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\subsection{Compton Polarimeter Upgrade}

\begin{center}
{\bf The Compton Polarimeter Upgrade}
\end{center}

\begin{center}
Contributed by Sirish Nanda\\
\end{center}


\subsubsection{Overview}
The Hall A Compton Polarimeter provides electron beam polarization
measurements in a continuous and non-invasive manner via Compton
scattering of polarized electrons from polarized photons trapped in a
Fabry-Perot cavity. A schematic layout of the Compton polarimeter is
shown in Fig.\ref{fig:cpt_sch_setup}. The electron beam is transported
through a vertical magnetic chicane consisting of four dipole
magnets. At present, a high-finesse Fabry-Perot cavity injected by a
240~mW, 1064~nm infrared laser serves as the photon
target. Intra-cavity power remains stable at about 500~watts. The
Compton back-scattered photons are detected in an electromagnetic
calorimeter consisting of lead tungstate crystals. The recoil
electrons, separated from the primary beam by the fourth dipole of the
chicane are detected in a silicon micro-strip detector. The electron
beam polarization is deduced from the counting rate asymmetries of the
detected particles.

\begin{figure}[htp]
\begin{center}
% \psfig{file=compton-upgrade-setup.eps,width=4in}
\showfig{width=4in}{compton_upgrade-setup}
\caption[Compton Upgrade: Schematic of the Compton polarimeter]{%
Schematic layout of the Hall A Compton polarimeter.}
\label{fig:cpt_sch_setup}
\end{center}
\end{figure}

\subsubsection{Upgrade}
A performance upgrade of the Compton Polarimeter, in accordance with
the conceptual design report,\cite{cpt_cdr} is presently under
implementation. The upgrade is motivated by upcoming high accuracy
experiments with very demanding requirement on the beam
polarization. Defining $k$ as the photon energy and $E$ as the
electron beam energy, the Figure Of Merit (F.O.M.) of a Compton
measurement scales with $k^2\times E^2$, making high accuracy
polarimetry a real challenge at low energy. The proposed upgrades are
thus driven by the requirement of the Lead Parity
experiment\cite{cpt_prex} of $1\%$ relative accuracy at 850 MeV. The
present infra-red system, while capable of achieving such accuracies
at higher energy ($\ge 6$ GeV), falls far short at lower energies. We
plan to upgrade the existing Fabry-Perot cavity operating at 1064~nm
(IR) with about 1.5~kW power to a 532~nm (green) cavity with similar
power.

In addition, improvements to the electron detector and photon
calorimeter combined with the development of a new integrated method
shows promise of absolute accuracies approaching 1\% at 0.85~GeV beam
energy. The main parameters of the green Compton upgrade are shown in
Table.\ref{tab:cpt_upg_parm}
\begin{table}[htp]
\begin{center}
\begin{tabular}{|l|l|l|}
\hline
 Parameter & Present & Upgrade \\\hline\hline
Wavelength (nm) & 1064   & 532  \\
Cavity Power (W) & 1500   & 1500  \\
Cavity Q & 1.0$\times 10^{11}$ & 1.8$\times 10^{11}$ \\
FOM ($\sigma$.A$^2$) @.85Gev & 0.57 & 2.2 \\ 
Energy Range (GeV) & 2 - 6 & 0.8 - 6 \\ 
\hline\hline
$\delta$P$_e$/P$_e$ @.85Gev & - & 1\%\\
\hline\hline
\end{tabular}
\end{center}
\caption[Compton Upgrade: Design parameters of the upgrade compared to present system]{%
Main design parameters of the green Compton polarimeter upgrade compared 
to the present infra-red system.}
\label{tab:cpt_upg_parm}
\end{table}

The conceptual design of the green Compton polarimeter utilizes much
of the design philosophy and the existing infrastructure of the
present Compton polarimeter. In the following, the subsystems under
upgrade are described.

\subsubsection{Electron Detector }
 
The electron detector upgrade is being carried out by Laboratoire de
Physique Corpusculaire IN2P3/Universit� Blaise Pascal,
Clermont-Ferrand. The new detector has 4 planes of 192 silicon
micro-strip with 240 $\mu m$ pitch. The expected resolution is about
100 $\mu m$.  A high precision vertical motion of 120 mm for the
detector has been incorporated to the design so as to facilitate
covering the recoil electrons corresponding to the Compton edge over a
broad range of energies. Motion control system as well as remote
control computer interfaces for the detector have been successfully
developed by the EPICS group . Illustrated in
Fig.~\ref{fig:edet-strips-cad} is the electron detector silicon
micro-strip assembly.
\begin{figure}[htp]
\begin{center}
% \psfig{file=edet-strips-cad.eps, width=2.1in}
\showfig{width=2.1in}{compton_edet-strips-cad}
\caption[Compton Upgrade: The new electron detector silicon micro-strip assembly]{%
The new electron detector silicon micro-strip assembly.}
\label{fig:edet-strips-cad}
\end{center}
\end{figure}
The electron detector as well as its support structure and vacuum
chamber were fabricated and later assembled at Clermont-Ferrand in
early 2008. The package was delivered to Jefferson Lab in summer
2008. Installation of the new detector in the Hall A beam line was
carried out by a team visiting from Clermont-Ferrand and Jefferson Lab
engineering services group during August-September
2008. Fiducialization of the detector with respect to the vacuum
chamber and alignment of the detector chamber was carried out by the
Jefferson lab survey and alignment group. The new installation
successfully completed vacuum test and basic electronics signal
checkout in October 2008. The CEBAF electron beam was successfully
transported through the Compton chicane at 5.9 GeV through the new
installation.
\begin{figure}[htp]
\begin{center}
% \psfig{file=edet-installed.eps, width=4.1in}
\showfig{width=4.1in}{compton_edet-installed}
\caption[Compton Upgrade: The new electron detector installed in Hall A beam line]{%
The new electron detector installed in Hall A beam line.}
\label{fig:edet-installed}
\end{center}
\end{figure}

Commissioning of the electron detector is in progress. Alexander
Camsonne is in the process of developing CODA interface to the new
ETROC trigger module supplied by Clermont-Ferrand.  Analysis software
for the electron detector is under implementation by Richard Holmes,
Syracuse University.


\subsubsection{Photon Detector }
 
The new photon detector consists of a single GSO crystal, 60 mm in
diameter and 150 mm in length. The GSO crystal was delivered by
Hitachi Chemicals Ltd. to Carnegie Mellon University (CMU) in March
2008. The compete photon calorimeter as shown in
Fig.~\ref{fig:gso-assembled}, was put together at CMU lead by Gregg
Franklin.  During the summer of 2008, the calorimeter was tested at
the HIGS facility of the Free Electron Laser Lab at Duke
University. Absolute energy calibration of the calorimeter was
performed over a range of photon energies from 2 to 20 MeV at
HIGS. Better than 2\% energy resolution was obtained with the
calorimeter.
\begin{figure}[htp]
\begin{center}
% \psfig{file=gso-calorimeter.eps, width=4.1in}
\showfig{width=4.1in}{compton_gso-calorimeter}
\caption[Compton Upgrade: GSO Calorimeter assembled and tested at Carnegie-Mellon Univerisity]{%
GSO Calorimeter assembled and tested at Carnegie-Mellon University.}
\label{fig:gso-assembled}
\end{center}
\end{figure}

The GSO calorimeter was installed in Hall A beam line in November
2008. Commissioning of the detector with the existing Compton
Polarimeter DAQ in counting mode, as well as the Flash ADC based
integrating DAQ is in progress. Preliminary spectrum of Compton
scattered photons from 5.9 GeV polarized electron beam obtained with
the detector is shown in Fig.~\ref{fig:gso-spectrum}.  The spectrum
shows a sharp Compton edge indicative of the high energy resolution of
the new detector. Spin asymmetry as a function scattered photon
energies are shown in Fig.~\ref{fig:gso-asym}. The red and blue points
are for right-handed and left handed polarization of the laser beam
while the green points are background data with the laser off. Data
analysis is being carried out by Diana Parno, CMU and Richard Holmes,
Syracuse U.

\begin{figure}[htp]
\begin{center}
% \psfig{file=gso-spectrum.eps, width=4.1in}
\showfig{width=4.1in}{compton_gso-spectrum}
\caption[Compton Upgrade: Compton scattering spectrum of 5.9 GeV electrons with the new GSO Calorimeter]{%
Compton scattering spectrum of 5.9 GeV electrons with the new GSO Calorimeter.}
\label{fig:gso-spectrum}
\end{center}
\end{figure}
\begin{figure}[htp]
\begin{center}
% \psfig{file=gso-asym.eps, width=4.1in}
\showfig{width=4.1in}{compton_gso-asym}
\caption[Compton Upgrade: Asymmetry of Compton scattering measured with the GSO calorimeter at 5.9 GeV]{%
Asymmetry of Compton scattering measured with the GSO calorimeter at 5.9 GeV.}
\label{fig:gso-asym}
\end{center}
\end{figure}
\subsubsection{Fabry-Perot Cavity}
A schematic layout of the optical setup for the upgrade is shown in
Fig.~\ref{fig:cpt_optics}. The heart of the upgrade plan is to replace
the infrared cavity with a 532~nm green cavity fed by the new
laser. Recent advances in the manufacturing of high reflectivity and
low loss dielectric mirrors as well as availability of narrow line
width green lasers facilitates the feasibility of our challenging
design goal.  High gain cavities at 532~nm have been successfully
constructed by the PVLAS\cite{cpt_bregant} group that has geometry and
gain comparable to our proposed design here.

\begin{figure}[hpt]
\begin{center}
% \psfig{file=optics_green.eps,width=3.5in}
\showfig{width=3.5in}{compton_optics_green}
\caption[Compton Upgrade: Optics table of the green Compton polarimeter]{%
Optics table of the green Compton polarimeter.}
\label{fig:cpt_optics}
\end{center}
\end{figure}

Significant progress has been achieved in the development of the green
Fabry-Perot cavity during 2008 with participation from Abdurahim
Rakhman, graduate student from Syracuse University. Earlier in 2007 we
were successful in lock acquisition with the stock CVI mirrors,
Prometheus green laser, and the home-made Pound-Drever-Hall feedback
servo system electronics system. However, cavity power was
substantially below expectations due to high loss in the mirrors. This
was attributed to lack of clean room conditions in the ARC laser lab.

In summer of 2008, a major effort was launched to convert the laser
lab into a clean room. The HVAC system for the room was modified to
achieve positive pressure in the room. A clean tiled acoustic ceiling
as well as HEPA fan filter units were installed in the lab. This led
to achieving Class 1000 level environment for the whole lab.  For
cavity mirror handling, cleaning and storage, a class 100 clean bench
was procured from Air Clean Systems. In addition, a portable soft-wall
clean room with additional HEPA filter units were installed
surrounding the optics table and the clean bench. This improved the
environment in the optics table and cavity to better than Class
100. Illustrated in Fig.~\ref{fig:cpt_cleanroom} is the new clean room
environment of the laser lab.
\begin{figure}[htp]
\begin{center}
% \psfig{file=cpt_cleanroom.eps,width=4in}
\showfig{width=4in}{compton_cpt_cleanroom}
\caption[Compton Upgrade: Clean room setup in the Compton Polarimetry Lab]{%
Clean room setup in the Compton Polarimetry Lab.}
\label{fig:cpt_cleanroom}
\end{center}
\end{figure}


New super-polished cavity mirrors substrates with better than 0.5
$\AA$ finish were ordered from General Optics in the Summer of
2008. Three sets of low loss HR at 532~nm coatings were also
ordered. The transmittance of the three sets of mirrors was specified
to be 50, 100, and 240 ppm with less than 10 ppm scattering and
absorption loss. The new cavity mirrors were delivered to the Lab in
October 2008. Shown in Fig.~\ref{fig:cpt_gsi_substrate} is the
measured data for surface roughness of the substrates as deviation
from the specified surface over the central region of the mirror
covering a circle of 1 mm in perimeter. The rms deviation was measured
to be 0.2$\AA$, better than our specification. Transmittance of the 50
ppm mirrors as a function of wavelength is show in
Fig.~\ref{fig:cpt_gsi_50ppm_t}.

\begin{figure}[htp]
\begin{center}
% \psfig{file=cpt_gsi_substrate.eps,width=4in}
\showfig{width=4in}{compton_cpt_gsi_substrate}
\caption[Compton Upgrade: Surface deviation of the mirror substrates fabricated by General Optics]{%
Surface deviation of the mirror substrates fabricated by General Optics.}
\label{fig:cpt_gsi_substrate}
\end{center}
\end{figure}

\begin{figure}[htp]
\begin{center}
% \psfig{file=cpt_gsi_50ppm_t.eps,width=4in}
\showfig{width=4in}{compton_cpt_gsi_50ppm_t}
\caption[Compton Upgrade: Transmittance of the high finesse GSI mirrors as a function of wavelength]{%
Transmittance of the high finesse GSI mirrors as a function of wavelength.}
\label{fig:cpt_gsi_50ppm_t}
\end{center}
\end{figure}

The 240 ppm mirrors were mounted in the cavity in November 2008. In
order to improve the electronics noise figures experienced in the
feedback servo electronics, Lawrence Lee, a summer student from
Rutgers University, and Dan Sexton from Jefferson Lab FEL electronics
group, participated in the enhancement of the Cavlock electronics. We
successfully acquired lock on the TEM$_{00}$ cavity mode. Illustrated
in Fig.~\ref{fig:cpt_short_lock} is the oscilloscope traces of the
cavity signals demonstrating the beginning of a lock sequence. The
upper blue trace is the photodiode signal from laser power reflected
from the cavity, the lower yellow curve is the photodiode signal from
the laser power transmitted through the cavity, the red trace is the
error signal fed back to the laser to sustain lock. As seen from these
curves,there are some oscillations in the transmitted and reflected
power. Work is in progress to further tune the Cavlock servo
electronics to improve the stability of the lock. We measured the
intra-cavity power to be 150 Watts with a gain of about 3000.  While
the mirrors performed quite well, the coupling efficiency to the
cavity was not optimal. We are in the process of tuning optics to
enhance coupling efficiency.

\begin{figure}[htp]
\begin{center}
% \psfig{file=cpt_short_lock.eps,width=4in}
\showfig{width=4in}{compton_cpt_short_lock}
\caption[Compton Upgrade: Scope trace of cavity lock acquisition with the GSI 240~ppm mirrors and the Prometheus laser]{%
Scope trace of cavity lock acquisition with the GSI 240~ppm mirrors and the Prometheus laser.}
\label{fig:cpt_short_lock}
\end{center}
\end{figure}

\subsubsection{Conclusion}

We have demonstrated that the newly acquired GSI mirrors, the
conversion of the laboratory to clean room environment, and
improvements to locking electronics, high gain cavity performance is
possible. We have also successfully coupled the IR beam of the
Prometheus laser through a fiber to the IPG Ytterbium fiber amplifier
to pump the PPLN crystal and produce more than 1 Watt of green
power. Future plan include acquiring lock of this fiber amplified beam
coupled to the GSI mirror cavity with higher cavity power levels.

\begin{thebibliography}{9}
\bibitem{cpt_cdr} S. Nanda and D. Lhuillier, Conceptual Design Report  for 
Hall A Compton Polarimeter Upgrade, Unpublished.  
\bibitem{cpt_prex}Jlab Experiment E06002, Paul Souder (Syracuse),   Robert Michaels (JLab),   Guido Urciuoli (INFN) spokespersons.
\bibitem{cpt_bregant} M. Bregant \etal, arXIV:hep-ex/0202046 v1 28 Feb 2002.
\end{thebibliography}
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