Report of RF-Gun Review
KEKB Accelerator Review
2016.6.14
Y.Honda• RF-Gun Review Committee- member: T.Shintake(OIST), M.Poelker(JLab), S.Gilevich(SLAC), K.Oide(KEK), Y.Honda(KEK)• Past reviews- (1)Feb.2015, (2)Nov.2015, (3)Jun.2016- (3) held Last week‣ member: Y.Honda, K.Oide, M.Poelker(video)

Past review (1st, Feb.2015)•••

Situation at the time: 3-2 RFGun started up quickly (2012), but the new A-1 Gun seems to have problems...
We reviewed the technical choise and plan of each items.
RF-Gun Cavity-•‣

Fine for 5nC/b, emittance 10 mmmrad, smaller aperture

QTW(Quasi-Traveling-Wave) RF-Gun (A-1 Gun)‣‣

Original design, possibility for 10nC/b, emittance 6mmmrad
Took time for RF Conditioning still not achieving designed E-ﬁeld

Support QTW development

Cathode-•DAW(Disk-And-Washer) RF-Gun (3-2 Gun)

LaB6

‣

Well known material, higher QE but shorter lifetime

Ir5Ce‣New one, seems to have possibility in future, stable and longer lifetime

Support IrCe development

Laser--

Nd-based multi-pass ampliﬁer (3-2 Gun)

‣Simple system but does not satisfy the ﬁnal goal✴

not possible to do temporal shaping, diﬃcult to go 50Hz rep-rate

Yb-disk cascaded ampliﬁer (A-1 Gun)‣‣‣Has upgradability, for example CPA and temporal shaping.
Issues in the hand-made oscillator (stability and synchronization)
A cascaded system of many stages can be chaos

Moving to a new laser room for much space and better environment

Seems Diverging

Committee feedback•••••

Impressed hard works and creativity of the group.
Seems R&D is diverging, need to select items for limited resources.
Phase-I starts soon, R&D stage to operation stage-Leave R&D items for Phase-II&III‣

Without temporal shaping, Nd-based is a promising scheme

Simplify the system-

Commercial components when possible‣

oscillator, ampliﬁer unit, etc.

2-bunches generation by a delay-line scheme is simpler
Reconsider Nd-based system
Still many items for operation stage-Transport system‣

Vacuum transport, pointing stability

Diagnostic system‣

Virtual cathode monitoring, prepare many monitoring points in the system

1st page of the review

Past review (2nd, Nov.2015)••••

Situation at the time: New laser system under development, commissioning with thermionic gun.
Status for responding the 1st review
Beam commissioning-RF-Gun Cavity-•

Started with a thermionic gun
QTW RF-Gun‣

Cathode plug design improvement, then RF conditioning went wellCDS(Cut-Disk-Structure) RF-Gun‣‣

New design shown, wider aperture, lower RF power
0-degree laser injection is possible

Laser-2-bunch(96ns separation) will be done by a splitter & delay-line scheme.

‣Regenerative ampliﬁer is promising

Designed a system for Nd-based and Yb-based regenerative ampliﬁer‣

Under construction in the new laser room

Existing Yb-based system‣‣

Oscillator improvement
High gain multi-pass ampliﬁer, reduced number of stages

Committee feedback•••••

Impressed progress in limited time
Need improvement in beam tuning-

Beam loss and optics mismatch
Identify sources of beam ﬂuctuation
RF-Gun cavity-

Successful conditioning of improved QTW is great.
Supported the idea of CDS
Cathode-Support investigating potential of IrCe (not to shift to CsTe)
Laser-Appeares to be more focused work than before, this is good.
Stability greatly improved, but still not enough‣

Identify the ﬂuctuation source, environmental improvement

More diagnostics should be installed.

••

Last review (3rd, Jun.2016)
Beam commissioning--•

Beam jitter and emittance investigation
RF-Gun Cavity-•

Stable RF-Gun operation and successful HER injection

QTW RF-Gun‣Stable beam operation

RF conditioning station‣

Proved choke structure, this is for future cathode plug designing

CDS(Cut-Disk-Structure) RF-Gun‣‣Under fabrication. 0-degree laser injection, better for laser pointing jitter.
To be installed in the new beam line

Cathode-Possibility of Ir7Ce2 or single crystal IrCe for higher QE
Laser

-

Existing Yb-based system‣

Stable operation for a month (no system change)

New laser for Phase-II‣‣

Nd regenerative ampliﬁer
10MHz oscillator for further simpliﬁcation

Development for Phase-III‣‣‣

Yb-disk regenerative ampliﬁer
Stretcher and compressor for CPA , Yb-RodCryo Yb R&DCommittee feedback•••••

Congraturate stable RF-Gun operation and HER injection in Phase-I
Beam commissioning-There are puzzles in emittance/jitter between right after the gun and at the end of the linac
If beam jitter dominated the projected emittance, improvement of laser pointing jitter is essential‣

In this case, 0-degree injection at CDS gun has an advantage.

Try 2-bunch injection
RF-Gun cavity-After RF conditioning, QTW gun working well.
Need clear explanation on beneﬁts of CDS gun‣

You can play in the 2nd beam line, but still need cleariﬁcation.

Cathode-Clear goal of QE should be shown. Critical number that changes/relaxes the laser system design.
Seems separated man-power, so doing R&D for Phase-III should be ﬁne.
Laser

-Existing Yb multi-pass system worked stably.

‣‣

Contradiction to our past recommendation of Nd system. Great achievement.
Feedback system for stabilizing should be tested.

Regenerative ampliﬁer to be developed will beneﬁt simplifying and beam proﬁle improvement.

‣Most critical study item at present for Phase-II

Detail of 2-bunch generation by splitter&delay system should be shown.
Engineering design of transport system should be shown.
Diagnostic system is still poor.

‣Virtual cathode monitoring, prepare many monitoring points in the system

Committee feedback• Beam operation- Now the issues are not only in the Gun but entire Linac‣ Linac and Ring operation team should join the study of emittance/jitter.
- Gun team should prepare more monitors to investigate the origin.
‣ Shot-by-shot laser position, intensity, Laser proﬁle, etc.
‣ These should be synchronized with other beam monitors, BPM etc.
- Present procedure of beam tuning, not just the result.
‣ For example, Phase optimization for short bunch and low emittance• Lots of works for laser experts, seems some items are put oﬀ.
- Laser transport line and diagnostics- Beam optics and diagnostics- For these items, support from other team may help.
• Need discussion based on real data
- Many schematic ﬁgures were presened but quantitative discussion was limited.
- Show data. This may come from issues in poor diagnostic, again.

Summary• We appreciate great works of RF-Gun and Laser team.
• We congratulate successful commissioning of HER injection by RF-Gun.
• Now it seems the development work seems organized than before.
• Keep the motivation and improvement works for Phase-II and III.
• Now the issues are in the beam operation of total accelerator system.

Supplement slides• Report supplied from committee members

Congratulations!
• Successful injection to the HER.
• ~100% injection eﬃciency• Good stability during operation for days.
• Now you can see the exit of the long tunnel!

1.724	  +/-­‐	  0.07	  nCT. Natsui

http://4.bp.blogspot.com/-x5q3h0qoZdc/VaAhlHMV-VI/AAAAAAAAFMo/serhWx916QY/s1600/DSC̲2501.JPG

Some issues to be overcome by Phase-II•

2 bunch injection•While the emittance measurement right after the gun
seems OK (~20 μm), signiﬁcant growth of emittanceand jitter have been observed through the linac andbeam transport line.

XY

Sector	  C4	  wire

176±32.3

34.2±7.6

Sector	  C3	  wire(no	  B)100±13834.2±16.5

Sector	  5 4	  wire

51.6±31.0

114±34.4

Sector	  5 3	  wire(no	  C)106±24.9

Sector	  5 3	  wire(no	  D)76.5±38.9

BT4	  wire211±110.6

BT3	  wire(no	  D)334±52.1

BT3	  wire(no	  C)133±22.2

138±45.6

N. Iida•

There are some puzzles in the emittance/jitter:•While the horizontal position jitter right after the gun seems
much higher than the vertical, the Q-scan at the screen does
not show the diﬀerence (if it is true, the jitter may not be theissue).
X
1σ	  :	  0.44	  mm	  y1σ	  :	  0.23	  mm	  Horizontal	  (projec2on)Ver2cal	  (projec2on)

A1	  chicane

28.3	  (31.8)	  26.4	  (29.4)	  A1	  M

20.3	  (20.8)	  17.7	  (18.3)	  B	  sector	  dump

48.5	  (52.7)	  21.7	  (22.2)	  T. Natsui•There are some puzzles in the emittance/jitter:•The observed phase space of the jitter in the BT line hasvery similar shape to the measurement of wire, why?

•Similar observation should be done for the thermal gun.

T. Mori••More eﬀorts, not only by the Gun team, but by entire Linac
people are necessary to solve the issue of emittance/jitter
growth:•identify the source(s)•understand the mechanism (via wake? nonlinearity in the Jarc?…)

As the ring operation halts until Phase-II, the ring operationteam should join the linac study.

Laser Issues (time structure is very challenging
from laser perspective, many options to consider)§ Gain media: Nd vs Yb vs Ti:sap§ Multi-pass ampliﬁer vs Regenerative Ampliﬁer§ Choice of master oscillator laser: commercialvs diﬀerent in-house manufactured§ 50 Hz operation§ 2-bunch time structure
Factors that inﬂuence decisions:§ Reliability, stability…the focus of Phase1§ Enhanced capability(complexity)…the focus
for next phases§ The rf gun and the photocathode inﬂuencelaser choices

The balance between preparing for production
running and performing R&D for future phases§ Underground laser system, with Yb gain
with enhanced capabilities
medium and multipass ampliﬁer shown to be
successful for phase1 (in mild contradiction topast recommendation that urged Nd gainmedium)§ Laser operating > 1 month withoutmaintenance!! Great achievement§ Group has beneﬁted from talented
troubleshooting and implementation of eﬀective
improvements: stable master oscillators,
tailored preampliﬁers, semiconductor opticalampliﬁer for pulse picking and ASE reduction

The balance between preparing for production
running and performing R&D for future phases
enhanced
Still to with
do forPhase1: capabilities (cont.)
§§

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2 bunch formation
The optical transport line needs work (isthere design and engineering support forthis work? Can this be taken oﬀ the gun/laser group “plate”?)
More diagnostics are needed to determine
the origin of laser instability (timing vspointing)
Virtual cathodeFeedback applied to multipass amp?

The balance between preparing for production
running and performing R&D for future phases§ Undergroundlaser, capabilities
a “bare bones”
systemwith enhanced(cont.)
for Phase1. Design is ﬁxed, not changing§ Above ground laser represents theultimate laser for Super KEKB§ Can the gun/laser group suﬃciently
maintain and improve the bare bones
Phase1 laser system while conducting
R&D, particularly the eﬀort devoted toconstructing reliable regenerative
ampliﬁer?
§ The regenerative ampliﬁer appears to be
the most critical R&D topic now…

§

You will beneﬁt during a review, if you say“up front” why you are installing the CDS RFGun. People on review committee who are
not intimately aware of speciﬁc issues may
think you are “playing”. The gun work can bea distraction from laser work?
Speciﬁcally, state the beneﬁts of the CDS
gun over the QTW gun: e.g. more charge,
normal incidence illumination ofphotocathode which provides more QE?, for§§§Other questions/Issues

Is one master oscillator better than theothers? e.g., for positron production, withrequired timing shift, Why are there still
three+ choices for MO?
If the regenerative ampliﬁer is thepreferred ﬁnal choice, please provide aschematic diagram of the beamsplitter/delay line for 2-bunch formation
Photocathode QE presently 10-4. Help thecommittee appreciate your eﬀort toincrease QE to 10-3. e.g., give us a sense
for how important this eﬀort is. The top
priority or the 20th priority

Other questions/Issues§State speciﬁcally how the gun/laser groupwill prepare for Phase2. Priority list thejobs that must be done….