\documentclass[acus]{tech-lrd}
\usepackage{epsfig}

\begin{document}
\technotenumber{FE-EE-022}
\technoteseries{FE3000}
\technotedate{February 11, 2002}
\title{Behavior of the SNS Front End Emittance Device}
\author{Lawrence R. Doolittle, LBNL}
\maketitle

{\it \small
 This technical note is intended exclusively for internal
use inside the SNS Front-End Systems Group.  The primary purpose of the
technical note series is to promote rapid dissemination of results for
work in progress.  No formal review has taken place to ascertain the
accuracy of its contents, or the consistency/compatibility of the information
with other SNS work.}

\begin{abstract}
The SNS Front End 2.5 MeV Emittance Device was first tested
with beam from the RFQ on 
February 1, 2002.  The hardware is simple in concept, but has
some non-obvious behavior that needs to be understood before
the output data can be used to construct renderings of phase space
intensities, and compute emittances.  This note documents the wiring,
channel mapping, offset subtraction, channel sensitivity calibration,
and probable slit scattering.
\end{abstract}

\section{Introduction}

The SNS Front End emittance device is made up of a
slit (0.044\thinspace mm opening) and a 32-wire harp
(0.5\thinspace mm pitch, 0.1\thinspace mm diameter),
each of which are on a motorized translation stage.
The beamline assembly can be rotated (as a unit, once let
up to air pressure) to collect data on different axes.
The wires are connected to a 32-channel preamplifier,
and then the signals are sent to a VME-based 32-channel
waveform digitizer, and the data is made available over
the network through EPICS.

We maintain a consistent wire and channel nomenclature,
with the wires numbered from 1-32, starting with 1 farthest
away from the flange.
Channel 32, closest to the flange, is internally and intentionally
connected not only to wire 32, but also to the harp's back plate.

\begin{table*}[p]
\begin{center}

\begin{tabular}{cccccccc}
\input table1.tex
\end{tabular}
\end{center}
The  top   half of the table refers to preamplifier board A, and P6 of the ICS145.

The bottom half of the table refers to preamplifier board B, and P7 of the ICS145.

Notes:

1 - Connection to two wires, feedback resistor changed
    to 25.5\thinspace k$\Omega$.

2 - Not used.

3 - Connected to slit with 25\thinspace $\Omega$ load, bypassing preamplifier.

4 - Connected to both wire 32 and backplate, feedback resistor
    changed to 5.52\thinspace k$\Omega$.

\caption{Channel cross reference}
\label{channel-xref}
\end{table*}

\section{Preamplifier}

\begin{figure}[htb]
\begin{center}
\mbox{\epsfig{file=preamp.simple.ps,width=0.8\columnwidth}}
\end{center}
\vspace{-2mm}
\caption{Schematic of one preamplifier channel}
\label{schem}
\end{figure}

The harp wires are connected to a head-mounted preamplifier
through 0.4\thinspace m of 0.4\thinspace mm diameter Formvar wire
in vacuum (carefully mounted to minimize cross-capacitance between channels),
a 32-pin Astro Seal 21927-04 vacuum feedthrough,
and another 0.07\thinspace m of air-side wiring.
The preamplifier has an inverting current-to-voltage topology,
and is built around an OPA655 (selected for its very low
input current noise), see figure~\ref{schem}.
Most of the feedback resistors used to set the gain are
precision ($\pm$0.1\%) 51\thinspace k$\Omega$,
metal film surface mount components.
Metal film resistors give nearly
perfect noise characteristics, unlike carbon film resistors.
Some channels have different gain resistors, as discussed later.
The preamplifier is built as two 16-channel boards,
% each labeled on the silkscreen:
% \begin{quote}
% {\tt
%     SNS MEBT Emittance Preamp\hfil\break
%     Larry Doolittle, LBNL\hfil\break
%     Rev.~2, 26-Oct-2001\hfil\break
% }
% \end{quote}
% The board was built
according to two mantras: ``Keep it simple, stupid,''
and ``Test components, calibrate systems.''

With a 2.2\thinspace pF parallel feedback capacitor, this circuit
acts as a two-pole low pass filter with -3\thinspace dB bandwidth
of about 2.5\thinspace MHz.
Examination of the filter's response to a square wave using an oscilloscope
shows no ringing and 120\thinspace ns rise time.

These preamplifiers
purposefully have no offset adjustment; the OPA655 has an intrinsic
voltage offset ($\pm$2\thinspace mV) small enough to not interfere
with acquisition,
and the pulsed nature of the beam allows easy offset measurement
in software.  Adding trim components would harm the high frequency
behavior of the preamplifier, increase failure modes, increase the labor
to assemble and test the boards, and probably still not eliminate the
need for a software-based offset trim.

Table 1 cross references which pins are used to connect the channels
from the harp wires, through the preamplifier, and into
the data acquisition board.  A wire harness forms the ``Y'' between the
air-side of that connector, and the two DB-25 input connectors
for preamplifier boards A and B.

\section{Data Acquisition}

The ICS145 data acquisition card contains 32 channels
of 16-bit sigma-delta digitizer, each based on an AD9260, which we
run at a nominal 2 MS/s.  The 16-bit bipolar output
signal is conventionally represented as an integer
between -32768 and 32767, which corresponds to $\pm$1\thinspace V
on the input terminals of the ICS145.
For ordinary channels with their inverting amplifier and
51\thinspace k$\Omega$ gain set resistor,
full scale is $\mp$19.6\thinspace $\mu$A.

The 0.5\thinspace $\mu$s per time step starts at the rising edge of
the TTL ``RF On'' trigger signal provided from the
Klystron gallery.  For the first $\sim$15\thinspace $\mu$s,
the RFQ is filling, and no real beam current arrives at the detector.
Thus, the first $\sim$30 samples are a clean zero input current
baseline which should be used to measure channel offsets.
The next $\sim$15\thinspace $\mu$s represent the transient when
the RFQ is nearly full, but the field is not stable
and therefore neither is the beam.

Acquisitions are triggered by the hardware on every
(5\thinspace Hz nominal) beam pulse, and even hit the
dual-port RAM on the ICS145.  The emittance scan software
only grabs this data when the slit and collector are in their desired
positions, and then moves on.

\section{Adapting to the real world}

Channel 32 (the back plate)
is AC coupled with a 1.0\thinspace $\mu$F capacitor, allowing
the plate to be biased.  Most runs have used a bench power
supply set to +40\thinspace V, with a 5\thinspace k$\Omega$ series resistor
(much higher than the input impedance of the current-mode amplifier).

The current on the harp back plate is much
higher than that on individual wires, so
a 6.19\thinspace k$\Omega$ resistor was added in parallel with its
51\thinspace k$\Omega$ gain set resistor, making full scale on
this channel (32) $\mp$181\thinspace $\mu$A.

Three pairs of wires (5 and 6, 17 and 18, 24 and 25)
are shorted within the harp assembly itself.  While the
harp assembly tested good at the factory, these defects
were discovered on the bench just before the unit was inserted
into the beamline.  The shorts could have developed either in
shipping or during handling at \hbox{LBNL}.  The manufacturer will repair
the unit before MEBT tests begin.

If two amplifiers are left connected
to these wire pairs, the amplifiers are still nominally stable,
but large currents flow between them.
So we disconnected one of each pair of amplifiers, leaving
two wires connected to one amplifier, and the other
amplifier unused.
Amplifiers 5, 18, and 24 now collect current from two wires,
not one, and would be at risk of saturating unless their gain
were reduced.  We paralleled two 51\thinspace k$\Omega$
resistors on these channels,
so that their full scale is doubled to $\mp$39.2\thinspace $\mu$A.
Channels 6, 17, and 25 are not connected to any wire.

The slit current is routed into the data acquisition system, using the
otherwise unused channel 17.  This connection bypasses
the current preamplifier (and its signal inversion), so the signal
level is set by the two 50\thinspace $\Omega$ terminating resistors
in the circuit, for a full scale of $\pm$40\thinspace mA.

\section{Measurements with Beam}

\begin{figure}[tb]
\begin{center}
\mbox{\epsfig{file=examp-time.eps,width=1.0\columnwidth}}
\end{center}
\vspace{-4mm}
\caption{Example current waveforms}
\label{examp-time}
\end{figure}

Example current waveforms acquired during a beam pulse (shot number 50
of MEBT\_Dia0100.mda) are shown in figure~\ref{examp-time}.
Since the preamplifiers invert the signal, the negative going
wire signals represent positive current collected by the wire.
The H$^-$ beam itself is of course negative, so the observed polarity
means that the wires' secondary electron emission coefficient is greater
than unity.  The currents picked up by the slit and back plate
are negative, as expected.  Remember that the slit current is
not inverted because its signal is not preamplified.

\begin{figure}[tb]
\begin{center}
\mbox{\epsfig{file=wires.eps,width=1.0\columnwidth}}
\end{center}
\vspace{-4mm}
\caption{Wire signals for fixed slit}
\label{wires}
\end{figure}

To measure the responsiveness of individual wires,
we took scans with the harp taking 0.5\thinspace mm steps, while
the slit was held fixed.  By plotting each wire's response
as a function of its approximate position (harp position plus
the wire number times 0.5\thinspace mm), as shown in figure~\ref{wires},
we can see that each wire responds about the same as every other wire.
The $y$ axis value for each data point is computed as the average
from time point 140 to time point 180, with a baseline subtracted
based on the average of time points 1 through 25, and normalized to
the value of channel 17, the slit current.

\begin{figure}[htb]
\begin{center}
\mbox{\epsfig{file=gauss-amp.eps,width=1.0\columnwidth}}
\end{center}
\vspace{-4mm}
\caption{Signal strengths measured for each channel}
\label{gauss-amp}
\end{figure}

These curves were fit to a Gaussian with $\sigma$=1.25\thinspace mm.
The amplitudes of these Gaussian fits (shown in figure~\ref{gauss-amp})
are particularly interesting,
since they represent a wire sensitivity factor, by which measurements
should be scaled to get a self-consistent measurement of actual H$^-$
wire current.  There are systematic per-wire variations that can be
explained by differences in secondary electron emission.
A few wires (different on each run) have substandard data
because of arcs in the source and \hbox{LEBT}.  More experience with the
system needed before reliable calibration numbers can be determined.

\begin{figure}[tb]
\begin{center}
\mbox{\epsfig{file=gauss-cen.eps,width=1.0\columnwidth}}
\end{center}
\vspace{-4mm}
\caption{Deviations of wire centroid from nominal 0.5\thinspace mm pitch}
\label{gauss-cen}
\end{figure}

The center position of each Gaussian fit is shown in figure~\ref{gauss-cen}.
The outliers at channels 5, 18, and 24 are expected---these are the
shorted wire pairs, that should appear at ``channel'' 5.5, 17.5, and 24.5.
The overall trend of the curve represents a 1\% inconsistency between
the harp step (0.5004\thinspace mm) and the wire pitch spacing.
Most or all of this deviation can be explained because the
slit was not perfectly fixed during the run.
As a temporary workaround to avoid a software quirk,
the slit moved 0.0025\thinspace mm when the harp moved 0.5004\thinspace mm.

\begin{figure}[tb]
\begin{center}
\mbox{\epsfig{file=slitscatter.eps,width=1.0\columnwidth}}
\end{center}
\vspace{-4mm}
\caption{Broad tail in collected current}
\label{slitscatter}
\end{figure}

Other interesting physics shows up if we replot the data of figure~\ref{wires}
over a broader scan dimension, and expanded vertical scale,
shown in figure~\ref{slitscatter}.  The long tails, which have a
$\sigma\approx 20$\thinspace mm, seem much too broad to represent
a tail in $x'$ from the RFQ, especially after the 400\thinspace mm
drift between the end of the RFQ and the slit.  This data does appear
consistent with slit scattering, and it might make sense for emittance
analysis software to deconvolute the measured data to account for this
effect.

The current step that appears between -30 and -12\thinspace mm
in figure~\ref{slitscatter} is apparently due to secondary electrons
emitted from the grounded front plate of the harp.  These electrons
are attracted towards the +40\thinspace V
backplate, and some fraction land on the (virtually) grounded wires.
The edge appears when the beam centroid moves about
13\thinspace mm higher than wire 32, and is bigger on higher numbered wires.

\section{Acknowledgements}
The mechanical design of this emittance gear is primarily due to Steve
Virostek and Matt Hoff, based on beam physics requirements from John Staples.
The harp itself was purchased from Princeton Scientific.
Electronic assembly and wiring was provided by Nathan Ybarrolaza,
Co Tran, and Bill Lau.  EPICS software to support the ICS145 and motion control was
written by Carl Lionberger and Ernest Williams.  Beam operations were
led by Rainer Thomae.  All the Front End technicians helped in one
way or another, and Don Syversrud deserves special mention.  Emittance
software of John Staples and Saeed Assadi
necessarily incorporates the data manipulation described in this note.

\end{document}
