\documentclass{article}
\usepackage{annrep}
\usepackage{psfigure}
\usepackage{epsfig}
\usepackage{graphicx}
\usepackage{multicol}
\addtolength{\textwidth}{10mm}
\addtolength{\oddsidemargin}{-5mm}
\addtolength{\evensidemargin}{-5mm}
\addtolength{\textheight}{3\baselineskip}
\addtolength{\topmargin}{-\baselineskip}
\newcommand{\etal}{{\it et al.\/}}
\newcommand{\NIM}{\rm Nucl. Inst. Meth.} 
\begin{document}
%
%   comment out the \Declare... for LaTex. Leave it in for pdflatex
%
%\DeclareGraphicsExtensions{.jpg,.pdf} %make jpg and pdf file with pdflatex,
%
\newpage 

\subsection{The Hall A Compton Polarimeter (Contributed by S.~Nanda)}
\subsubsection{Overview}
The Hall A Compton polarimeter has been in production during most of the running time 
in the hall this year. Continuous measurement of beam polarization during acceptable
beam conditions for the spin duality 
(E01-012) and the GDH sum rule (E97-110) experiments have been carried out successfully this year.
The primary limitation to the operation of the polarimeter bas been due to unacceptable backgrounds
in the Compton detectors arising from  beam halo during simultaneous beams in Halls A and C.


During the year, the Compton polarimeter configuration has seen substantial  upgrades
in its hardware as well as software, improving overall performance and reliability 
of the system. In the following sections,  developments in  the Compton Polarimeter,
results from recent experiments, and future plans are presented. Generally, the 
polarimeter performed reasonable well down to 2.5 GeV beam energy. 



\subsubsection{Helicity Timing}
In order to handle counting rates as high as 100 kHz, 
the Compton data acquisition design relied on the 15 Hz toggle signal to flip-flop between 
two acquisition processors for consecutive helicity states.
The introduction of the new helicity timing scheme for the G0 Experiment posed a challenge 
for the running of the Compton polarimeter data acquisition. In going from pseudo-random 
pairs of helicity assignments to quartets, the G0 scheme replaced the old 
15~Hz pair signal with the quartet (QRT) signal shown in Fig.\ref{fig:cpt_helicity_timing}.  
Following discussions with the G0 group, it was decided that a pair-sync (PS) signal will be 
implemented to the G0 timing generator module to maintain compatibility with 
the Compton polarimeter. The PS signal Fig.\ref{fig:cpt_helicity_timing} was delivered to Hall 
A at 14.8 Hz by January 2003 and the Compton polarimeter was tested successfully
with the new signal, in both normal and delayed reporting modes. 
 
\begin{figure}[htp]
\begin{center}
\psfig{file=cpt_helicity_timing.ps,width=5in}
\caption{ }
\label{fig:cpt_helicity_timing}
\end{center}
\end{figure}

 
\subsubsection{Detectors}

In order to extend the dynamic range of the Compton polarimeter to lower energies, 
last summer the electron detector was opened to bring the micro-strips
closer to the primary beam. A $200 \mu m$ aluminum foil was also installed
in front the first of the four planes of micro-strip to shield it against
the synchrotron radiation emitted in the third dipole of the chicane.\\
The detector was operated successfully during the GDH experiment at only  3~mm
away from the beam centroid with minor upgrade of the tuning procedure
of the Compton chicane. As show on Fig.\ref{fig:cpt_run_7572} the four planes are
\begin{figure}[htp]
\begin{center}
\psfig{file=cpt_7572_rate.ps,width=5in}
\caption{Singles counting rates in the four planes of the electron detector as a function of 
micro-strip number.  }
\label{fig:cpt_run_7572}
\end{center}
\end{figure}

\begin{figure}[htp]
\begin{center}
\psfig{file=cpt_7563_rate.ps,width=5in}
\caption{Coincidence counting rates in the four planes of the electron detector as a function of 
micro-strip number.  }
\label{fig:cpt_run_7563}
\end{center}
\end{figure}

now working. A beam halo, characterized by its exponential shape, has been
measured typically 10 times higher than the Compton rate at 3mm and comparable
to the Compton rate at 7mm. This background is totally removed when asking a
coincidence with the photon detector in the trigger (Fig.\ref{fig:cpt_run_7563}).
A possible origin of this halo could be the continuous offset of the diode
laser at the source which doesn't turn off completely between two beam pulses.\\
The data collected show that if the beam halo can be reduced, operating the
electron detector that close to the primary beam would allow accurate
measurements, based on the semi-integrated method \cite{Stephie}, down to 2.5 GeV.\\

In addition, a new set of  four planes of silicon micro strip detectors have been procured from Canberra 
Systems. They have been tested and mounted on a high precision frame. The new asspembly
servers as  the spare electron detector.

\subsubsection{Data Acquisition}
The data acquisition and analysis packages for the Compton polarimeter have been integrated 
into a single Linux platform which provides more efficient handling of Compton data.The 
new machine, named compton.jlab.org, is a modern dual processor Pentium server, 
running RedHat Linux 7.3.  

In the earlier system,
a Sun machine, adaqs1, was used for data acquisition, online  monitoring of events, and EPICS  
control of the hardware. This machine approaching obsolescence, has been problematic recently. 
The off-line analysis was carried out in an older PC, adaqcp, 
with poor performance. Typical analysis times exceeded acquisition times by a factor 1.5,
even at modest beam currents of 10~$_\mu$A generally used in the $^3$He polarized target experiments. 
Hence, in a continuous running scenario, the results of the Compton polarimeter were 
not readily available. 

The original acquisition software developed by CEA, Saclay, was specific to SunOS. It relied on 
a non-standard version of CODA, with DD event monitoring system, that is no longer supported by 
Jefferson Lab. Migration of the
acquisition software to Linux was, hence, a major project requiring significant development 
effort. The task was carried out by the  Compton group and 
the CODA group in consultation with Damien Neyret of Saclay. David Abbott of the CODA group
developed the CODA source tree for the RedHat Linux 7.3 system. Carl Timmer of the CODA group
developed the crucial TCL application interface to the ET event monitoring system. Additional effort went into 
incorporating MySQL database support in CODA. With these developments incorporated into CODA
and tested in the Compton set up, the CODA group has now released the new version as
CODA 2.2.5.

Furthermore, the data analysis software, has been upgraded to ROOT based programs 
from the original PAW based system. Several improvements have been made to the software 
package eventSelection, the main analysis engine, to analyze coincidence 
data in more reliably. The package has been ported to the new Compton machine. The analyzer
is capable of analyzing electron-only of electron-photon coincidence data.

With the performance upgrades, the new system
has reduced the analysis time to 0.1 of acquisition times, facilitating quasi-online 
polarization results.

\subsubsection{Control Software}
A process of migration of the EPICS control software, originally developed by the Saclay group,
to Jefferson Lab EPICS software group was launched this year. We envision a phased transition 
of the package until it is fully taken over by the EPICS group. 
A second single board computer, serving as a slave IOC for this development has 
been installed in the Compton controls VME crate to facilitate the porting process. The 
new IOC uses the common server for Hall A managed by the EPICS group.

The first phase of the migration, namely controls of the electron detector, has been 
successfully implemented and tested. 

\subsubsection{Recent Data}
Compton polarimeter data taken for the GDH experiment between June 31 and August 05, 2003
has been analyzed for both
electron only and electron-photon coincidence triggers.
Figure \ref{cpt_res} show preliminary results
obtained  at 2.8 GeV. The two methods are in good agreement although they have
different systematic errors.\\
\begin{figure}[htp]
\begin{center}
\psfig{file=cpt_res.epsi,width=4in}
\caption{Preliminary beam polarization measurement
performed at 2.8 GeV between June 31 and August 05, 2003. Blue points
are
results of the coincidence analysis, Red points are electron only. Only
statistical errors are represented.}
\label{fig:cpt_res}
\end{center}
\end{figure}


\subsubsection{Upgrade Plan}

The upgrade of the Compton polarimeter is motivated by upcoming high accuracy experiments
with very demanding requirement on the beam polarization. Defining $k$ the photon
energy and $E$ 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 of $1\%$ relative accuracy at 850 MeV. Sub-percent accuracy can be reached at
higher beam energy although some limitations may show up from the operating energy range
of the detectors.The best accuracy achieved so far is taken as a reference
point to discuss the hardware upgrades and the expected budget error.

We are presently developing a conceptual design to upgrade the present Hall A Compton 
Polarimeter by doubling the laser frequency with commercially available components.
The existing Fabry-Perot cavity operating at 1064~nm (IR)
with about 1.5~kW power will be replaced by  a 532~nm (green) cavity
with the same power. With this upgrade, the Figure of Merit ($\sigma <A>^2$)
will increase twice compared to the IR system, over a broad range of electron
beam energies. In addition, the upgrade will double the opening angle
between the primary beam and the scattered electrons which in turn will extend the
operating range of the Compton to lower beam energy.
Associated improvements to the electron detector and photon calorimeter designed to
enhance the accuracy of beam polarization measurement at lower beam energy, shows promise
of absolute accuracies well below 2\% at 0.85~GeV beam energy.


\begin{thebibliography}{9}
\bibitem{Stephie} S. Escoffier, {\it Mesure pr\'ecise de la polarisation
du faisceau
d'\'electrons a TJNAF par polarimetrie Compton pour les expériences Gep
et N-Delta},
PhD thesis, CEA-Saclay.%\bibitem{ransome} R. Ransome {\it et al.}, in preparation
\end{thebibliography}
\end{document}

