Status of the Photocathode R&Dfor ATF RFgun

N.Terunuma (KEK)

Why we use Cs2Te photocathode?

Pure metal cathode.Q.E. 10-5 ~ 10-4

Cs2Te Q.E. ~ 10-1 , a few %

OthersNeed more study.

We prepared the Cs2Te formation system and the Cathode loadlock system for ATF.

For ATF, Shorter preparation time for the beam operation.--> Preparation system connected to the RFGUN.

RFGUNCavity

Cathode loading mover

RFGUN

Laser

Cathode Load-lock System

Cleaning&Pickup Storage

Cathodeformation

Xe light

Quantum EfficiencyQ.E.= Number of electrons / Number of photons

Offline measurementXe lamp with monocrometer~266nm.Measure the current on cathode.

Online measurementRFGUN operation, Laser~266nm.Extracted beam intensity detected by faraday cup.

Cathode and Cs-Te sources

Te

Cs

Evaporation SystemCathode Plug (Cu)

15mm

Half cell endplate with Cu plug

Argon ion bombardment

WhenNew cathode is installed.Clean the used cathode.Sputter depth ~ 50nm .

Step 1: Cleaning

Ion Source

Cleaningchamber

Formation: Step 1, Cleaning

Te is heated by a Tungsten filament.We can control the Te thickness.

Step 2: Evaporation of Te

Formation: Step 2, Te evaporation

Cs is evaporated on the Te film and its thickness is controlled by monitoring the Q.E. with Xe system.

Step 3: Evaporation of Cs

Formation: Step 3, Cs evaporation

Cathode holder with Mask 8mm

R&DFor the ATF operation…

Prepare the new cathode on Monday.Beam Test: a week or more.Check Q.E. by calibrated lights (Xe,266nm) every a few days.

For the basic study… in shutdown weeks.

This year, we had two months beam operation.Two Cu plugs Two Mo plugs

QE of Cu#1 Cathode

ATF

1.3x10-6 Pa at RFGUN

Q.E.0.4%

Yellow area: RF on

Cu cathode after the Beam TestTwo cathode plugs were used for a month operation.

Cu#1 cathode (left)Used for the cavity aging.Many RF breakdowns.

Cu#2 cathode (right)Used stable laser operation.

Mo Cathode

Two plugs ware used for a month operation.Their surface were polished as a mirror.

Small damage was found after the beam operation.

Q.E. of Cs2Te on Mo plugs

• Q.E. before RF power is supplied reaches > 15%.

• Q.E. dropped quickly when RF power was supplied.

• It shows stable Q.E. after RF power supplied.

• Cs2Te on Molybdenum plugs shows higher Q.E. ~3% than that on Copper plugs ~0.4%.

Dark vs.

Dark Current

Power (MW)

SummaryWe had two months beam operation.

Cs2Te on Cu plugsQ.E. ~0.4%.Damages on the Cu surface.

Cs2Te on Mo plugsQ.E. ~3%. Stable.Small damages compared to Cu case were observed.

Next Establish the formation method and its reproducibility.Check Q.E. under longer beam operations.