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{\large {\bf Update}}\\[2mm]
Status of Charissa at Strasbourg
}}\\[7mm]
\makebox[2.5cm][l]{Author:} Steve Chappell\\[2mm]
\makebox[2.5cm][l]{Date:} 11 November 1996\\[2mm]
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\noindent
Present : S.P.G. Chappell, R. Cunningham, J. Lilley, B. Bilwes.\\
Circulation : Charissa Staff, B. Bilwes \& N. Rowley.\\

%\begin{verbatim}

The Charissa beamline at Strasbourg was commissioned with a series of
tests followed by a short feasibility study during the period 21$^{\rm
st}$-25$^{\rm th}$ October 1996. The
outcome of these tests is presented in this document with a summary of
the current status of Charissa's experimental capability at Strasbourg.
Please refer to the report CH/Tech/96/04 by Bob Cunningham for
technical details regarding the beamline equipment.

\section{Beam Tests}

A 104 MeV $^{16}$O beam was provided for the initial testing of the
beamline equipment. Firstly, the tuning of the beam from the Vivitron
through the 8 mm (vertical) $\times$ 3 mm (horizontal) diagnostic box
aperture was investigated. All elements upstream of the diagnostic box
are controlled by the Vivitron operators. It was not possible to focus
the beam through this aperture without the quadrupole (QSW) upstream of
the switching magnet steering the beam in the vertical direction. When
the beam was set-up through this quadrupole such that application of
power to the magnets did not introduce any steering, the beam was too
high at the diagnostic box aperture position and had to be steered down
a little. When that was done, however, it was clear that the spot size
was not bigger than the aperture because, even though the beam
fluctuated in intensity and position, the current intercepted by the
aperture did fall to zero for a good fraction of the time.  These tests
indicate that the components of the Vivitron beamline prior to the
Charissa line are not optimally aligned and also that the angle of the
beam directed towards QSW is not optimum.

During the commissioning, run, there was only one set of H/V steerers
(the ones upstream of QSW) available for steering the beam after the
object waist for QSW. Normally, there should be two sets: one before
and one after QSW. Having both of them available would, in principle,
(but not easily) allow a better line to be chosen for the beam entering
the switcher and then on to the diagnostic box. As it is,  reproduction
of beams using old machine parameters is imperfect (since one relies
upon some vertical steering of the beam upstream of the quadrupole
where the electric steerers have an imprecise readout). The setting up
of beam to this point is then more difficult than is desirable. If the
diagnostic box aperture is used to define the beam for all experiments
this does not present any problem.  A little practise by the operators
should hopefully speed up the beam set-up time.

\subsection{The Charissa Beam Line}

All elements of the Charissa line worked correctly. A little steering
was required on the electric X-Y steerers (in Y). This reflects the
problem discussed above. A small correction to the beam was required as
it emerged from the diagnostic aperture angled vertically.  The beam
line was set up through the magnetic axis of the quadrupole without
difficulty (i.e. non steering quadrupole) through a 2mm $\times$ 2mm
scintillator aperture at the target position and onto the chamber
Faraday cup. The beam transmission on the line (between the diagnostic
and Faraday cups) was $\simeq$ 100\%. The beam optics were found to be
slightly better than those calculated (by Tom Aitken using emittance
information more appropriate for heavier ions than oxygen) where the
calculations indicate a 3 mm $\times$ 3 mm beam spot at the target and
are representative of what may be expected for heavier ions.  The beam
divergence was also tested by winding the line slits in and reading off
the current at various distances. The beam size at the slits was
approximately 7 mm (vertical) by 5 mm (horizontal). These slits are
positioned immediately downstream of the last quadrupole doublet in the
line at a distance of 4 m from the target.

The line was also tested with two other beams. The magnet settings were
found to scale approximately with the relative analyzing magnet $B$
fields when going to 84 MeV $^{16}$O and then 50 MeV $^{7}$Li. A
summary of the beams tested is given in table \ref{beams}. The energies
of these beams are calculated (as provided by the Vivitron operator).
The range of beam dump currents shown represents the large fluctuation in
measured beam current. This fluctuation is dramatic and periodic and
believed to result from the join in the belt charging system. The
fluctuations can be reduced by defocussing the beam through the
machine, which also reduces the maximum intensity and presumably the
beam quality (in terms of energy). The absolute energy calibration is
not at all clear. The analyzing magnet object point does not appear to
be well defined and probably accounts for the fact that improved
stability can be gained by analyzing a defocussed beam.

The beam energy change from 104 MeV $^{16}$O to 84 MeV took just over 1
hour and highlighted some possible difficulties in procedure. A reduced
beam current is desirable during set-up so as to avoid destroying the
diagnostic scintillator (and detectors when tuning down the beam line).
It is usual to increase the beam current after the initial tuning. This
control over intensity is difficult to achieve at present and not
usually performed. In addition, the analyzing magnet was not ``cycled''
to avoid hysteresis. The ``odd'' beam energies are a result of
requesting 100 and 80 MeV beams without emphasizing the importance of
the absolute energy. The result was the setting of the terminal voltage
as the priority not the energy of the analyzed beam. With some
experience it is expected that the time to change beam energy could be
halved at best. A change of beam species requires at least one day and
is subject to the availability of the source operator.
\begin{table}
\begin{center}
\begin{tabular}{c|c|c}
\hline
Beam &  Calculated Energy & Rough Beam Dump Current\\
\hline\hline
$^{16}$O (6+)   & 104 MeV & 0-25 enA\\
$^{16}$O (6+)   & 84 MeV  & 0-8 enA\\
$^{7}$Li (3+)   & 50 MeV  & 0-10 enA\\
\hline
\end{tabular}
\end{center}
\caption{Summary of the Beams tested.}
\label{beams}
\end{table}

\subsection{Transmission Tests}

A request was made for the rough variation of beam transmission with
machine terminal voltage to be measured. A summary of the results is
given in Table \ref{trans}. Note that two different injection currents
were used in the tests using $^{16}$O and that the analyzed beam is
measured immediately after the analyzing slits.  There is clearly a
large drop in beam intensity as the terminal voltage is reduced. The
machine transmission for $^{16}$O at 15 MV is $\simeq$10$\times$ that
at 9 MV.
\begin{table}
\begin{center}
\begin{tabular}{c|c|c|c|c}
\hline
Terminal Voltage &  Injected Current & Analyzed Current\\
\hline\hline
  Ion   &  terminal volts  &  LE beam    &  HE beam    &  analyzed beam\\
        &       (MV)       &   (enA)     &   (enA)     &     (enA)\\

$^{16}$O &       9          & 100    &   50/ 90   &     4/  8\\
        &      10         &  100    &   70/120    &    5/ 15\\        
        &      11          & 100    &  110/130    &   10/ 25\\
        &      12         &  100    &  110/150    &   10/ 30\\
        &      12         &  200    &  280/320    &   10/110\\
        &      13          & 200    &  300/350    &   10/130\\
        &      14         &  200     & 300/350    &   20/150\\
        &      15         &  200     & 300/400     &  20/150\\
& & & &\\
$^{7}$Li   &     12.5     &      36    &     20      &     10(max)\\
        &      11      &      30    &     17     &       9\\
        &      10      &      32     &    18      &      8\\
        &       9      &      28     &    16      &      6\\
\hline
\end{tabular}
\end{center}
\caption{Beam Transmission tests for the analysis of $^{16}$O and 7Li ions.}
\label{trans}
\end{table}

\subsection{Particle detection}

Two surface barrier detectors were mounted in the chamber during the
tests. One was used to measure beam scattered from the target (and
possibly elsewhere), the other to measure $\alpha$-particles following
the activation of a $^{208}$Pb target with a $^7$Li beam in a
feasibility study of the (possible) breakup modified Lithium fusion
cross section. The results of this study will be reported by J.
Lilley. The beam energy resolution was better than could be measured in
these tests. The measured resolution was entirely accounted for by
contributions from the electronics and detector. Negligible background
scattered beam was observed.

\section{Problems to be Solved Before Running}

Some work needs to be carried out on Charissa equipment at Strasbourg
before any experiments can be mounted. In addition some minimum
improvements to the beam are required before a wider programme of
experiments can begin (although feasibility studies and certain
specific experiments should be possible). These problems are outlined
in the sections below.

\subsection{Charissa Equipment}

\begin{enumerate}
\item{The arm drive mechanism needs some maintenance. The control system does not supply the $+12$ V to the drive motors when requested.}
\item{Some backlash on the target angle movement needs checking.}
\item{The present chamber cables are too thick and hinder arm movement. These need replacing.}
\item{Chamber arm patch panels need to be installed for the cabling.}
\item{No electronics in Strasbourg!}
\item{No D.A. system in Strasbourg! Even obtaining a single channel
spectrum is near impossible using equipment available locally.}

\end{enumerate}

\subsection{The Beam}

\begin{enumerate}
\item{The current stability needs improving. Large fluctuations of
current on target are typical. The resulting signal pile-up and D.A.
rate clogging would present serious problems and prevent running.}
\item{Cannot run the machine at low terminal volts (Not clear how
serious the technical difficulty for low voltage operation is). Above $\sim$ 9
MV is the preferred operating range. Low beam energies therefore
require gas stripping (at present unavailable).}
\item{Little control over beam intensity. Difficult to stop back the
beam at the source. More control over this is required to
provide the optimum (and adjustable) beam currents on target.}
\item{Beams are of low intensity where the transmission drops rapidly
with terminal voltage.}
\item{Only a few beam species have actually been produced. It is
estimated (by the source operator) that beam transmissions for the
Vivitron (inclined field tube) would be typically 1/2 that given by a
HV tube (i.e. the MP) and 1/4 that for an NEC tube. Clearly some
empirical data are required.}
\item{It is not clear what the energy calibration is! One needs to be
able to specify a beam and an energy and have at least some numerical
idea as to the expected error.}
\end{enumerate}

\subsection{Miscellaneous Difficulties}
Some of the other perceived difficulties and operating restrictions are
listed here.
\begin{enumerate}
\item{Local Maintenance and Support are required for both running and
idle periods to keep the beamline equipment in good service. A local
``expert'' on the Charissa equipment would be useful. There appears to
be no clear delegation of responsibility for this at present.}
\item{With all experimental lines in the same hall access time for setting up requires careful scheduling.}
\item{The Vivitron operators based in the control room have limited
knowledge of the source and can perform remote operations on it only.
In fact the source can only be set-up for new beam etc. during the
hours (approx. 7am to 5pm) when the source operator is on duty. Careful
timetabling of experiments is therefore required.}
\item{Learn French! Seriously, communication is a BIG problem. Without
a local guide to who's who running would be impossible.}
\end{enumerate}

\section{Summary \& Recommendations}

The commissioning of the Charissa beamline at Strasbourg is complete.
The minimum work remaining before an experiment can be mounted includes
the installation of a D.A. system and associated electronics. Some
minor investigation and maintenance of faults in the chamber control
system are also required. At the present time only a few beam species
have been produced at the Vivitron with severely reduced intensities at
the lower terminal voltages 9-10 MV. Extreme fluctuations in beam
current at the target would also make any experiment extremely
difficult. These problems are under investigation. However, even when
they are solved it remains clear that experiments requiring many
changes of beam energy or species will not be possible. There is little
empirical data available upon which to base a feasibility report for
running at the Vivitron. A programme of tests of desired beams is
required. Beyond the technical difficulties there is also the wider
problem of communication between the user and operator. Language
differences aside, the beam requirements for charged particle
spectroscopy were found to be not well appreciated by the personnel
providing the beam during the commissioning period. Some discussion is
required to overcome this problem.

Neil Rowley has called for proposals or at least letters of intent to
be submitted as soon as possible. Direct requests for specific beams
will provide a framework for improvements to be made and hopefully some
of the desired criteria will be met in the near future. It is strongly
recommended that Charissa proposals be forwarded NOW in order to
indicate our requirements and allow some room for the laboratory to
plan and manoeuvre.  IN2P3 will be discussing the status of the
Charissa programme at Strasbourg in April '97. The directors of the
Vivitron have requested that (if possible) Charissa make some further
level of commitment (of equipment and running) by this date.

%\end{verbatim}
 
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