Abstract
Detailed experimental studies of the first operation of an $X$-band (8.547 GHz) rf photoinjector are reported. The rf characteristics of the device are first described, as well as the tuning technique used to ensure operation of the $1\frac{1}{2}$-cell rf gun in the balanced $\ensuremath{\pi}$-mode. The characterization of the photoelectron beam produced by the rf gun includes: measurements of the bunch charge as a function of the laser injection phase, yielding information about the quantum efficiency of the Cu photocathode ( $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ for a surface field of $100\mathrm{MV}/\mathrm{m}$); measurements of the beam energy (1.5--2 MeV) and relative energy spread ( $\ensuremath{\Delta}\ensuremath{\gamma}/{\ensuremath{\gamma}}_{0}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.8\ifmmode\pm\else\textpm\fi{}0.2%$) using a magnetic spectrometer; measurements of the beam 90% normalized emittance, which is found to be ${\ensuremath{\varepsilon}}_{n}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.65\ensuremath{\pi}\mathrm{mm}\mathrm{mrad}$ for a charge of 25 pC; and measurements of the bunch duration ( $<2\mathrm{ps}$). Coherent synchrotron radiation experiments at Ku-band and Ka-band confirm the extremely short duration of the photoelectron bunch and a peak power scaling quadratically with the bunch charge.
Highlights
Compact, high-gradient (.100 MeVm) rf photoinjectors capable of producing relativistic (.5 MeV) electron beams with extremely low emittance (,1p mm mrad), high charge (1 nC), and subpicosecond duration are currently being developed [1,2,3,4,5] for a variety of applications ranging from the Linear Collider (NLC) to compact, prebunched free-electron masers [6]
This signal is sent to a 1 kW traveling wave tube amplifier (TWTA), followed by a Stanford Linear Accelerator Center (SLAC)
As mentioned in the Introduction, one method that can be used to lock the phase of the rf to the pulse train of the laser is to run a master clock and use feedback to adjust the oscillator to keep the output in phase with the clock. Another possibility, which does not require any feedback and is considerably easier to implement, is used in this experiment: the 79.138 MHz pulse train from the oscillator is sampled with a photodiode, this signal is fed into the phase-locked dielectric resonance oscillator (PDRO), which raises the frequency by a factor of 108 yielding a 8.547 GHz signal that is precisely locked in phase with the laser pulses
Summary
High-gradient (.100 MeVm) rf photoinjectors capable of producing relativistic (.5 MeV) electron beams with extremely low emittance (,1p mm mrad), high charge (1 nC), and subpicosecond duration are currently being developed [1,2,3,4,5] for a variety of applications ranging from the Linear Collider (NLC) to compact, prebunched free-electron masers [6]. At 1 Å (12 keV), protein crystallography is revolutionizing the pharmaceutical industry by allowing the design of new drugs, as exemplified by the protease inhibitors used to decrease the viral load in HIV patients [8] This type of radiation can be produced by Compton scattering [9] using a high-quality electron beam and a tabletop terawatt laser for an advanced x-ray source. A detailed theoretical and computational study of the scaling of rf injector beam performance with frequency has been performed [11], with the conclusion that X-band represents an optimum balance between beam quality and accelerated charge and should reach very high brightness These higher frequency rf photoinjectors offer the possibility of operating at larger peak accelerating fields, alleviating the detrimental effects of space charge, which scale as g21, where the identity g 2 1 eym0c2 defines the relativistic mass factor g of the electrons at the potential y.
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