Status of the ATF Extraction Line Laser-wire Page: 2 of 7
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2 Laser system
A mode-locked seed laser (Nd:VAN) produced by Time-Bandwidth (GE-100-XHP series) is
used to deliver a 600mW train of light pulses with a wavelength of 1064 nm and a repetition
rate of 357 MHz. The Nd:VAN crystal is diode pumped and a semiconductor saturable absorber
(SESAM)[1, 2] is used to start and stabilise the pulse forming process. The SESAM mirror
forms one end of the laser cavity. It is mounted on a translation stage so that the length of the
cavity can be adjusted by a picomotor and a piezo-electric crystal to cancel the effects caused
by changes in ambient conditions. A photo-diode is used to monitor the output of the cavity.
A timing stabiliser (Time Bandwidth CLX-1100) [3, 4] is used to phase-lock the signal from the
photo-diode to the ATF 357 MHz RF signal (This is achieved by adjusting the cavity length).
The pulse train produced by the seed laser is injected in a regenerative amplifier custom-
built by Positive Light (model RGN). This system amplifies one of the nanoJoule seed pulses to
approximately 600 milliJoules. A Faraday isolator is used to prevent reflected pulses damaging
the seed laser. Two Pockels cells are used to select the pulse that will be amplified. The first
Pockels cell is located before the Faraday isolator, it is used to chop the end of the pulse. The
second Pockels cell is located inside the amplifying cavity. When this second Pockels cell is
deactivated the pulses do not reach the amplifying rod in the cavity. When this Pockels cell is
switched on, it will trap one pulse inside the cavity. This pulse is reflected through an Nd:YAG
amplifying rod and bounces back and forth in the cavity. After a number of pulse round trips
in the cavity (usually 10-15) a third Pockels cell is switched on, this changes the polarisation of
the pulse inside the cavity and this pulse is extracted by a polariser making a 45 degrees angle
with the beam trajectory.
After extraction the pulse passes a second Faraday isolator that protects the cavity. It is
then transmitted through a spatial filter and then through 2 Nd:YAG linear amplifier that
amplify the pulse by a factor of 10 each, bringing it to more than 600 mJ. Before the exit of the
laser system a KD*P crystal doubles the frequency of the laser pulse, bringing its wavelength
to 532 nm with an efficiency close to 50% at full power. At this stage the pulses have a length
of 200-300 ps.
The timing of the Pockels cells is critical to ensure that one and only one pulse is amplified
and to improve the contrast ratio of the pulse by removing any pre and post pulses. This
timing is controlled by a signal and delay generator (SDG-II from Positive Light). A photo-
diode monitors the signal in the amplifying cavity. A second photo-diode, located after the
spatial filter, monitors the signal extracted from the cavity. These two photo-diodes are used
to tune the timing of the 3 Pockels cells and thus improve the extracted signal.
The amplifying rod located inside the cavity and the two linear amplifiers are optically
pumped by flash lamps. The signal used to trigger the flash is taken from the ATF extraction
kicker charge signal. The kicker fire signal is used to trigger the SDG and thus the amplified
laser pulse.
3 Laser delivery optics and focusing lens
The laser system described above is located on top of the ATF shielding. A set of mirrors are
used to bring the laser light from the output of the amplifier to the focusing light. The total
length travelled by the light is of the order of 10 meters. A scanning system and diagnostic
tools such as photo-diodes and camera will be installed along this path.2
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Delerue, Nicolas; Gannaway, Fred; Howell, David; Blair, Grahame; Boorman, Gary; Driouichi, Chafik et al. Status of the ATF Extraction Line Laser-wire, article, February 22, 2006; Menlo Park, California. (https://digital.library.unt.edu/ark:/67531/metadc877822/m1/2/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.