Creation of a Novel X ray Generator

1
Creation of a Novel X ray Generator

• James Hostetter
• Louisiana State University and
• SLAC National Accelerator Laboratory

2
Why X rays?
3
Why short pulses?
4
Capabilities and Advantages

• 10s of KeV X rays
• 1 picosecond pulses
• 100 MHz pulses
• 108 photons/s at 8 KeV
• Compact
• Relatively cheap to build
• Complementary to LCLS
• Always available to PULSE researchers

5
Current, Future and Past X ray Sources

• Shorter pulses means studying faster processes
• More photons means bigger signal
• May be unnecessary
• May be harmful
• LCLS is by far the brightest and fastest pulse
  source, but runs at very low repetition rate.

6
X-ray Production

• Cathode creates e-
• Potential difference accelerates electrons
  towards anode
• Collision with anode causes emission of X-rays
• E q V

7
Better X-ray Production

• Laser allows for pulsed x rays
• RF cavity compresses electron bunches
• Shorter pulses
• Brighter pulses

8
Photocathode basics

• GaAs
• Must be cesiated
• Must be exposed to NF3 or O2
• Must have an ultra-high vacuum (UHV)
• Photocurrent decays
• 10 Quantum Efficiency (QE)
• QEe-/photon
• QE Ih?

Gate Valve
Loadlock
Laser
Cesiator
Manipulator
Heater
Ion pump
Pe
9
Photocurrent Decay

• I I0e-t/t I0 (1-t/t)
• Half-life2.36 days

10
Photocurrent vs. Power
11
Conclusion

• Significant Progress in the design of the x-ray
  generator.
• Understanding the photocathode is proceeding well
• Decay on the order of days
• Linear dependence of QE on power

Future Work

• Scaling to short laser pulse and higher average
  current (1mA)
• Finish Electron gun design and sample hand-off

12
Acknowledgements

• David Reis
• Shambhu Ghimire
• Takashi Maruyama
• SULI program