Per mill level control of the circular polarisation of the laser beam for a Fabry-Perot cavity polarimeter at HERA

We report on the highest precision yet achieved in the measurement of the polarization of a lowenergy, O(1 GeV), continuous wave (CW) electron beam, accomplished using a new polarimeter based on electron-photon scattering, in Hall C at Jefferson Lab. A number of technical innovations were necessary, including a novel method for precise control of the laser polarization in a cavity and a novel diamond microstrip detector which was able to capture most of the spectrum of scattered electrons. The data analysis technique exploited track finding, the high granularity of the detector and its large acceptance. The polarization of the 180 µA, 1.16 GeV electron beam was measured with a statistical precision of < 1% per hour and a systematic uncertainty of 0.59%. This exceeds the level of precision required by the Q weak experiment, a measurement of the weak vector charge of the proton. Proposed future low-energy experiments require polarization uncertainty < 0.4%, and this result represents an important demonstration of that possibility. This measurement is the first use of diamond detectors for particle tracking in an experiment. It demonstrates the stable operation of a diamond based tracking detector in a high radiation environment, for two years.

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“…Ref. [37] used an implementation of the optical reversibility theorem that is equivalent to maximizing the signal in the RPD of Fig. 4.…”

Section: Analysis and Resultsmentioning

confidence: 99%

Precision Electron-Beam Polarimetry at 1 GeV Using Diamond Microstrip Detectors

Narayan¹,

Jones²,

Cornejo³

et al. 2016

Phys. Rev. X

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“…Other factors include analyzing power and photon detector gain shift can also affect the systematic errors in the final electron beam polarization [24]. Recent studies at HERA [20] and JLab Hall C [7] show that it is possible to get < 0.3% level precision for the intra-cavity laser polarization with proposed techniques. So far this result is only based on scattered photon analysis.…”

Section: Discussionmentioning

confidence: 99%

“…Recently optical cavities have been widely employed in many areas such as X-/γ-ray generation via inverse-Compton scattering [17,18], high harmonic generation [19] and electron beam polarimetry [14,20]. According to the best of our knowledge, there are only three facilities in the world have ever built a Compton polarimeter with an optical cavity; Mainz Microtron (MAMI) [21], Hadron Electron Ring Accelerator (HERA) [20] and JLab [14]. In Hall A of JLab, a Compton back-scattering polarimeter had been operational since 1999 [22].…”

Section: Introductionmentioning

confidence: 99%

A high-finesse Fabry–Perot cavity with a frequency-doubled green laser for precision Compton polarimetry at Jefferson Lab

Rakhman

Hafez

Nanda

et al. 2016

Nuclear Instruments and Methods in Physics Research Section A:

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A high-finesse Fabry-Perot cavity with a frequency-doubled continuous wave green laser (532 nm) has been built and installed in Hall A of Jefferson Lab for high precision Compton polarimetry. The infrared (1064 nm) beam from a ytterbium-doped fiber amplifier seeded by a Nd:YAG nonplanar ring oscillator laser is frequency doubled in a single-pass periodically poled MgO:LiNbO 3 crystal. The maximum achieved green power at 5 W infrared pump power is 1.74 W with a total conversion efficiency of 34.8%. The green beam is injected into the optical resonant cavity and enhanced up to 3.7 kW with a corresponding enhancement of 3800. The polarization transfer function has been measured in order to determine the intra-cavity circular laser polarization within a measurement uncertainty of 0.7%. The PREx experiment at Jefferson Lab used this system for the first time and achieved 1.0% precision in polarization measurements of an electron beam with energy and current of 1.06 GeV and 50 µA.

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“…An ellipsometer located at the output of the cavity is used to measure the laser beam polarisation. It is described in details in a companion article [26] to the present one. The principle of the measurement is to send a light beam of any unknown polarisation through first the QWP1 and then the cavity, and from there it is guided with two mirrors (Mo1 and Mo2) to go through a holographic beam sampler (HBS) and another quarter wave plate (QWP2).…”

Section: The Rod Temperature Can Be Varied Thanks To a Peletier Modulmentioning

confidence: 99%

“…Left and right laser beam polarisations: The circular laser beam polarisation S 3 has been measured using the optical ellipsometer with an uncertainty of 0.3% [26]. This has been however checked by a devoted scan measurement: in a period of time short enough to expect that the electron polarisation will only change slowly, five different MOCO position combinations have been used to measure the polarisations.…”

Section: Correlated Systematic Errorsmentioning

confidence: 99%

A high precision Fabry-Perot cavity polarimeter at HERA

et al. 2010

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A Fabry-Perot cavity polarimeter, installed in 2003 at HERA for the second phase of its operation, is described. The cavity polarimeter was designed to measure the longitudinal polarisation of the HERA electron beam with high precision for each electron bunch spaced with a time interval of 96 ns. Within the cavity the laser intensity was routinely enhanced up to a few kW from its original value of 0.7 W in a stable and controllable way. By interacting such a high intensity laser beam with the HERA electron beam it is possible to measure its polarisation with a relative statistical precision of 2% per bunch per minute. Detailed systematic studies have also been performed resulting in a systematic uncertainty of 1%.To cope with the physics program after the upgrade, a fast and high precision longitudinal Compton polarimeter using a continuous wave laser resonating in a Fabry-Perot cavity (LPOL cavity) was proposed [4], constructed and installed near the existing longitudinal Compton polarimeter (LPOL) [5]. In addition to the LPOL, the transverse polarisation of the electron beam is also measured by another polarimeter (TPOL) [6].With respect to the prior HERA LPOL and TPOL polarimeters, the higher statistical precision of the LPOL cavity is achieved by increasing firstly the power of the continuous wave laser by more than two orders of magnitude compared to the TPOL and secondly the frequency of the electron-photon(laser) interaction to 10 MHz compared to 0.1 kHZ of the pulsed laser of the LPOL. A new Data Acquisition System (DAQ), synchronised to the HERA beam clock, has been developed accordingly which operates without any trigger at 10 MHz. This is one of the novelties of the experiment described in this article.The HERA Fabry-Perot cavity is similar to a device that has been used successfully to measure the polarisation of the CEBAF LINAC electron beam [7,8,9]. One major difference between the HERA and CEBAF LPOL cavities is the dynamical regime. Whereas the luminosity of Compton scattering is relatively low at CEBAF, it reaches much higher values at HERA. That is, the average number of scattered Compton photons is close to one per bunch in the latter case and much smaller in the former. The HERA dynamical regime, denoted as 'few photon mode' in this article, has been used successfully for the first time by the LPOL cavity to measure the electron beam polarisation. An important point to mention is that a huge effort was made to reduce the unforeseen high level of synchrotron radiation emitted by the electron beam at the cavity location downstream of the HERMES experiment, by adding many protections (cf Sect. 3.1 for more detail) to avoid damaging any optical or electronic component [10]. Especially fragile were the 'supermirrors', whose high quality, which was maintained until the end of data taking, was the key to obtaining the foreseen high power of the laser,The main purpose of this article is to describe the LPOL cavity experiment and to report about the electron polarisation measurement. The article is org...

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Per mill level control of the circular polarisation of the laser beam for a Fabry-Perot cavity polarimeter at HERA

Cited by 6 publications

References 24 publications

Precision Electron-Beam Polarimetry at 1 GeV Using Diamond Microstrip Detectors

Precision Electron-Beam Polarimetry at 1 GeV Using Diamond Microstrip Detectors

A high-finesse Fabry–Perot cavity with a frequency-doubled green laser for precision Compton polarimetry at Jefferson Lab

A high precision Fabry-Perot cavity polarimeter at HERA

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