\documentclass{chowto}

\title{Beam Kicker System for Hall C Moeller Polarimeter}
\howtotype{user} % ``expert'', ``user'', ``reference''
\category{beamline}
%\maintainer{} % Optional

\author{Chen Yan}

\date{November 26, 2003}


\begin{document}


\begin{abstract}

The kicker system of Hall C Moeller polarimeter uses a fast 
intermmitant beam kick technique to control exposure time 
and cooling time of the iron wire target in order to operate 
polarimeter at higher beam current, also to achieve on-line 
Moeller measurement.

\end{abstract}


\section{Introduction}

The first Hall C kicker system was installed in October 2003. The 
kicker magnet, located at the begining of Hall C arc, will bend 
beam vertically across an iron wire target. The beam sweeps the 
iron wire within in 1 $\mu$s. The kick frequency can be continuously 
adjusted from 10 kHz down to 1/10 Hz. The beam heating effect on 
the target can be precisely restricted by proprely adjusting time 
interval of trigger signals. A 20 $\mu$s gate signal simultaneously 
generated from the kick pulse can be used to alternate data 
aquisistion of G0 experiment and Moeller measurement in a time-share 
mode. The estimated maximum beam current is 20 to 40 $\mu$A. 

\section{Kicker Magnet}

The kicker magnet is located at station $\#$ 4070 - right after Hall C 
superharp IHA3C07A. The location is very close to the beginning of Hall
C arc achromat transport line, where double focusing is achieved. For 
preliminary test, one Hall C fast raster magnet is temporarily installed 
as kicker magnet. The magnet inductance is 88 $\mu$H, which determines 
the shortest time for a single traversal (10 $\mu$s). The distance from 
kicker magnet to Moeller target is about 44 m. If the optical 
transportation in vertical direction can be simplified as free drift, 
the corresponding bending angle for 1 mm displacement on Moeller 
target is about 0.02 mr. The field integral value of the fast
raster magnet is 80 Gauss cm per Ampere.

The magnet is positioned in vertical direction. If one faces the magnet
in front of the girder 3C07, the winding direction is anticlockwise. 
Excited by a positive current pulse, the magnet generates a pulse field 
along the normal of magnet plane. By applying left-hand role, electron 
beam will be moved up.


\section{Kicker Driver Electronics}

A diagonal half of H-bridge is configurated as the main component of 
kicker driver. When a trigger pulse comes, a 10 $\mu$ width gate is 
generated from controller and the half bridge becomes conductive. The 
magnet is then connected to HV power supply. The magnet current rises 
with a time constant $\tau$ = L/R $\sim$ 7.33 ms. As the gate width is 
much shorter than $\tau$, the current is rising in approximately linear
mode (linearity is better than 98\%).
 
Magnet current starts to drop down from its peak value as the fall 
edge of gate pulse arrives. The ramping up and ramping down process 
is complete in 20 $\mu$s for each trigger signal. The 20 $\mu$s TTL 
signal is sent to data aquisition electronics of both Moeller and 
Hall C as strobe of data stream.


\section{System Configuration}

The first kicker system is manually controlled. The basic functions 
of such a system fulfills operation requirement of system commissioning.

\begin{itemize}
\item{Magnet - on the first hall C superharp granite table 3C07 
      at station $\#$ 4070}
\item{Power driver - inside the lead shielding by the 3C07. 
      Two signals are generated from the power module: the kicker 
      current analog signal from Pearson current probe and the 
      synchronized 20 $\mu$s gate signal for alternatively strobe 
      data aquisition systems of Moeller measurement and experiments.}
\item{Manual Controller - in the rack CH03B14 in Hall C counting house,
      left potentiometer is used to adjust voltage of HV unit. Left 
      LCD window displays voltage value, the right LCD window shows 
      the load current of HV unit. The POWER ON switch also controls 
      the remote AC power relay unit of the power module. On the rear 
      panel, a LEM socket gives a 5 V level output indicating kicker 
      system "ON" for MCC.}
\item{Kick Frequency Generator - WAVETEK function generator model 29 
      on CH03B14 rack, generates TTL level trigger frequency.}
\item{Kick Waveform Monitor - Tektronix TSD 211 storage oscilloscope 
      in rack CH03B13 in Hall C counting house displays the single 
      kick current waveform synchronized by the full gate signal. 
      On the scrteen, readout of kick amplitude and frequency is 
      automatically displayed.}   
\end{itemize}    

\section{System Calibration and Conversion Formula}

\subsection{Determination of kick amplitude from HV setting}

\begin{itemize}
\item{The scale of Pearson current probe output is 1 volt per 10 A}
\item{The kick amplitude is determined by HV setting voltage. Current
      amplitude is read out from Pearson current probe. The magnet
      current to HV voltage setting conversion factor is 
      $\Delta I/ \Delta V = 0.121 {\rm A/V}$}
\item{From Jeff Karn's DC mapping data, the field interal of the kicker
      magnet (i.e. FR magnet) is 80 Gauss cm per Ampere}
\item{From bending power formula, 
      \[
      \int Bdl [{\rm kG m}] = 33.356 p [{\rm GeV}/c] \theta [{\rm rad}]
      \]}
\item{Convert field integral to magnet current (Pearson probe output
      in volt), we have:
      
      \[ 
      V[{\rm volt}] = 7.7 \times \Delta h [{\rm mm}] \times p[{\rm Gev}/c]  
      \]}      

      here, $\Delta$h is the vertical kick amplitude on Moeller 
      target in mm and V is HV voltage in volt.
\end{itemize}               


\subsection{Kick frequency calculation}

Based on the target thermo-equilibrium, the kick operational frequency
can be simply estimated. The major constraint is the temperature rise of
the iron wire target. Here we assume:

\begin{itemize}
\item{The iron wire target is 1 cm in length, 25 $\mu$m in diameter}
\item{Kick ramp is 10 $\mu$s, beam exposure time is 2 $\mu$s for two 
      traversals in a single kick.}
\item{Allowable temperature gradient is 200 $^{o}$C/cm along the 
      wire target}
\item{Beam deposit power on the wire can be expressed as: 

      \[ 
      P_{\rm beam}[{\rm W}] = I_{{\rm beam}}[\mu A] \times \Delta E[{\rm MeV}],
      \]

      $I_{\rm beam}$ is the effective beam current = 

      \[ 
      I_{0} \times d_{\rm wire}/d_{\rm beam}
      \] } 

\item{The cooling power through heat transfer from wire target to the 
      frame:
       
      \[ 
      P_{cool} [W] = - k[{\rm W}/({\rm cm} {}^{\circ}{\rm C}] \times A[{\rm cm}^{2}] \times
      dT/dx[{}^{\circ}{\rm C/cm}],
      \]
      
      here, $k$ is thermo-conductivity of pure iron.  A is double 
      wire cross section (for two ends).}
\end{itemize}  

By applying 

      \[ 
      t_{exp} \times P_{beam} = t_{\rm cool} \times P_{\rm cool},
      \]

the estimated value of kick ferequency can be found as 

      \[ 
      I_{beam} [\mu {\rm A}] \times f_{\rm kick} [{\rm Hz}] = 2000.
      \]

\section{Supplementary Kick Diagnostic Device}

Downstream 14 feet from Moeller target, a Russian FEU115M timing
PMT was installed on the straight section of Moeller triple pipe
with very forward angle. The PMT will pickup radiations generated 
when beam across the target. We expect the signals are bright enough
for observation on the scope when the kicker gate output signal
is applied as the trigger. This device is used for verification
of kicker function.

The LeCroy HV4032A high voltage unit is located in CH03B14 rack.
To set high voltage for the pickup PMT, turn the power key at 
first, then push HV ON switch. Left window displays the channel
and right window for HV. Push CH together with UP or DOWN arrow
to set the right channel number. Push HV to set the desired
high voltage value.

The HV channel for this PMT is 5. Normally applied HV is 2000 V.


\end{document}

% Revision history:
% $Log: moeller_kicker.tex,v $
% Revision 1.1  2003/11/26 19:06:36  saw
% Initial checkin from Chen Yan
%
