You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
the aps rf thermionic gun serves both as an injector for the aps @xcite storage ring as well as a high - brightness source for sase fel experiments as part of the aps leutl project .
tuning of the gun as a high - brightness source was accomplished using ctr from the rf gun beam accelerated to 40 mev .
the gun , linac , and ctr setup are shown in figure  [ fig1 ] .
the beam emerges from the 1.6 cell @xmath0 mode rf gun and proceeds to the alpha magnet via a beamline containing focusing , steering , a kicker , and an entrance slit .
the alpha - magnet vacuum chamber contains a scraper that is used to remove the low energy / high emittance tail from the beam .
after the alpha - magnet , the beam traverses some focussing and correction elements , then proceeds through a 3-m slac s - band accelerating waveguide to the ctr foil .
the ctr foil is mounted on an actuator along with a yag crystal , which is used to focus the beam to a small beam spot at the foil position .
the ctr is collected by a lens and sent to a michelson interferometer with a golay detector mounted on one arm .
the autocorrelation of the ctr is performed by moving one arm of the interferometer while recording the golay detector output .
the golay detector output can be maximized at the peak of the autocorrelation scan and used to adjust rf gun power and phase , beam current , and alpha - magnet current so as to minimize the bunch length out of the alpha magnet .
once this is done , the ctr signal is a good relative measure of the bunch length .
to prepare the rf gun to produce a high - brightness beam one must first scan the alpha - magnet current to find the minimum bunch length .
typically the rf gun is powered anywhere from 1.5 to 1.7 mw , and the heater current is adjusted to produce 1 to 2 nc in a train of 23 s - band bunches . the gun power and beam current
are kept constant during the scans .
prior to the scan , the beam is focused on the yag using quads before and after the alpha magnet , with the alpha magnet `` close '' to the setting required for minimum bunch length . during the scan
, the rf gun phase must be adjusted linearly to compensate for path length changes in the alpha magnet . to maximize scan resolution ,
the interferometer is set to maximize the golay detector signal .
figure  [ fig3 ] shows a typical alpha - magnet scan showing a peak at 175 amperes .
the curve represents the output of the golay detector from a gated integrator amplifier .
once the minimum bunch length has been found , an alpha - magnet scraper scan is performed .
simulations show a microbunch profile that has a low - emittance , high - energy core beam and a high - emittance , low - energy tail . the scraper scan is performed to optimize removal of the low - energy tail .
figure  [ fig4 ] shows a typical scraper scan where the ctr signal is plotted vs scraper position .
the edge of the core beam is at approximately 9.5 cm .
figure  [ fig5 ] shows a plot of ctr signal vs beam current , as measured by a beam position monitor ( bpm ) adjacent to the ctr foil , taken during the scraper scan .
included with the data is a quadratic fit , showing the expected quadratic dependence of the coherent radiation on the number of particles .
once the scraper position is determined , the interferometer is used to measure the autocorrelation of the digitized gated integrator ctr signal .
figure  [ fig6 ] shows the autocorrelation measured for a beam of 1 nc in 23 s - band micropulses .
autocorrelation processing begins with taking the fast fourier transform ( fft ) of the autocorrelation , which gives the square of the bunch spectrum .
the method of lai and sievers is then used to reconstruct the phase spectrum from the amplitude spectrum by computing a principal value integral .
once the phase spectrum is obtained , an inverse fft is performed to derive the microbunch profile .
additional processing is performed to correct for the reduced response of the golay detector at low frequencies ( long wavelengths ) .
since any bunch spectrum approaches low frequencies quadratically , a quadratic fit is performed for frequencies from the golay detector 3-db point to a user - selectable higher frequency , typically including 3 to 5 frequency points  @xcite .
the fit is then used to extrapolate quadratically to dc from the golay detector 3-db point .
figure  [ fig7 ] shows the amplitude spectrum derived from the measured autocorrelation and the corrected spectrum for low frequencies .
the main effects of this low - frequency correction is to broaden the derived bunch profile and flatten the dips in the autocorrelation adjacent to the peak .
these dips are unphysical since the autocorrelation is always positive .
figure  [ fig8 ] shows the derived bunch profile from the the corrected autocorrelation spectrum .
the overall profile contains a high - current peak ( @xmath1 100 amperes ) , a lower current shoulder , and is overall about 400 fs wide .
this beam was used for sase measurements .
the beam prepared as described above was accelerated to 217 mev .
the emittance was measured in the transport line using the standard three - screen technique , giving a normalized emittance of approximately 10 @xmath0 mm .
the energy spread is estimated to be  0.1% .
the beam was transported to the undulator hall and passed through five aps undulators with diagnostics stations between them .
figure  [ fig9 ] shows the measured photon intensity ( corrected for spontaneous background ) at each undulator diagnostic station .
the solid line is an exponential fit to the data showing a gain length of 1.3 m for both undulator radiation and coherent transition radiation data  @xcite , in agreement with a calculation using the previously listed peak current , emittance , and energy spread .
the rf thermionic gun beam was quite stable once tuning was completed .
one limitation of the beam is that the microbunch length is on the order of the electron slippage length . the final saturated power is therefore expected to be lower for this beam .
the authors thank j. lewellen , s. milton , and j. galayda for useful comments and suggestions .
m. borland , `` an improved thermionic microwave gun and emittance - preserving transport line , '' proc .
1993 pac , may 17 - 20 , 1993 , new york , 3015 - 3017 .
j. lewellen et al . , `` operation of the aps rf gun , '' proceedings of the 1998 linac conference , anl-98/28 , 863 - 865 ( 1999 ) .
r. lai and j. sievers , `` determination of bunch asymmetry from coherent radiation in the frequency domain , '' aip vol .
367 , 312 - 326 ( 1996 ) .
milton et al . , `` observation of self - amplified spontaneous emission and exponential growth at 530 nm , '' ( submitted to phys .
a. h. lumpkin et al .
, `` first observation of z - dependent electron beam microbunching using coherent transition radiation , '' ( submitted to physical review ) .
we describe use of the advanced photon source ( aps ) rf thermionic gun  @xcite , alpha - magnet beamline , and linac  @xcite to produce a stable high - brightness beam in excess of 100 amperes peak current with normalized emittance of 10 @xmath0 mm - mrad . to obtain peak currents greater than 100 amperes 
 , the rf gun system must be tuned to produce a fwhm bunch length on the order of 350 fs . 
 bunch lengths this short are measured using coherent transition radiation ( ctr ) produced when the rf gun beam , accelerated to 40 mev , strikes a metal foil . 
 the ctr is detected using a golay detector attached to one arm of a michelson interferometer . 
 the alpha - magnet current and gun rf phase are adjusted so as to maximize the ctr signal at the golay detector , which corresponds to the minimum bunch length . 
 the interferometer is used to measure the autocorrelation of the ctr . 
 the minimum phase approximation  @xcite is used to derive the bunch profile from the autocorrelation . 
 the high - brightness beam is accelerated to 217 mev and used to produce self - amplified spontaneous emission ( sase ) in five aps undulators installed in the low- energy undulator test line ( leutl ) experiment hall  @xcite . 
 initial optical measurements showed a gain length of 1.3 m at 530 nm .