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\documentclass{chowto}

\title{Beam Quality Check and Monitor}
\howtotype{reference}
\author{Liguang Tang}
\category{beamline}
\date{June 17, 2003}
\begin{document}

\begin{abstract}

This Howto outlines the basic methods and general Hall C beam 
line equipment to check and monitor the beam quality needed by 
experiments.

\end{abstract}

\section{General equipment in Hall C beam line to verify quality}

The general equipment in Hall C line that can be used to check 
or monitor the beam tune and beam quality include Superharps, 
Beam Position Monitors (BPMs), and Optical Transmission Radiation 
(OTR) monitor or Synchrotron Light Interferometer (SLI). In 
addition, fast feedback lock systems to lock beam energy and beam 
position on target are installed for the Hall C beam line using 
the readouts from specific BPMs and arc optics.


\section{Superharps}

Superharps are used to measure the beam size or profile at the 
specific locations alone the beam line where beam quality can be 
characterized.  At each such location, there is a standard pair 
of superharps with known separation distance (1.817 meters), 
sandwiched a BPM in the middle. For example, at location C07, 
Superharp C07A is at upstream of the BPMC07 and Superharp C07B 
is at downstream of the BPMC07 with equal distance.

The four important locations are: (1) C07 (entrance of the arc), 
(2) C12 (middle of the arc), C17 (exit of the arc), and (4) H00 
(2.5 meter before target).  

For a normal archromatic tune (as most of the experiments use), 
the size of the beam at C07 and C17 should be around 100$\mu$m 
in both x and y directions.  At the middle point of the arc, the 
momentum dispersion is maximized about 3-4 cm\%, depending on the 
required beam energy.  This means that the size of beam in x 
direction should be $< 300 \mu{\rm m}$, corresponding to an energy 
spread $< 10^{-4}$, while the size in y direction be around 
$100 \mu{\rm m}$. Harp scan at the location of H00 will verify the beam 
size on target.  Beam size at H00 can be specified to MCC based 
on the need of the experiment.  MCC will tune the multipole 
magnets to meet the requirement.  Around $100 \mu {\rm m}$ (FWHM) has been 
achieved in the past.

The centers of the beam profiles extracted from scanning the pair 
of superharps at the specfic location can provide the beam incident 
angles at that location.

The superharps cannot be used continuously.  Harp scan should be 
done to verify the beam tune.  The frequency of such scan depend 
on the need of experiments.


\section{BPMs and fast feedback locks}

The BPMs are used to measure the center position of the beam alone 
the line.  The BPMs located at C07, C12, C17, and H00 are able to 
be read at a frequency of 1kHZ in order to install the fast 
feedback lock systems.  They provide important information about 
stability of the central beam energy and position on target.  

Under a specific tune of the arc, an optics for the given beam 
energy is defined.  A position shift indicated by the BPM at C12 
represents a central energy shift, if positions at C07 and C17 are 
relatively stable.  For experiments that require high precision 
beam energy, monitoring the stability of these positions are 
important.  The readout of these BPMs can also be added into data 
stream using a suitable readout frequency.  Energy shift correction 
can be made offline using arc optics matrix.

With the same principle, energy fast feedback lock system reads 
these positions at a frequency of 1kHZ and calculates the needed 
energy correction.  Then correction is added to the last cavity 
module in the south linac.  Such system is installed on both 
Hall A and C arcs.  Only one of these two can be used to control 
the beam stability.  Practically, the hall with the lock on its 
arc has the best energy stability and $\sigma < 2x10^{-5}$ was 
achieved in the past.  For other halls the energy shift is larger 
but less than $10^{-4}$.

The BPMs, H00A and H00B, are in front of the target.  They are 
3.455 and 1.637 meters upstreams from the target, respectively, 
and are used to indicate the beam position on target and the 
incident angles.  Checking and monitoring these positions allow 
experiments maintain their desired beam position on target and 
the stability.

A position fast feedback lock is also installed in order to 
stablize the beam position on target.  This lock is independent 
for each hall.  Since the position stability is coupled with the 
beam energy stability, whichever the hall having the enegry lock 
on will also have the best position on target stability 
($< 200 \mu{\rm m}$.


\section{OTR and SLI}

OTR is a device which detects the emitted radiation from a thin 
($10 \mu{\rm m}$) Carbon foil as beam passing through.  It can measure 
both the center position and the size of the beam.  Although its 
accuracy is less than superharp, it can be used continuously as 
a monitor.  

An OTR system is installed at the C12 position, middle point of 
the Hall C arc.  Thus, its continuously meausred beam size 
provides the intrinsic energy spread with respect the central 
value.  Thus, it measures another important beam energy quality.  
Since this spread cannot be corrected as that of central drift 
that can be corrected by the fast feedback system, if the size 
measured at C12 exceeds the desired energy spread, MCC should be 
contacted.  

Although OTR uses thin foil, it was seen that it did produce halo 
in the beam.  Thus, a new device will be installed at the same 
location (C12) soon, replacing the existing OTR. The new device 
is SLI.  This device does not have any material in beam but can 
measure the size of the beam as OTR does.  By using it at the 
location of C12 to measure continuously the beam size, the 
intrinsic energy spread can be checked and monitered constantly.


\end{document}

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