\documentclass{chowto}

\title{Description of the Hall C Raster Systems}
\howtotype{reference}
\author{Chen Yan}
\category{beamline}
\date{April 15, 2003} 

\begin{document}

\begin{abstract}

A brief description of Hall C two raster systems (target raster and
polarized target raster) is summarized.

\end{abstract}

\section{History}

The Lissajour pattern target raster system has been used for about 8 years
since 1994. The operation is based on power resonance loop. Lissajous pattern
is generated from two independent sinusoidal current signals. Its simplicity
makes the system very reliable. No major maintenance was performed during
past 8 years.

The major problem of Lissajous raster is the uniformity of the raster pattern.
Along the boundaries of raster pattern, there is density enhancement produced 
by slowdown behavior of sinusoidal waveform at turning point. This large
increase in deposited beam energy in these areas eventually causes overheating
of the target material. By experimental measurements - a luminosity scan with 
HMS, we found the luminosity decreases gradually by increase of beam curent.
That indicates that local overheating effect near the boundaries and the corners
of Lissajous pattern contributes an uncertainty in the target length which, in
turn, effects the acuracy of the experimental data.

Therefore, besides upgrade target circulation system - increase stream velocity,
generation of uniform raster density is the first requirement of new raster 
system design.

\section{Operational principle}

Based on an H-bridge technique, four branches of power HexaFETs form a two 
by two switch. The raster air-core magnet is located at the center of H-bridge.
A high voltage power supply powers the two far rails of the switch. By
switching the HexaFETs in the right order and at the right frequency, a 
triangular current waveform is generated. The timing property of the switch is
determined by the H-Bridge controller. The magnetic field of the raster magnets
follow the shape of the current waveform and this steers the beam using
a triangular waveform rather than a sinusoidal waveform.
 
\section{Major specification}

\begin{tabular}{ll}
Raster frequency in kHz                & 24.96/25.08                     \\
Maximum peak-to-peak current in Ampere & 82                              \\
Maximum rigidity in GeV/c)             & 20/$\Delta x_{1/2}$  for Hall C \\
                                       & 35/$\Delta x_{1/2}$  for G$_{0}$\\
Maximum bending power in mr            & 0.97/p[GeV/c]        for Hall C \\                                           
                                       & 1.70/p[GeV/c]        for G$_{0}$\\
Shape of raster pattern                & rectangular                     \\
Shape of raster trace                  & triangular                      \\
Dwelling time at vertex in ns          & 50                              \\
Dwelling/half period  in  {\%}         & 0.25                            \\
Trace linearity  in  {\%}              & 98.2                            \\
Density uniformity  in {\%}            & 95                              \\
Operational mode                       & free run                        \\
                                       & helicity synchronization        \\
                                       & line synchronization            \\
Pattern on-line display                & current probe 2D or 3D          \\
                                       & field pickup  2D or 3D          \\
\end{tabular}

\section{Special feature of the system}

\subsection{Rule of raster frequency selection}

Two raster ferequencies are selected in this way: the two numbers are integer
of 416 $\times$ 60 and  418 $\times$ 60. These two frequencies are close
enough to generate a very fast rolling picture. It ensures the beam can travel
as long as possible distance at unit time. The two frequencies can also
be easily locked at single signal source such as line or helicity frequencies.

\subsection{Sweeping properties}

Compared with Lissajous raster, the H-bridge raster driver provide highly
homogeneous density distribution with 98$\%$ linearity and 95 $\%$ uniformity
that approaches theoretical limits. The linear sweep velocity is a constant
at 1000 m/s. The return time at the raster peak is about 50 ns, the dwelling
time is almost negligible. The maximum rigidity allows operation at 11 GeV/c
in the near future.

\subsection{Rastered beam position}

The system is equipped with phase-lock module. Under phase-lock option, the
two initial beam positions (in both x and y) will exactly start at the front 
edge of external signals.  The relatively broad range of phase-lock
function have made full success to synchronize helicity signals during G$_{0}$ 
experiment in 2002. By proper digitizing either current or filed pickup 
signals, users will get full knowledge about instant bean location.
                                                                                                                         
\subsection{Dual control system}

The system is equipped with dual control systems - manual controller and
EPICS control. Manual controller is installed in counting house. Users 
can operate the system and set up demand parameters as a stand-alone 
unit. When the EPICS control is runing, the manual controller automatically
display the setting parameters as monitor.

\subsection{On-line raster pattern display}

Chris Slominski developped a
\htlink{on-line raster pattern display program}{http://devsrv.acc.jlab.org/controls_web/LLAPPS/AppDoc/beamRaster/gui/doc/manuals/user.html}. It 
can provide 2D and 3D histigram display with calibrated scale. Users 
could use this feature to correlate with data stream as instant beam 
position flags. This program is available in the accounts \verb|cvxwrks@cdaqh1| or \verb|cdaq@cdaqs1/2/3| by clicking the left mouse button and selecting
``Raster Pattern Display''.
see Hall C logbook.
\begin{figure}[htp]
\begin{center}
\includegraphics{beamline_raster-scope.ps}
\caption{Raster display program}
\end{center}
\end{figure}

\section{Operation}

Users call MCC to ask them to set right raster size. The raster control
screen display in Hall C counting house is passive. Users could
check raster status, verify raster size from the parameters of the screen.  

\section{Polarized traget raster}
The polarized target experiments ask a round shape 2.5 cm in diameter 
spiral raster pattern on polarized target cell. The original raster system 
was based on the power resonance driver. The resonance loop gives a high Q
value that limits bandwidth very much. The resultant modulation ferequency 
is 1 Hz. Also the uniformity of the raster pattern is not satisfied by
experiments.

The new design (pulse width modulation amp) will solve the problem. It will 
provide 100 Hz fundamental and 30 Hz modulation with absolutely density 
uniformity. The commissioning of new system will be scheduled at the end 
of 2003.

\end{document}

% Revision history:
% $Log: beamline_raster.tex,v $
% Revision 1.2  2003/04/15 21:18:38  saw
% (saw) Add example pic and point to raster display docs
%
