT
This makes the electric field discharge
limit much higher.

Careful alignment will be important.

Heat diffusion becomes much better.
AR coating will eliminate reflection fromoutput side, stable operation.

Degrada`on of RF‐Gun cavity

Photocathode and RF‐gun• Commi2ee congratulates the team for discovery ofIrCe phptocathode• Commi2ee congratulates the team for the inven`onof quasi‐traveling wave side coupled RF‐gun, which
will preserve emi2ance from the cathode torela`vis`c energy• Drive laser issues appear to dominate the group’s
a2en`on, making it diﬃcult to work on gun andphotocathode issues…
• Review Commi2ee will provide a detailed summary
report. We intend to evaluate SuperKEKB laser
speciﬁca`ons against other exis`ng laser systems