RF	  gun	  review	  @	  KEK,	  Nov	  	  19	  and	  20,	  2015	  2nd	  RF-­‐Gun	  Workshop	  	  Commi4ee	  Feedback	  Yosuke	  Honda	  (KEK)	  Katsunobu	  Oide	  (KEK)	  Ma4hew	  Poelker	  (JLab)	  Tsumoru	  Shintake	  (Chair,	  OIST)	  	  not	  present	  Sasha	  Gilevich	  (SLAC)	  	  RF-­‐Gun	  Cavity&Cathode	  Development	  • Comi4ee	  congratulate	  successful	  operaXon	  of	  thermionic	  gun	  and	  A-­‐1	  RF	  gun.	  
• With	  new	  cathode	  plug	  design	  funcXoned	  well,	  and	  smooth	  high	  power	  condiXoning	  was	  observed.	  	  Comment	  on	  Cathode	  R&D	  • Ir5Ce	  cathode	  "should	  have"	  higher	  potenXal,	  	  while	  obeserved	  QE	  =	  4.4x10-­‐5	  was	  close	  to	  bare	  copper.	  	  • It	  may	  help	  to	  reconsider	  the	  laser	  cleaning	  as	  the	  	  "cathode	  acXvaXon".	  	  • Before	  starXng	  R&D	  on	  Cs2Te	  cathode,	  team	  shold	  put	  some	  eﬀort	  to	  invesXgate	  "laser	  acXvaXon"	  on	  Ir5Ce	  cathode.	  • CongratulaXons	  on	  much	  progress	  since	  last	  review,	  only	  8	  months	  ago	  – Much	  improved	  master	  oscillators,	  commercial	  and	  in-­‐house	  manufactured	  – Greatly	  reduced	  ASE	  using	  tailored	  preamps	  – Semiconductor	  opXcal	  ampliﬁer	  excellent	  pulse-­‐picker	  and	  preamp	  for	  10	  MHz	  pulse	  train	  • Impressive	  level	  of	  creaXvity	  and	  enthusiasm	  for	  work	  • Virtual	  photocathode	  is	  only	  truly	  useful	  if	  located	  near	  the	  gun	  • Work	  toward	  construcXng	  a	  vacuum	  or	  nitrogen-­‐ﬁlled	  opXcal	  transport	  line	  and	  employ	  an	  imaging	  system	  that	  reduces	  sensiXvity	  to	  poinXng	  instability	  • InvesXgate	  changing	  the	  size	  of	  the	  laser	  beam	  at	  photocathode	  serve	  as	  a	  useful	  adjustment	  to	  reduce	  beam	  loss	  in	  ﬁrst	  accelerator	  secXon.	  
• ConXnue	  to	  install	  more	  diagnosXcs	  at	  each	  laser	  subsystem	  to	  isolate	  the	  origin	  of	  instabiliXes	  • Build	  diagnosXcs	  that	  help	  diﬀerenXate	  between	  three	  instabiliXes:	  amplitude,	  Xming	  and	  poinXng,	  e.g.,	  a	  Xming	  instability	  at	  stage	  X	  can	  cause	  amplitude	  instability	  at	  stage	  Y	  • Because	  the	  pathlength	  of	  some	  subsystems	  is	  so	  long,	  extra	  sensiXve	  to	  mechanical	  vibraXons.	  	  Correlate	  instabiliXes	  to	  mechanical	  vibraXon:	  install	  piezo	  geophone	  in	  laser	  room	  to	  idenXfy	  vibraXon	  frequencies.	  Eliminate	  or	  isolate	  vibraXons.	  
• Ask	  management	  to	  approve	  the	  purchase	  Menlo	  oscillator	  at	  1064	  nm.	  	  It’s	  only	  money.	  
• Are	  there	  be4er	  ways	  to	  drive	  pockels	  cell	  in	  regeneraXve	  ampliﬁer,	  with	  less	  “ringing”?	  
• You	  will	  learn	  a	  great	  deal	  if	  you	  are	  allowed	  to	  commission	  rf	  gun	  during	  Phase	  1	  • Laser	  development	  eﬀort	  appears	  more	  focused	  and	  coordinated	  now,	  compared	  to	  last	  review,	  this	  is	  good	  • You	  conXnue	  to	  reduce	  the	  number	  of	  technical	  challenges	  but	  some	  concern	  that	  the	  list	  is	  sXll	  too	  large.	  	  Can	  you	  “down	  select”	  and	  reduce	  the	  number	  of	  projects	  you	  are	  pursuing?	  	  For	  example,	  can	  you	  decide	  now,	  Nd:YAG	  versus	  Yb:YAG?	  	  With	  just	  months	  to	  go,	  can	  you	  work	  on	  both	  to	  prepare	  for	  Phase1	  operaXon?	  

Stability of the electron beam•The stability of the electron beam is worse
than the laser power ﬂuctuation:•Identify the reason by observing pointing
stability, etc., and taking the correlation withthe beam data.
•If it is caused by the pointing stability, pleaselook at the ampliﬁcation eﬀect due to thetransverse impedance of the RF cavity. In
that case, future upgrade of the RF gun
cavity should take this into account.

Beam Transport to Linac• Currently a beam loss by about 30% has been
observed at the accelerating structure next to theRF gun.
• Understand the reason of the beam loss:• It can be a simple optical mismatch: build up anoptics model after the RF gun. A standard optics
calculation as in the thermionic beam line will be
useful at least after the gun.
• Improve the optics matching after the gun.
Choose the polarity of the quadrupole doublets
properly.
• It can be caused by the ﬂuctuation of the beam:
look at pulse-to-pulse correlation of the BPMs andlaser power, timing, etc.

The further development of RF gun
cavity•The idea of CDS Rf gun cavity seems
excellent.
•The size of the beam hole can be bigger.
Optimize it by considering the transverse
impedance and the rf focusing, etc. Even an
APS-like structure with a larger hole maybeneﬁt in this sense. Note that a smaller beam
size at the output is not always better,
considering the matching to the next section
to the linac.

Stability of the laser• The progress after the last review on the stability of
the laser is remarkable: now there is no uncontrolled
drop of the laser power for days.
• However, the stability of the intensity, is notsatisfactory yet as a usable beam in the linac. It isreported that the beam intensity ﬂuctuates
0.7+/-0.3 nC. Please identify the source of theﬂuctuation: Looking at the time structure with higher
sampling rate may reveal the source.
• The ﬂuctuation of the piezo feedback for phasestabilization indicates some sources in 10 - 30 Hz
frequency range (although an aliasing cannot beexcluded).

Comment on laser•Environment issues, such as covering andtemperature stabilization, should be considered. It
will be needed anyway later. No reason to wait these
items after completion of all the system.
Oscillator timing stability. Not only the feedback
voltage to piezo, a direct timing stability data should
be taken. A fast piezo, which means small one, may
be essential to have a good stability. It is known thestability of LD driver power aﬀects phase noise ofﬁber oscillator, there is a way to apply feedback
control to the LD to cure phase noise. Putting an EO
phase modulator in the oscillator may be a solution
to have a high bandwidth feedback.

••Origin of shot by shot ﬂuctuation should besurveyed. Measurement in each stage andcorrelation analysis should be done.
Need to show experimental data of position stability,
proﬁle stability, ...
Laser transport line will be very important for beam
operation. It should not be compromised. Try toreduce number of mirrors, girder stability should beimportant.
Virtual cathode should be placed at the mostdownstream after transport. Spatial shaping should
be considered also at the place.
Arrangement of delay line for producing 2-bunch
was not shown in detail.

••••Thank you
for excellent tour andpresentations.#