The next generation of e+/e- colliders will require a very intense flux of
gamma rays to allow high current polarized positrons to be produced. This can
be achieved by converting polarized high energy photons in polarized pairs into
a target. In that context, an optical system consisting of a laser and a
four-mirror passive Fabry-Perot cavity has recently been installed at the
Accelerator Test Facility (ATF) at KEK to produce a high flux of polarized
gamma rays by inverse Compton scattering. In this contribution, we describe the
experimental system and present preliminary results. An ultra-stable
four-mirror non planar geometry has been implemented to ensure the polarization
of the gamma rays produced. A fiber amplifier is used to inject about 10W in
the high finesse cavity with a gain of 1000. A digital feedback system is used
to keep the cavity at the length required for the optimal power enhancement.
Preliminary measurements show that a flux of about $4\times10^6 \gamma$/s with
an average energy of about 24 MeV was generated. Several upgrades currently in
progress are also described

Akagi, T.

Araki, S.

Bonis, J.

Chaikovska, Iryna

Chiche, R.

Cizeron, R.

Cohen, M.

Colin, J.

Cormier, E.

Cornebise, P.

Delerue, Nicolas

Flaminio, R.

Funahashi, S.

Honda, Y.

Jehanno, D.

Labaye, F.

Lacroix, M.

Marie, R.

Miyoshi, S.

Nagata, S.

Omori, T.

Peinaud, Y.

Pinard, L.

Shimizu, H.

Soskov, V.

Takahashi, T.

Terunuma, T.

Urakawa, J.

Variola, A.

Zomer, F.

English

arXiv

http://accelconf.web.cern.ch/AccelConf/IPAC2011/papers/tupo002.pdfInternational audienceThe next generation of e+/e- colliders will require a very intense flux of gamma rays to allow high current polarized positrons to be produced. This can be achieved by converting polarized high energy photons in polarized pairs into a target. In that context, an optical system consisting of a laser and a four-mirror passive Fabry-Perot cavity has recently been installed at the Accelerator Test Facility (ATF) at KEK to produce a high flux of polarized gamma rays by inverse Compton scattering. In this contribution, we describe the experimental system and present preliminary results. An ultra-stable four-mirror non planar geometry has been implemented to ensure the polarization of the gamma rays produced. A fiber amplifier is used to inject about 10W in the high finesse cavity with a gain of 1000. A digital feedback system is used to keep the cavity at the length required for the optimal power enhancement. Preliminary measurements show that a flux of about $4\times10^6 \gamma$/s with an average energy of about 24 MeV was generated. Several upgrades currently in progress are also described

Delerue, N.

Chaikovska, I.

HAL-CEA

HIGH FLUX POLARIZED GAMMA RAYS PRODUCTION: FIRSTMEASUREMENTS WITH A FOUR-MIRROR CAVITY AT THE ATFN. Delerue∗, J. Bonis, I. Chaikovska, R. Chiche, R. Cizeron, M. Cohen, J. Colin, P. Cornebise,D. Jehanno, F. Labaye, M. Lacroix, R. Marie, Y. Peinaud, V. Soskov, A. Variola, F. Zomer,LAL, Universite´ Paris-Sud XI, Orsay, FranceE. Cormier, CELIA, Universite´ de Bordeaux, Bordeaux, FranceR. Flaminio, L. Pinard, LMA, Lyon, FranceS. Araki, S. Funahashi, Y. Honda, T. Omori, H. Shimizu, T. Terunuma, J. Urakawa,KEK, Tsukuba, JapanT. Akagi, S. Miyoshi, S. Nagata, T. Takahashi, Hiroshima University, Hiroshima, JapanAbstractThe next generation of e+/e- colliders will require a veryintense ﬂux of gamma rays to allow high current polarizedpositrons to be produced. This can be achieved by convert-ing polarized high energy photons in polarized pairs intoa target. In that context, an optical system consisting of alaser and a four-mirror passive Fabry-Perrot cavity has re-cently been installed at the Accelerator Test Facility (ATF)at KEK to produce a high ﬂux of polarized gamma rays byinverse Compton scattering. In this contribution, we de-scribe the experimental system and present preliminary re-sults. An ultra-stable four-mirror non planar geometry hasbeen implemented to ensure the polarization of the gammarays produced. A ﬁber ampliﬁer is used to inject about10W in the high ﬁnesse cavity with a gain of 1000. A digi-tal feedback system is used to keep the cavity at the lengthrequired for the optimal power enhancement. Preliminarymeasurements show that a ﬂux of about 4×106 γ/s with anaverage energy of about 24 MeV was generated. Severalupgrades currently in progress are also described.INTRODUCTIONFuture electron-positrons colliders will require a ﬂux ofpositrons much higher than what can be produced with cur-rent methods. One of the scheme that has been proposedto achieve such ﬂux is to use an intense ﬂux of polarizedhigh energy gamma rays [1] which can be converted intopolarized electron-positron pairs. Although promising thismethod requires the demonstration that such high intensityﬂux of gamma rays can be produced. Polarized positronscan be produced by Compton interaction between an elec-tron beam and a laser. However given the low cross-sectionof this process, it is desirable to recycle both the photonsand the electrons by, for instance, using an electron ringand a Fabry-Pe´rot cavity (FPC). We decided to investigatesuch a scheme by installing a four-mirror FPC on the Ac-celerator Test Facility (ATF) [2] damping ring at KEK inJapan.∗delerue@lal.in2p3.frEXPERIMENTAL SETUPLaser SystemThe laser seed includes a modiﬁed commercial oscillatoremmiting at 1031 nm with a repetition rate of 178.5 MHz.We use a standard chirp pulse ampliﬁcation (CPA) archi-tecture in order to limit the non-linearities in the ampliﬁer.The ampliﬁer is built around a commercial microstructureYb-doped double clad ﬁber. The system delivers recom-pressed pulses of around 60 ps and up to 50 W averagepower. The complete optical layout is shown on Figure 1. Laser Oscillator EOM OI Stretcher Unit Optical Pumping Compressor Unit   " # 	 !!!Fabry-Pérot Cavity     Transmited Beam Reflected Beam Figure 1: Optical layout of the experiment.Fabry-Pe´rot Cavity (FPC)The FPC is built using the experience gained at LAL inprevious experiments [3, 4]. The FPC is formed by 2 con-cave mirrors with a radius of curvature of 0.5m and 2 ﬂatmirrors arranged in a non-planar tetrahedron geometry (seeFigure 2). The two concave mirrors form a 52μm× 76μmwaist at which the laser beam crosses the electron beamwith a collision angle of 8 degrees. The mirrors have avery high reﬂectivity (1 - 1060 ppm for one of them andTUPO002 Proceedings of IPAC2011, San Sebastián, Spain1446Copyrightc ○2011byIPAC’11/EPS-AG—ccCreativeCommonsAttribution3.0(CCBY3.0)02 Synchrotron Light Sources and FELsA14 Advanced Concepts1 - 330 ppm for the others) leading to a cavity ﬁnesse of theorder of 3000 (that is a power enhancement of about 1000).The duration of a round trip in the FPC is 5.6 ns. Great carehad to be taken while designing the cavity to ensure that itis compatible with the ultra-high vacuum requirements ofthe ATF (3× 10−8 mbar).Figure 2: Layout of the 4-mirror cavity. The electron beamis represented by the blue arrow and the blue line enteringto the right of the cavity. The laser beam is representedby the red arrow and the red line entering to the left of thecavity. The blue cylinders near the mirrors contain the 12actuators used to remotely adjust the X, Y and Z positionsof the mirrors.All the mirrors are mounted on actuators carts or tablesthat allow a ﬁne adjustment of the cavity length. Further-more one of the mirrors is mounted on a piezoelectric trans-ducer (PZT) that allows to synchronize the length of theFPC with the repetition frequency of the ATF by using adigital phase lock loop as shown on Figure 3.	 	   ADC ($*#  ($*#  - -   %& 	!  		 (+,$*# (+,$*# 	 ADC FIR " "FIR ()*  Figure 3: Digital phase lock loop used to synchronize theFPC with the ATF clock.In order to constructively stack the pulses in the FPC,it is mandatory to control the pulse to pulse phasingwith an interferometric accuracy. This is done using thePound-Drever-Hall method [5] implemented in a VIRTEX-II FPGA board as shown on Figure 4. The error signal gen-erated by this method is used to adjust the length of thelaser oscillator.Using this double digital feedback system we have beenable to keep the FPC locked on the ATF and the laser oscil-lator locked on the FPC for several hours.ATFA detailed description of the ATF at KEK can be foundin the literature [2]. At the ATF the 1.28 GeV electron 	         SOS1 SOS2 DDS   FIR SOS3 SOS4 MUX ADC DAC Φ 	 ADC  	  DAC   Figure 4: Digital feedback to adjust the laser of the lasercavity depending on the PDH signal.bunches are separated by 2.8 ns. Although various ﬁll pat-terns can be achieved in the ATF damping ring (DR), thedata presented here were taken in single bunch single trainmode in which the same electron bunch returns to the sameposition after 165 RF buckets (one complete revolution ofthe ATF DR equals to 462 ns). Given that a round trip inthe FPC is 5.6 ns this means that electrons collide with thelaser pulse stored in the FPC every other turn.CalorimeterDuring the collisions the gamma rays are detected usinga fast scintillation detector made of barium ﬂuoride (BaF2)coupled with a Photomultiplier Tube (PMT). To eliminatethe slow component of scintillation an optical ﬁlter is in-stalled in front of the PMT. The data acquisition is per-formed using a LeCroy WS454 oscilloscope (1GS/s, 500MHz bandwidth). Geant4 simulations have been used tostudy this calorimeter.DATA TAKING AND DATA ANALYSISThe FPC was commissioned in October 2010 and Comp-ton collisions were recorded on the ﬁrst attempt. Wepresent here a brief analysis of some of the data taken.During data taking we record the waveforms from thePMT as well as the 357 MHz ATF clock and laser powertransmitted by FPC measured by a photodiode. A fullwaveform contains approximately 200 000 samples spacedby 1 ns. On the day where the data presented here weretaken the average power stored in the cavity was 160 W.The 357 MHz clock is used to deﬁne the beginning of theperiods in time during which the collisions occurred. So,the length of the period is naturally 924 ns. For each periodwe deﬁne the gate which contains the Compton peak to cal-culate consequently its height and integral. Different qual-ity cuts such as a cut on the shape of the electronic pulse,a cut on the arrival time of the signal etc. were applied toProceedings of IPAC2011, San Sebastián, Spain TUPO00202 Synchrotron Light Sources and FELsA14 Advanced Concepts 1447 Copyrightc ○2011byIPAC’11/EPS-AG—ccCreativeCommonsAttribution3.0(CCBY3.0)restrict the analysis to a high purity sample.The shape of the detector’s response creates a linear re-lation between the total charge and the maximum chargemeasured. This can be seen as a correlation between thecalculated peak height and peak integral as shown on Fig-ure 5.−0.04 −0.03 −0.02 −0.01 0−20−100x 10−11Peak integral [V s] Peak height [V] Figure 5: Peak height vs. peak integral for the high puritydata sample.RESULTSThe spectrum of the gamma rays is shown on Figure 6.It corresponds to the distribution of the energy deposited in−25 −20 −15 −10 −5 0020040060080010001200Peak height normalized by laser power [a.u.]dN dI10.36Figure 6: Gamma ray spectrum.the calorimeter expressed by the peak heights normalizedby the laser power.Since we took data over a wide range of power storedin the FPC, we can study the gamma ray production fordifferent ranges of that power. On Figure 7, the averageof the peak height distribution goes towards higher energywith increasing FPC power as expected.Using cosmic rays for calibration, we can estimate theaverage number of scattered gammas per bunch crossing.According to this calibration, a peak height of 1 mV isequivalent to 34 MeV of energy deposited in the calorime-ter. Assuming the mean energy of scattered gammas to be24 MeV, approximately 4 gammas are produced in averageper bunch crossing (for an average laser power stored in thecavity of 160 W). As the repetition frequency of the colli-sions is about 1 MHz the ﬂux of gamma rays achieved is−12 −10 −8 −6 −4 −2 0050100150200250300350Peak height [mV] dN dI10.34mV  22% of max LP, −1.38 mV25% of max LP, −1.47 mV31% of max LP, −1.81 mV42% of max LP, −2.12 mV49% of max LP, −2.67 mV54% of max LP, −2.86 mV59% of max LP, −3.19 mV62% of max LP, −3.2 mV100% of max LP, −3.21 mVFigure 7: Gamma spectrum for different FPC stored power.Different colors correspond to the spectra taken at differentFPC powers. In the legend, the fraction of the maximumFPC stored power and the mean of each of these spectraare shown.about 4× 106 γ/s (this does not take into account the 0.75duty cycle of the ATF).FUTURE PLANSOur experiment has been suspended due to the earth-quake that struck Japan in March 2011 however we planto resume soon. To increase the Compton ﬂux we intend toreplace the mirrors of the cavity by mirrors with a higherreﬂectivity, giving a ﬁnesse of about 30 000. We also planto improve the laser system and to replace our gamma raysdata acquisition chain with a faster one.Numerical simulations have shown that with the ﬂuxachieved so far we do not expect signiﬁcant effects on thedynamics of the stored beam [6]. However, with these fu-ture improvements, we intend to study the impact of Comp-ton collisions on the beam dynamics in the DR.This effort is an important step toward Compact X-raysources such as ThomX [7] or polarized positrons sourcefor the next generation of e+/e- colliders.REFERENCES[1] T. Omori et al., Nucl. Inst. Meth. A500 (2003) 232.[2] ATF Collaboration, Phys. Rev. ST Accel. Beams, 13-4 (2010)042801[3] S. Baudrand et al., JINST 5 (2010) 06005 [arXiv:1005.2741[physics.ins-det]].[4] V. Brisson, et al., NIM A 608,1, Supplement 1,1st Septem-ber 2009, Pages S75-S77, DOI:10.1016/j.nima.2009.05.038.[5] An introduction to Pound-Drever-Hall laser frequency stabi-lization, Eric D. Black, DOI:10.1119/1.1286663.[6] Effect of Compton Scattering on the Electron Beam Dynam-ics at the ATF Damping Ring, I. Chaikovska et al., Proc. ofIPAC2011, WEPC051[7] The ThomX Project, A. Variola et al., Proc. of IPAC2011,WEOAA01TUPO002 Proceedings of IPAC2011, San Sebastián, Spain1448Copyrightc ○2011byIPAC’11/EPS-AG—ccCreativeCommonsAttribution3.0(CCBY3.0)02 Synchrotron Light Sources and FELsA14 Advanced Concepts

High flux polarized gamma rays production: first measurements with a four-mirror cavity at the ATF

HAL-IN2P3

HIGH FLUX POLARIZED GAMMA RAYS PRODUCTION: FIRSTMEASUREMENTS WITH A FOUR-MIRROR CAVITY AT THE ATFN. Delerue∗, J. Bonis, I. Chaikovska, R. Chiche, R. Cizeron, M. Cohen, J. Colin, P. Cornebise,D. Jehanno, F. Labaye, M. Lacroix, R. Marie, Y. Peinaud, V. Soskov, A. Variola, F. Zomer,LAL, Universite´ Paris-Sud XI, Orsay, FranceE. Cormier, CELIA, Universite´ de Bordeaux, Bordeaux, FranceR. Flaminio, L. Pinard, LMA, Lyon, FranceS. Araki, S. Funahashi, Y. Honda, T. Omori, H. Shimizu, T. Terunuma, J. Urakawa,KEK, Tsukuba, JapanT. Akagi, S. Miyoshi, S. Nagata, T. Takahashi, Hiroshima University, Hiroshima, JapanAbstractThe next generation of e+/e- colliders will require a veryintense ﬂux of gamma rays to allow high current polarizedpositrons to be produced. This can be achieved by convert-ing polarized high energy photons in polarized pairs intoa target. In that context, an optical system consisting of alaser and a four-mirror passive Fabry-Perrot cavity has re-cently been installed at the Accelerator Test Facility (ATF)at KEK to produce a high ﬂux of polarized gamma rays byinverse Compton scattering. In this contribution, we de-scribe the experimental system and present preliminary re-sults. An ultra-stable four-mirror non planar geometry hasbeen implemented to ensure the polarization of the gammarays produced. A ﬁber ampliﬁer is used to inject about10W in the high ﬁnesse cavity with a gain of 1000. A digi-tal feedback system is used to keep the cavity at the lengthrequired for the optimal power enhancement. Preliminarymeasurements show that a ﬂux of about 4×106 γ/s with anaverage energy of about 24 MeV was generated. Severalupgrades currently in progress are also described.INTRODUCTIONFuture electron-positrons colliders will require a ﬂux ofpositrons much higher than what can be produced with cur-rent methods. One of the scheme that has been proposedto achieve such ﬂux is to use an intense ﬂux of polarizedhigh energy gamma rays [1] which can be converted intopolarized electron-positron pairs. Although promising thismethod requires the demonstration that such high intensityﬂux of gamma rays can be produced. Polarized positronscan be produced by Compton interaction between an elec-tron beam and a laser. However given the low cross-sectionof this process, it is desirable to recycle both the photonsand the electrons by, for instance, using an electron ringand a Fabry-Pe´rot cavity (FPC). We decided to investigatesuch a scheme by installing a four-mirror FPC on the Ac-celerator Test Facility (ATF) [2] damping ring at KEK inJapan.∗delerue@lal.in2p3.frEXPERIMENTAL SETUPLaser SystemThe laser seed includes a modiﬁed commercial oscillatoremmiting at 1031 nm with a repetition rate of 178.5 MHz.We use a standard chirp pulse ampliﬁcation (CPA) archi-tecture in order to limit the non-linearities in the ampliﬁer.The ampliﬁer is built around a commercial microstructureYb-doped double clad ﬁber. The system delivers recom-pressed pulses of around 60 ps and up to 50 W averagepower. The complete optical layout is shown on Figure 1.????????? Laser Oscillator EOM OI Stretcher Unit Optical Pumping Compressor Unit ??????????????? ?????????????? ??????????????????????? ???????????? ????????????????????Fabry-Pérot Cavity ?? ?? ??????????? ?????????? Transmited Beam Reflected Beam Figure 1: Optical layout of the experiment.Fabry-Pe´rot Cavity (FPC)The FPC is built using the experience gained at LAL inprevious experiments [3, 4]. The FPC is formed by 2 con-cave mirrors with a radius of curvature of 0.5m and 2 ﬂatmirrors arranged in a non-planar tetrahedron geometry (seeFigure 2). The two concave mirrors form a 52μm× 76μmwaist at which the laser beam crosses the electron beamwith a collision angle of 8 degrees. The mirrors have avery high reﬂectivity (1 - 1060 ppm for one of them andTUPO002 Proceedings of IPAC2011, San Sebastián, Spain1446Copyrightc ○2011byIPAC’11/EPS-AG—ccCreativeCommonsAttribution3.0(CCBY3.0)02 Synchrotron Light Sources and FELsA14 Advanced Concepts1 - 330 ppm for the others) leading to a cavity ﬁnesse of theorder of 3000 (that is a power enhancement of about 1000).The duration of a round trip in the FPC is 5.6 ns. Great carehad to be taken while designing the cavity to ensure that itis compatible with the ultra-high vacuum requirements ofthe ATF (3× 10−8 mbar).Figure 2: Layout of the 4-mirror cavity. The electron beamis represented by the blue arrow and the blue line enteringto the right of the cavity. The laser beam is representedby the red arrow and the red line entering to the left of thecavity. The blue cylinders near the mirrors contain the 12actuators used to remotely adjust the X, Y and Z positionsof the mirrors.All the mirrors are mounted on actuators carts or tablesthat allow a ﬁne adjustment of the cavity length. Further-more one of the mirrors is mounted on a piezoelectric trans-ducer (PZT) that allows to synchronize the length of theFPC with the repetition frequency of the ATF by using adigital phase lock loop as shown on Figure 3.??????? ????? ??????????? ?????? ADC ?????? ????? ?????? ?????? ? ? ???? ????? ???????? ???????? ?????? ?????????????????????????????? ???????? ???????? ????????????????? ADC FIR ????? ???FIR ???????? Figure 3: Digital phase lock loop used to synchronize theFPC with the ATF clock.In order to constructively stack the pulses in the FPC,it is mandatory to control the pulse to pulse phasingwith an interferometric accuracy. This is done using thePound-Drever-Hall method [5] implemented in a VIRTEX-II FPGA board as shown on Figure 4. The error signal gen-erated by this method is used to adjust the length of thelaser oscillator.Using this double digital feedback system we have beenable to keep the FPC locked on the ATF and the laser oscil-lator locked on the FPC for several hours.ATFA detailed description of the ATF at KEK can be foundin the literature [2]. At the ATF the 1.28 GeV electron????????? ??? ????????????? ?? ? ???? ??????? ?? ?? ??????????????? SOS1 SOS2 DDS ?? ?? FIR SOS3 SOS4 MUX ADC DAC Φ ???????????? ADC ??????? ??????????????? ????????????? DAC ? ???????? Figure 4: Digital feedback to adjust the laser of the lasercavity depending on the PDH signal.bunches are separated by 2.8 ns. Although various ﬁll pat-terns can be achieved in the ATF damping ring (DR), thedata presented here were taken in single bunch single trainmode in which the same electron bunch returns to the sameposition after 165 RF buckets (one complete revolution ofthe ATF DR equals to 462 ns). Given that a round trip inthe FPC is 5.6 ns this means that electrons collide with thelaser pulse stored in the FPC every other turn.CalorimeterDuring the collisions the gamma rays are detected usinga fast scintillation detector made of barium ﬂuoride (BaF2)coupled with a Photomultiplier Tube (PMT). To eliminatethe slow component of scintillation an optical ﬁlter is in-stalled in front of the PMT. The data acquisition is per-formed using a LeCroy WS454 oscilloscope (1GS/s, 500MHz bandwidth). Geant4 simulations have been used tostudy this calorimeter.DATA TAKING AND DATA ANALYSISThe FPC was commissioned in October 2010 and Comp-ton collisions were recorded on the ﬁrst attempt. Wepresent here a brief analysis of some of the data taken.During data taking we record the waveforms from thePMT as well as the 357 MHz ATF clock and laser powertransmitted by FPC measured by a photodiode. A fullwaveform contains approximately 200 000 samples spacedby 1 ns. On the day where the data presented here weretaken the average power stored in the cavity was 160 W.The 357 MHz clock is used to deﬁne the beginning of theperiods in time during which the collisions occurred. So,the length of the period is naturally 924 ns. For each periodwe deﬁne the gate which contains the Compton peak to cal-culate consequently its height and integral. Different qual-ity cuts such as a cut on the shape of the electronic pulse,a cut on the arrival time of the signal etc. were applied toProceedings of IPAC2011, San Sebastián, Spain TUPO00202 Synchrotron Light Sources and FELsA14 Advanced Concepts 1447 Copyrightc ○2011byIPAC’11/EPS-AG—ccCreativeCommonsAttribution3.0(CCBY3.0)restrict the analysis to a high purity sample.The shape of the detector’s response creates a linear re-lation between the total charge and the maximum chargemeasured. This can be seen as a correlation between thecalculated peak height and peak integral as shown on Fig-ure 5.−0.04 −0.03 −0.02 −0.01 0−20−100x 10−11Peak integral [V s] Peak height [V] Figure 5: Peak height vs. peak integral for the high puritydata sample.RESULTSThe spectrum of the gamma rays is shown on Figure 6.It corresponds to the distribution of the energy deposited in−25 −20 −15 −10 −5 0020040060080010001200Peak height normalized by laser power [a.u.]dN dI10.36Figure 6: Gamma ray spectrum.the calorimeter expressed by the peak heights normalizedby the laser power.Since we took data over a wide range of power storedin the FPC, we can study the gamma ray production fordifferent ranges of that power. On Figure 7, the averageof the peak height distribution goes towards higher energywith increasing FPC power as expected.Using cosmic rays for calibration, we can estimate theaverage number of scattered gammas per bunch crossing.According to this calibration, a peak height of 1 mV isequivalent to 34 MeV of energy deposited in the calorime-ter. Assuming the mean energy of scattered gammas to be24 MeV, approximately 4 gammas are produced in averageper bunch crossing (for an average laser power stored in thecavity of 160 W). As the repetition frequency of the colli-sions is about 1 MHz the ﬂux of gamma rays achieved is−12 −10 −8 −6 −4 −2 0050100150200250300350Peak height [mV] dN dI10.34mV  22% of max LP, −1.38 mV25% of max LP, −1.47 mV31% of max LP, −1.81 mV42% of max LP, −2.12 mV49% of max LP, −2.67 mV54% of max LP, −2.86 mV59% of max LP, −3.19 mV62% of max LP, −3.2 mV100% of max LP, −3.21 mVFigure 7: Gamma spectrum for different FPC stored power.Different colors correspond to the spectra taken at differentFPC powers. In the legend, the fraction of the maximumFPC stored power and the mean of each of these spectraare shown.about 4× 106 γ/s (this does not take into account the 0.75duty cycle of the ATF).FUTURE PLANSOur experiment has been suspended due to the earth-quake that struck Japan in March 2011 however we planto resume soon. To increase the Compton ﬂux we intend toreplace the mirrors of the cavity by mirrors with a higherreﬂectivity, giving a ﬁnesse of about 30 000. We also planto improve the laser system and to replace our gamma raysdata acquisition chain with a faster one.Numerical simulations have shown that with the ﬂuxachieved so far we do not expect signiﬁcant effects on thedynamics of the stored beam [6]. However, with these fu-ture improvements, we intend to study the impact of Comp-ton collisions on the beam dynamics in the DR.This effort is an important step toward Compact X-raysources such as ThomX [7] or polarized positrons sourcefor the next generation of e+/e- colliders.REFERENCES[1] T. Omori et al., Nucl. Inst. Meth. A500 (2003) 232.[2] ATF Collaboration, Phys. Rev. ST Accel. Beams, 13-4 (2010)042801[3] S. Baudrand et al., JINST 5 (2010) 06005 [arXiv:1005.2741[physics.ins-det]].[4] V. Brisson, et al., NIM A 608,1, Supplement 1,1st Septem-ber 2009, Pages S75-S77, DOI:10.1016/j.nima.2009.05.038.[5] An introduction to Pound-Drever-Hall laser frequency stabi-lization, Eric D. Black, DOI:10.1119/1.1286663.[6] Effect of Compton Scattering on the Electron Beam Dynam-ics at the ATF Damping Ring, I. Chaikovska et al., Proc. ofIPAC2011, WEPC051[7] The ThomX Project, A. Variola et al., Proc. of IPAC2011,WEOAA01TUPO002 Proceedings of IPAC2011, San Sebastián, Spain1448Copyrightc ○2011byIPAC’11/EPS-AG—ccCreativeCommonsAttribution3.0(CCBY3.0)02 Synchrotron Light Sources and FELsA14 Advanced Concepts

The next generation of e+/e- colliders will require a very intense flux of gamma rays to allow high current polarized positrons to be produced. This can be achieved by converting polarized high energy photons in polarized pairs into a target. In that context, an optical system consisting of a laser and a four-mirror passive Fabry-Perot cavity has recently been installed at the Accelerator Test Facility (ATF) at KEK to produce a high flux of polarized gamma rays by inverse Compton scattering. In this contribution, we describe the experimental system and present preliminary results. An ultra-stable four-mirror non planar geometry has been implemented to ensure the polarization of the gamma rays produced. A fiber amplifier is used to inject about 10W in the high finesse cavity with a gain of 1000. A digital feedback system is used to keep the cavity at the length required for the optimal power enhancement. Preliminary measurements show that a flux of about $4\times10^6 \gamma$/s with an average energy of about 24 MeV was generated. Several upgrades currently in progress are also described

DELERUE, N.

BONIS, J.

CHAIKOVSKA, I.

CHICHE, R.

CIZERON, R.

COHEN, M.

COLIN, J.

CORNEBISE, P.

JEHANNO, D.

LABAYE, F.

LACROIX, M.

MARIE, R.

PEINAUD, Y.

SOSKOV, V.

VARIOLA, A.

ZOMER, F.

CORMIER, E.

FLAMINIO, R.

PINARD, L.

ARAKI, S.

FUNAHASHI, S.

HONDA, Y.

OMORI, T.

SHIMIZU, H.

TERUNUMA, T.

URAKAWA, J.

AKAGI, T.

MIYOSHI, S.

NAGATA, S.

TAKAHASHI, T.

Oskar Bordeaux

High flux polarized gamma rays production: first measurements with a
  four-mirror cavity at the ATF

High flux polarized gamma rays production: first measurements with a four-mirror cavity at the ATF

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Available Versions

HAL-CEA

HAL-IN2P3

Oskar Bordeaux