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\begin{document}
\section{Upgrade of the inner barrel layer BI}
\label{sec:BI_layer_upgrade}

\subsection{Small-diameter monitored drift tube (sMDT) chambers}

Small-diameter muon drift tube (sMDT) chambers~\cite{sMDTperf1} with a
tube diameter of $15\mm$, half the tube diameter of the original MDT
chambers, have been developed since 2008 in order to cope with the
higher background rates at the HL-LHC. They allow for a higher number
of tube layers or for constructing thinner chambers, making space
e.g.\ for additional trigger chambers under the tight space
constraints in the muon spectrometer. Standard industrial aluminum
tubes are used, with the same $0.4\mm$ wall thickness as for MDT
tubes.  Figure~\ref{fig:tubes} shows a comparison of the MDT and sMDT
drift tubes. The sMDT chambers are operated in ATLAS with the same gas
mixture, gas pressure, and gas gain as the MDT chambers. Their high
voltage is reduced with respect to MDT chambers from $3080\V$ to
$2730\V$. Table~\ref{tab:parameters} shows a comparison of the MDT and
sMDT operating parameters.  Typical drift time spectra of MDT and sMDT
are shown in Fig.~\ref{fig:spectrum_gasgain} (left). The maximum drift
time of sMDT tubes is only $175\ns$ in contrast to about $720\ns$ for
MDT tubes, and the sMDT cross section exposed to the radiation is half
the size of that of an MDT. Altogether this results in an about 8
times lower occupancy and a nearly linear space-to-drift-time
relationship in the standard MDT drift gas Ar:CO$_2$ (93:7) at $3\bar$
which becomes rather non-linear at larger drift radii
(see Figure~\ref{fig:rtrelation_resolr}, left). In the linear range, 
the drift velocity is independent of the electrical field strength and 
insensitive to space charge fluctuations due to background irradiation 
and other environmental parameter variations. 


\begin{figure}[tb]
\centering
\includegraphics[width=0.75\textwidth]{tubes.pdf}
\caption{Picture of MDT (left) and sMDT (right) tubes.}
\label{fig:tubes}
\end{figure}

\begin{table}[tb]

\begin{center}
\caption{Material and operating parameters of ATLAS sMDT chambers
  compared to MDT chambers. The values of $500\Hzcm$ and $200\kHzt$
  are the maximum background rates expected in the barrel inner (BI)
  layer of the ATLAS muon spectrometer at the HL-LHC.}
\label{tab:parameters}
\vspace{2mm}

\begin{tabular}{|l|c|c|}

\hline
Type & MDT & sMDT \\
\hline\hline
Tube material & Aluminium & Aluminium \\
              & Aluman100 & AW 6060-T6/ AlMgSi \\
Tube inner \& outer surface & & Surtec 650 chromatization \\
Tube outer diameter & $29.970\mm$ & $15.000\mm$ \\
Tube wall thickness & $0.4\mm$ & $0.4\mm$ \\
Wire material & W-Re (97:3) & W-Re (97:3) \\
Wire diameter & $50\mum$ & $50\mum$ \\
with gold plating thickness & $3\%$ & $3\%$ \\        
Wire resistance/m & $44~\Ohmm$ & $44\Ohmm$ \\
Wire pitch & $30.035\mm$ & $15.099\mm$ \\
Wire tension & $350\pm 15\g$ & $350\pm 15\g$ \\
\hline
Gas mixture & Ar:CO$_2$ (93:7) &  Ar:CO$_2$ (93:7) \\
Gas pressure & 3 bar (abs.) & 3 bar (abs.) \\
Gas gain & $2\times 10^4$ & $2\times 10^4$ \\
Wire potential  & $3080\V$ & $2730\V$ \\
Maximum drift time & $720\ns$ & $175\ns$ \\
\hline
Average tube spatial resolution & $83\mum$ & $106\mum$ \\
without background irradiation & & \\
Average tube spatial resolution & $160\mum$ & $110\mum$ \\
at $500\Hzcm$ background rate & & \\
Drift tube muon efficiency  & $95\%$ & $94\%$ \\ 
without background irradiation & & \\
Drift tube muon efficiency  & $80\%$ & $90\%$ \\ 
at $200\kHzt$ background rate & & \\
Wire positioning accuracy & $20\mum$ r.m.s. & $10\mum$ r.m.s. \\
\hline
\end{tabular}

\end{center}

\end{table}

\begin{figure}[tb]

\includegraphics[width=0.49\textwidth]{spectrum.pdf}\hfill
\includegraphics[width=0.49\textwidth]{gasgain.pdf}

\caption{Left: measured drift time spectra of MDT (green) and sMDT
  tubes (gray) together with the prediction of a GARFIELD simulation
  for sMDT tubes (red line)~\cite{sMDTperf1}.  Right: Measurements of
  the gas gain of MDT and sMDT tubes relative to the nominal gas gain
  $G_0 =20000$ as a function of the $\gamma$ background rate at the
  Gamma Irradiation Facility (GIF) at CERN compared to predictions
  based on the Diethorn formula~\cite{sMDTperf3}.}
\label{fig:spectrum_gasgain}

\includegraphics[width=0.49\textwidth]{rtrelation.pdf}\hfill
\includegraphics[width=0.49\textwidth]{res_with_TS_new.pdf}

\caption{Left: Track radius-to-drift-time relationship of MDT drift
  tubes. The part with drift radii $r$ below $7.5\mm$, relevant for the
  sMDT tubes, is linear to good approximation.  Right: Spatial
  resolution after time-slewing corrections (TSC) as a function of the
  drift radius for MDT and sMDT drift tubes measured under the same
  operating conditions in the H8 muon beam at CERN without background
  irradiation~\cite{sMDTperf2,sMDTperf3}.  As expected, the results
  for 15 and $30\mm$ diameter tubes are in good agreement for drift
  radii below $7.5\mm$.}
\label{fig:rtrelation_resolr}

\end{figure}

A full-scale sMDT prototype chamber of trapezoidal shape has been
constructed and tested in the H8 muon beam and in the Gamma
Irradiation Facility (GIF) at CERN in 2010~\cite{sMDTprototype}. The
chamber has been operated in the ATLAS cavern in 2012.  In 2014, two
BME sMDT chambers~\cite{BMEproposal, BMEnote, BMEop} have been installed in the
elevator regions of the barrel muon spectrometer in sector 13,
sandwiched between two new thin-gap RPC chambers (doublets) each, and
they are in operation since the start of LHC Run 2. In January 2017,
12 BMG sMDT chambers~\cite{BMGconstr, BMGnote} have been installed in
the detector feet in sectors 12 and 14 of the
barrel~\cite{BMGproposal, BMGproduction}.

The construction of 16 BIS78 sMDT chambers with integrated thin-gap
RPC chambers (triplets) has started. They will be installed in LS2 at
the ends (outermost positions along the beam line, in the ATLAS-$Z$
coordinate) of the small BI sectors in order to improve the trigger
efficiency and the rate capability of the chambers in the transition
regions between barrel and endcaps at $1.0<|\eta|<1.3$
~\cite{BIS78proposal, BIS78note}. This Phase-I upgrade serves as a
pilot project for the complete replacement of the BIS1--6 MDT chambers
in LS3 by sMDT+RPC chamber modules that will have a very similar
design. The installation of the new RPCs is made possible by the
replacement of the BIS MDT chambers by sMDT chambers of about half the
height. As a side effect, the sMDT chambers have about an order of
magnitude higher rate capability than the MDTs.


\subsection{Performance of sMDT chambers}

The performance of MDT and sMDT chambers has been extensively studied
at GIF using the existing ATLAS MDT readout electronics with bipolar
shaping~\cite{sMDTperf1, MDTperf, sMDTperf2}. The new MDT and sMDT
amplifier-shaper-discriminator (ASD) chips for Phase II will have the
same specifications as those used in the present system. The MDT
chambers can be operated up to background rates of $500\Hzcm$ and
$300\kHzt$.  At background rates above $500\Hzcm$, the gas gain drops
by more than $20\%$ (see Fig.~\ref{fig:spectrum_gasgain}, right) which
results, together with the effect of space charge fluctuations, in a
rapid deterioration of the spatial resolution with increasing
background flux.  This limitation of MDT chambers is overcome by using
drift tubes with half the diameter of MDT tubes, leaving the operating
parameters, Ar:CO$_2$ (93:7) gas mixture at 3 bar pressure and a
nominal gas gain of 20000, unchanged~\cite{sMDTperf1}.

\begin{figure}[p]
\centering 
\includegraphics[width=0.7\textwidth]{resolution.pdf}

\caption{Average spatial resolution of MDT and sMDT drift tubes
  measured at GIF as a function of the $\gamma$ background rate using
  standard MDT readout electronics with bipolar shaping. A further
  improvement of the sMDT drift tube spatial resolution at high
  background rates and space charge densities can be achieved by
  employing additional fast baseline restoration (BLR) in order to
  suppress signal pile-up effects (blue curve)~\cite{blr}. 
  This is, however, not necessary and will not be implemented for Phase-II
  operation.}
\label{fig:resolution}
%\end{figure}

\vspace{1cm}

%\begin{figure}[p]
\centering
\includegraphics[width=0.7\textwidth]{efficiency.pdf}

\caption{Muon detection efficiencies of MDT and sMDT drift tubes with
  deadtime settings of $820\ns$ and $220\ns$, respectively, measured
  at GIF as a function of the $\gamma$ background counting rate using
  standard MDT readout electronics with bipolar shaping.  The
  efficiency is defined as the probability to find a hit on the
  extrapolated track within $3\sigma$ of the drift tube spatial
  resolution ($3\sigma$ efficiency).  The measurement results agree
  well with the expectations from detailed simulations of detector and
  electronics response.  A further improvement of the efficiency of
  the sMDT drift tubes at high background counting rates is possible
  by employing BLR in order to suppress signal pile-up effects (blue
  dashed curve)~\cite{blr}. This is, however, not necessary and will not
  be implemented for Phase-II operation.}
\label{fig:efficiency}
\end{figure}

As the space charge density inside the drift tubes is proportional to
the third power of the tube radius, $15\mm$ diameter drift tubes
exhibit a significant gain drop only at 8 times higher background
rates compared to $30\mm$ diameter drift tubes (see
Fig.~\ref{fig:spectrum_gasgain}, right).  At the same time, the
adverse effect of space charge fluctuations on the spatial resolution
is eliminated because the drift gas is linear to a good approximation
for drift radii below $7.5\mm$ (see Fig.~\ref{fig:rtrelation_resolr},
left). The radial dependence of the spatial resolution of 15 and
$30\mm$ diameter drift tubes from measurements in the H8 muon beam at
CERN without radiation background is shown in
Fig.~\ref{fig:rtrelation_resolr} (right) \cite{sMDTperf3}. Standard
MDT time-slewing corrections are applied in both cases.  Without
irradiation and associated space charge effects and with time-slewing
corrections, the average sMDT drift tube resolution is $106\pm 2\mum$
in contrast to $83\pm 2\mum$ for the MDTs as measured in the H8 muon 
beam at CERN~\cite{sMDTperf3}. The dependence of the average spatial 
resolution of MDT and sMDT drift tubes on the $\gamma$ background rate 
is shown in Fig.~\ref{fig:resolution}. The spatial resolution 
deteriorates quickly with increasing background flux for the MDTs, 
while it is affected only very little by space charge effects up to 
very high irradiation rates for the sMDTs.

At the same background rate, small-diameter drift tubes experience an
8 times lower occupancy than $30\mm$ diameter MDT tubes because of the
four times shorter maximum drift time (see
Fig.~\ref{fig:spectrum_gasgain}) and the two times smaller tube cross
section exposed to the radiation. Because of the much shorter maximum
drift time, the adjustable deadtime of the MDT readout electronics 
(which for the MDTs is set to a nominal value of $820\ns$, slightly above 
the maximum drift time, to prevent the detection of secondary ionization 
clusters) can be reduced to the minimum value of $220\ns$, just above 
the maximum drift time of the sMDT tubes. In this way, the masking of 
muon hits by preceding background pulses is strongly reduced, increasing 
the muon detection efficiency (defined as the probability to find a hit on 
the extrapolated muon track within 3 times the drift tube resolution,
i.e.\ the $3\sigma$ efficiency).  Figure~\ref{fig:efficiency} shows
the superior $3\sigma$ efficiency of sMDT tubes at high background
counting rates compared to MDT tubes.  Muon track segment
reconstruction efficiencies of almost $100\%$ and a segment position
resolution of better than $40\mum$ are achieved with 8-layer 
sMDT chambers at the maximum background rates expected at the HL-LHC.


\subsection{Drift tube design and fabrication}

The sMDT chamber design and construction procedures have been
optimized for mass production, and for providing a very high
mechanical accuracy in the sense wire positioning.  Standard
industrial aluminium tubes with $15\mm$ outer diameter and a wall
thickness of $0.4\mm$ are used. The tubes are chromatized 
on the in- and outside for cleaning purposes and reliable
electrical ground contact. The ground pins are screwed into the
triangular holes between adjacent tubes during the gluing of
the tube layers (see Figs.~\ref{fig:endplug1} and \ref{fig:endplug2}).
The drift tube design and fabrication procedures are the same as used
for the construction of the BMG sMDT chambers in
2016~\cite{BMGconstr,BMGproduction}.

\begin{figure}[p]
%\vspace{-18mm}
\includegraphics[width=\textwidth]{sMDT_endplug_exploded.pdf}
\caption{Exploded view of an sMDT endplug with interfaces for precise
  wire positioning and measurement, for gas and high-voltage supplies
  and for readout electronics~\cite{sMDTprototype}.}
\label{fig:endplug1}
%\end{figure}

\vspace{2mm}

%\begin{figure}[th]
\centering
\includegraphics[angle=-90,width=0.7\textwidth]{sMDT_endplug_cross_section_labels_2.pdf}
\caption{Cross section of an sMDT endplug with internal wire locator
  and external reference surface for tube and wire positioning during
  construction and for wire position measurement~\cite{BMGconstr}.}
\label{fig:endplug2}
\end{figure}

\begin{figure}[tb]
\centering 
\includegraphics[width=0.9\textwidth]{wiring_station.pdf}
\caption{Semi-automated drift-tube assembly station in a climatized clean room.}
\label{fig:wiring}
%\end{figure}

\vspace{1cm}

%\begin{figure}[tb]
\centering
\includegraphics[width=0.49\textwidth]{wire_crimping_and_tube_swaging_tool.pdf}\hfill
\includegraphics[width=0.49\textwidth]{wire_tensioning_and_crimping_tool.pdf}

\caption{Automated tools for mechanical swaging of the aluminum tubes
  onto the O-rings of the endplugs (left) and for crimping of the
  sense wire in heat-treated copper crimp tubelets, inserted into the
  central brass pins of the endplugs (right).  This operation takes
  place after feeding the wire through the tube by air flow, without
  any manual intervention, and after automated tensioning of the wire
  using a stepping motor controlled by a strain gauge (right).}
\label{fig:swaging_tensioning}
	
\end{figure}


The drift tubes are assembled using a semi-automated wiring station in
a temperature-controlled clean room (see Fig.~\ref{fig:wiring}).  The
endplugs are inserted into the tubes, and the sense wires are fed
through tubes and endplugs by means of air flow without any manual
intervention. After that, the endplugs are fixed by swaging (see
Fig.~\ref{fig:swaging_tensioning}). Finally, the wires are fixed in
copper crimp tubelets, inserted in the endplug central pins after
tensioning them to $350\pm 15\g$ (see
Fig.~\ref{fig:swaging_tensioning}, right), including over-tensioning to
$430\g$ for $10\s$. This tension results in a gravitational sag of
only $17\pm 1\mum$ (absolute tolerances) for $1\m$ long tubes. The
wires are positioned at each tube end to a few micron precision
with respect to a cylindrical external reference surface on the
central brass insert of the endplug. This insert also holds the
spiral-shaped wire locator on the inside of the tube.  The drift tubes
are sealed at the endplugs using two O-rings per endplug and
mechanical swaging of the tube walls. For the injection-molded endplug
insulators and gas connectors of the individual tubes, plastic
materials with minimum out-gassing have been selected which are also
immune against cracking.



\begin{figure}[p]
\centering
\includegraphics[width=0.9\textwidth]{assembly_station.pdf}
\caption{Assembly of a BMG sMDT chamber. The first multilayer has been
  completed, the alignment sensor platforms have been mounted on the
  inside of the multilayer, and the spacer frame is positioned on top,
  ready for the assembly of the second multilayer.  The wire grids at
  each chamber end are defined by the precisely machined pattern of
  holes into which the cylindrical endplug reference surfaces are
  inserted. The jigs have been stacked together with the tube layers.
  Special, precisely machined bars (with ribs) define the distance 
  between the two multilayers where the spacer frame inserted~\cite{BMGnote}.}
\label{fig:assembly}
%\end{figure}

\vspace{1cm}

%\begin{figure}[th]
%	\centering
\includegraphics[width=0.9\textwidth]{glueing.pdf}
\caption{Glue distribution between adjacent tubes of a tube layer
  during BMG chamber assembly (second multilayer) using an automated
  glue dispenser.}
\label{fig:glueing}
\end{figure}

\begin{figure}[p]
\centering
\includegraphics[angle=90,width=0.85\textwidth]{bis78_jig_drawing.pdf}
\caption{Drawing of the precision jigging for the BIS78 and BIS1--6
  chamber construction and illustration of the steps of the assembly
  procedure.  The brackets for the precise mounting of the alignment
  sensor platforms are an integral part of the jigging design.  The
  platforms for endcap alignment sensors connecting to the NSWs are
  mounted on the bottom of the first multilayer in the first assembly
  step and are only present for BIS78. The axial, praxial, and CCC
  barrel alignment sensors are mounted, together with the in-plane
  alignment system, on the outside of the top multilayer at the end of
  the chamber assembly.}
\label{fig:jig_drawing}
\end{figure}

\begin{figure}[tb]
\centering
\includegraphics[width=\textwidth]{bis78_comb_drawing.pdf}
\caption{Drawing of the assembly jig for BIS chambers with the
  precision-machined hole grids for the endplugs of the drift tubes.}
\label{fig:comb_drawing}
%
\vspace{1cm}

\includegraphics[width=0.9\textwidth]{bis7_jig_photo.pdf}
\caption{Photograph of the assembly jig for BIS chambers with the
  precision-machined hole grids for the endplugs of the drift tubes.}
\label{fig:bis7_jig_photo}
\end{figure}

Only materials already certified for ATLAS MDT chambers are used
for sMDT drift tubes and their gas connections in order to prevent
aging.  No out-gassing of the plastic materials of endplugs (PBTP
Crastin LW9330, reinforced with 30\% glass fiber) and gas connectors
(PBTP Crastin S600F20, unreinforced) has been observed.  The sMDT
tubes, including the plastic material of the endplugs, have been
irradiated with a $200\MBq$ ${}^{90}$Sr source over a period of 4
months with a total charge accumulated on the sense wire of $9\Ccm$,
15 times the requirement for the ATLAS MDT chambers for ten years of
operation at the LHC design luminosity,
without any sign of aging~\cite{sMDTperf3, aging}.

Typical production rates of 100 tubes per day have been achieved with
one assembly station operated by two technicians, at an average
(mechanical) failure rate of about $4\%$, which is mostly due to
occasional failures of the assembly devices~\cite{BMGnote}.  The
failure rate in the subsequent quality assurance tests of wire tension
($350\pm 15\g$), gas leak rate (better than $10^{-8}\mathrm{\ bar\cdot
  l/s}$ and leakage current (less than $2\mathrm{\ nA/m}$ at the
nominal operating voltage of $2730\V$) was as low as
$2\%$~\cite{BMGnote}, mostly due to too high leakage currents.
Similar behaviour is found for the ongoing drift tube production 
for the BIS~7/8 chambers~\cite{BIS78note}.

\subsection{sMDT chamber construction and test}

After passing the quality assurance tests, the drift tubes are
assembled into chambers in a climatized clean room. The endplug
reference surfaces are inserted into a grid of bores in the assembly
jigs at each chamber end. The bores define the wire positions with an
accuracy of better than $5\mum$ (see Fig.~\ref{fig:assembly}). The
tubes are glued to each other and to the spacer and support frame
using an automated glue dispenser (see Fig.~\ref{fig:glueing}). One
complete chamber can be assembled within two working days, including
the precise mounting of the alignment sensor platforms.  The precisely
machined assembly jig used for the BIS78 chambers, which will also be
used for the BIS1--6 chambers, is shown in
Figs.~\ref{fig:jig_drawing}, \ref{fig:comb_drawing}, and
\ref{fig:bis7_jig_photo}~\cite{BIS78note}. The same two-component epoxy glues as for
the MDT chamber construction are used, Araldite~2014 with low enough 
viscosity and long enough curing time to be deposited with the glue 
dispenser in the grooves between neighbouring tubes in an already 
assembled layer and DP~490 with high viscosity and high peeling strength 
to glue the multilayers to the spacer and support structures.  

After the gluing of each new tube layer, gold-plated ground connection 
screws are inserted, through holes in the jig, into the triangular gaps between 
adjacent tube layers not yet filled with glue,
scratching the chromatized tube walls and making reliable 
electrical contact as has been verified in accelerated aging tests.
The gaps are afterwards filled with the glue deposited for the glueing 
of the next tube layer, which runs through the glue gaps between neigbouring 
tubes and fixes and encapsulates the ground screws. 
Accelerated aging tests and beam tests of the BME chambers showed that reliable 
long-term, low-impedance ground contacts and interconnections between the tube walls 
and the tube layers are provided in this way.
The locations of the ground screws are shown in Fig.~\ref{fig:ground_screws_FC} (left).

On the multilayers of the BIS chambers, $0.2\mm$ thick chromatized aluminium ground foils 
with cutouts for the signal caps of the endplugs  will be mounted which serve as 
back planes of the Faraday cages, like in the MDT chambers, providing a common
ground connection between the tubes of a multilayer close to the tube walls.
The layout of the BIS Faraday cages is shown in Fig.~\ref{fig:ground_screws_FC} (right).
A laser cutting method has been developed which allows for the fabrications of the foils.
The concept has been successfully tested mechanically and electrically on the BMG 
module-0 chamber.

After mounting of the gas connections,
ground pins connecting to the readout and high-voltage distribution
boards are screwed onto the ground screws between the first and second and the 
third and fourth tube layer of a multilayer (see Fig.~\ref{fig:stacked_mezzanine_card}) 
providing good ground contacts between tube and Faraday cage walls and the electronics.

\begin{figure}[tb]
\centering
\includegraphics[width=0.49\textwidth]{BME_ground_screws_4.pdf}\hfill
\includegraphics[width=0.49\textwidth]{Faraday_cage_BIS.pdf}

\caption{Left: Pattern of the ground screws inserted in the gaps between adjacent sMDT 
tube layers. Right: Schematic view of the cross section through the Faraday cage of 
the BIS chambers consisting of $0.5\mm$ thick chromatized aluminium walls, 
a $0.2\mm$ thick chromatized aluminium ground foil serving as back plane and $0.5\mm$
thick chromatized aluminium shielding plates integrated between hedgehog and mezzanine cards
on the readout side (see Fig.~\ref{fig:stacked_mezzanine_card}).
The ground screws between adjacent tube layers and the ground pins connecting to the
signal and high-voltage distribution (hedgehog) cards in the top and bottom layer of
ground screws are indicated.}
\label{fig:ground_screws_FC}
\end{figure}


\begin{figure}[tb]
\centering
\includegraphics[width=0.9\textwidth]{sMDT_mezzanine_card_BMG.pdf}
\caption{sMDT signal distribution (hedgehog) board with stacked active
  readout electronics (mezzanine) card carrying three ASD chips in the
  middle layer and the HPTDC with programming FPGA in the top
  layer. The board is mounted on the gold-plated signal and ground
  pins of the drift tubes of a BMG chamber. The signal hedgehog
  boards also contain the protective network for the ASD chips and are
  shielded against the mezzanine cards by aluminum plates which are
  connected to the outer Faraday cage.  The coupling capacitors are
  encapsulated in injection-molded white plastic containers to prevent
  discharges and minimize leakage currents under the high voltage.
  The analog and digital ground planes of the ASD and TDC boards, 
  respectively, are connected separately to the Faraday cage.}
\label{fig:stacked_mezzanine_card}
\end{figure}

Like the BME sMDT chambers, but in contrast to the BMG chambers, the
BIS78 and BIS1--6 sMDT chambers will have in-plane alignment
monitoring systems, measuring longitudinal sag and torsion between the
readout and high-voltage ends of the chambers. In addition to the
axial, praxial, and CCC sensors of the barrel alignment system which
will be mounted on the outer multilayer of all new BIS sMDT chambers
in the same positions as on the present BIS MDT chambers, the BIS78
chambers also carry endcap alignment sensors on the inner multilayer
for connecting them to the NSWs (see Fig.~\ref{fig:BIS78_3D_sMDT}).
The alignment sensor platforms are mounted with $20\mum$ positioning
accuracy with respect to the sense wires on the outer tube layers
using the precision assembly jigging (see Fig.~\ref{fig:jig_drawing}).

\begin{figure}[p]
\centering
\includegraphics[angle=90,width=0.95\textwidth]{BIS78_3D_sMDT.pdf}
\caption{Drawings of a BIS78 sMDT chamber with the alignment sensors
  mounted on platforms on the outside of the two multilayers.}
\label{fig:BIS78_3D_sMDT} 
\end{figure}

\begin{figure}[p]
\centering
\includegraphics[width=0.9\textwidth]{CMM_measurement.pdf}
\caption{Measurement of the sense wire and alignment platform
  positions of an sMDT chamber (BMG, with cutouts for alignment
  rays) with a coordinate measuring machine (CMM). Individual wire
  positions are measured with an accuracy of $2\mum$. The geometry
  parameters of the sense wire grids at the tube ends can be fully
  reconstructed.  The wire positions on the readout and high-voltage
  distribution ends of the chamber are measured in the same coordinate
  frame, allowing for the reconstruction of global deformation
  parameters like torsion between the two ends.}
\label{fig:CMM}

\vspace{8mm}

\includegraphics[width=0.6\textwidth]{BMG3_residuals_r.pdf}
\caption{Residuals of the sense wire positions measured at both ends
  of a BMG sMDT chamber with 356 tubes with respect to the nominal
  wire grid.  The width of the distribution includes the accuracy of
  the coordinate measuring machine of about $2\mum$~\cite{BMGconstr}.}
\label{fig:residuals}
\end{figure}


\begin{figure}[p]
\centering
\includegraphics[width=0.49\textwidth]{Gassystem_BIS78_2.pdf}\hfill
\includegraphics[width=0.49\textwidth]{Gassystem_BIS78_1.pdf}
	
\caption{Schematics of the gas distribution system of the BIS78 and
  BIS1--6 sMDT chambers (left: 3D model, right: cross section).  The
  concept is the same as for the BME and BMG chambers. It uses
  injection molded plastic gas connectors made of Crastin S600F20
  (PBTB), the same materials as used for the endplug plastic
  parts~\cite{BMGconstr} (see Fig.~\ref{fig:endplug1}), to connect the
  tubes in columns perpendicular to the chamber plane to the aluminum
  gas bars mounted along each multilayer on the HV and RO side.}
\label{fig:bis78_gassystem}

\vspace{1cm}

\includegraphics[width=0.9\textwidth]{bis78_gassystem_photo.pdf}
\caption{Photograph of the test module of the BIS sMDT gas
  distribution system.}
\label{fig:bis78_gassystem_photo}

\end{figure}

\begin{figure}[tb]
\centering
\includegraphics[height=4.5cm]{sMDT_HVHH_BMG.pdf}\hfill
\includegraphics[height=4.5cm]{sMDT_ROHH_BMG.pdf}
\caption{sMDT high-voltage (left) and signal (right) distribution
  boards (hedgehog boards).  The termination resistors on the
  high-voltage side and the coupling capacitors on the readout side
  are encapsulated in injection-molded white plastic containers in
  order to prevent discharges and to minimise leakage currents under the high voltage.} 
\label{fig:hedgehog}

\end{figure}


After the gluing of the tube layers, the positions of the individual
endplug reference surfaces (and thus of the sense wires) are measured
at the two chamber ends with an automated coordinate measuring machine
(CMM) with a precision of about $2\mum$ (see Fig.~\ref{fig:CMM}). This
measurement, which takes about 1--2 hours, was performed for every BME
and BMG chamber and is planned as a regular spot check during the BIS
chamber series production.  In addition, the positions of the
alignment sensor platforms with respect to the wire grid can be
measured with the same accuracy.  The measurements show that sense
wire positioning accuracies of better than $10\mum$ (r.m.s.) have been
routinely achieved during BME and BMG chamber
construction~\cite{BMEop, BMGconstr}.  An ultimate wire positioning
accuracy of $5\mum$ (r.m.s.) has been achieved in the BMG chamber
construction, which is close to the precision of the assembly jigs
(see Fig.~\ref{fig:residuals}). All BMG sMDT chambers have a wire
positioning accuracy of better than $10\mum$ with an average of
$7\mum$~\cite{BMGnote, BMGconstr}.
 
After the wire position measurement, the parallel gas distribution
system is mounted, consisting of modular injection-molded plastic gas
connectors connecting tubes in columns perpendicular to the chamber
plane to the chromatized aluminium gas distribution 
bars~\cite{sMDTprototype, BMGnote, BIS78note} (see
Figs.~\ref{fig:endplug1} and \ref{fig:bis78_gassystem}).  Gas leak
rates below the limit of $2n \times 10^{-8}\mathrm{\ bar\cdot l/s}$, 
required for a chamber with $n$ tubes at $3\bar$ pressure, have been achieved 
for all BMG chambers~\cite{BMGconstr, BMGnote}. 
It has been observed for the BMG chambers, that neighbouring signal or 
high-voltage distribution boards (see next section), which touch each other 
because their dimensions exceed the fabrication tolerances, 
may tilt the gas connectors on the endplugs and introduce gas leaks. 
For the BIS78 chamber electronics production, the gaps between neighbouring 
electronics boards have been increased to prevent such conflicts. 

Minor modifications of the gas connectors and gas bars compared to the BMG chambers 
had to be implemented due to the small available space in the BIS layer~\cite{BIS78note}.  
The new injection-molded connectors have been produced and successfully
tested (see Fig.~\ref{fig:bis78_gassystem_photo}).


\subsection{sMDT chamber electronics}

After the installation of the gas distribution system, ground pins,
and Faraday cages, the high-voltage (HV) and signal distribution
boards (see Fig.~\ref{fig:hedgehog}), as well as the active readout
(RO) electronics cards (mezzanine cards) with $6\times 4$ channels
are mounted on the HV and RO ends of the chambers, respectively (see
Figs.~\ref{fig:HV_side} and \ref{fig:RO_side}). The decoupling
capacitors on the RO side and the terminating resistors on the HV side
are enclosed in plastic containers in order to guarantee HV
stability. The mezzanine cards, stacked on top of the signal
distribution boards, contain three 8-channel ASD chips and a TDC chip
which digitizes the threshold crossing times of the signal pulses,
multiplexes the data, and transmits them to the CSM (chamber service
module) via flat twisted-pair signal cables (the blue cables in
Fig.~\ref{fig:BMG_cables}) which fulfill the same specifications as
the original MDT signal cables.  The sMDT readout boards which will be
used for the BMG and BIS78 chambers until the end of Run 3 are shown
in Fig.~\ref{fig:stacked_mezzanine_card}).  They contain three of the
current MDT ASD chips together with a CERN HPTDC
chip~\cite{HPTDC_manual} as well as an FPGA for JTAG programming and
provide the same functionality as the original MDT mezzanine cards.
The same board layout, but with new versions of the ASD and TDC chips,
will be used for the sMDT chambers in Phase II.

\begin{figure}[p]
\centering
\includegraphics[width=0.9\textwidth]{HV_side.pdf}
\caption{HV distribution boards~\cite{sMDTprototype} for $6\times 4$
  channels mounted on the signal and ground pins of the drift tubes on
  the high-voltage side of a BMG chamber inside the Faraday cage.}
\label{fig:HV_side}

\vspace{2cm}

\includegraphics[width=0.9\textwidth]{RO_side.pdf}
\caption{Signal distribution and active readout electronics
  (mezzanine) boards~\cite{BMGproduction} with $6\times 4$ channels
  mounted on the signal and ground pins of the drift tubes on the
  readout side of a BMG chamber inside the Faraday cage (see also
  Fig.~\ref{fig:stacked_mezzanine_card}).}
\label{fig:RO_side}

\end{figure}

\begin{figure}[tb]
\centering
\includegraphics[width=0.9\textwidth]{BMG_cables.pdf}
\caption{Readout side of BMG chambers during commissioning at CERN
  with closed Faraday cages and flat (blue) twisted-pair signal cables
  connecting the mezzanine cards to the externally mounted readout
  boxes (CSMs).}
\label{fig:BMG_cables}
\end{figure}
	
\begin{figure}[tb]

\includegraphics[width=\textwidth]{BIS_sMDT_layout_2.pdf}
\caption{Layout example of the BIS sMDT layer with two neighboring
  BIS1 and BIS2 chambers.}
\label{fig:BIS_sMDT_layout}
		
\end{figure}


\subsection{BIS sMDT chamber design}


The design and construction of new BIS78 sMDT chambers with $2\times
4$ tube layers and integrated thin-gap RPCs for the replacement of the
current BIS7 and BIS8 MDT chambers in LS2 is a pilot project for the
complete replacement of the BIS MDT chambers by integrated sMDT and
RPC chambers for the HL-LHC.  There are 12 different types of BIS78
sMDT chambers with complex shapes, maximizing the coverage in the
overlap region with the New Small Wheels~\cite{sMDT_parameterbook}.
Drawings of a BIS78 sMDT chamber are shown in
Fig.~\ref{fig:BIS78_3D_sMDT}.  The design of the remaining new BIS1-6
sMDT and RPC chambers will be very similar to the BIS78 design,
somewhat simpler due to their rectangular shape and only two different
chamber types.  The parameters of the BIS1-6 sMDT chambers are
summarized in Tables~\ref{tab:bis16_parameters} and
\ref{tab:bis16_numbers}. A tentative BIS sMDT chamber layout is shown
in Fig.~\ref{fig:BIS_sMDT_layout}.  The same assembly procedure and
precision tooling will be used both for the BIS78 and for the BIS1--6
sMDT chambers.

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


  \caption{Geometrical parameters of the BIS1--6 sMDT chambers. 
   The right-handed chamber coordinate system is defined as for the ATLAS MDT chambers, 
   with the x axis pointing along the tubes and the z axis perpendicular to the tubes 
   within the chamber plane.}
\label{tab:bis16_parameters}
\vspace{2mm}

\begin{tabular}{|l|c|c|}
\hline
Type & BIS1 & BIS2--6 \\
\hline\hline
Number of chambers & 16 & 80 \\
Radial distance from beam axis (mm) & 4550 (4635)$^\ast$ & 4550 (4635)$^\ast$ \\
\hline
Chamber width in $z$ (mm)  & 1113 &  933 \\
Tubes width in $z$ (mm)    & 1081 &  901 \\
Chamber length in $x$ (mm) & 1839 & 1839 \\
Aluminum tube length (mm) & 1660 & 1660 \\
Assembled tube length (mm) & 1669 & 1669 \\
Active tube length (mm) & 1640 & 1640 \\ 
Active area/chamber (m$^2$) & 1.77 & 1.48 \\
\hline
Tube layers & $2\times 4$ & $2\times 4$ \\
Tubes/layer & 72 & 59 \\
Tubes/chamber & 576 & 472 \\
Spacer height (mm)  & 45.6 & 45.6 \\
Multilayer height (mm)   & 139 & 139 \\
Chamber height (mm) & 249 & 249 \\
\hline
Gas volume/chamber (l) & 151.4 & 124.1 \\
Chamber weight (kg)    & 100 &  86 \\
\hline
Mezz.~cards (24 ch.)/chamber  & 24 & 20 \\
Mezz.~cards/CSM~1 (inner ML1) & 12 & 10 \\
Mezz.~cards/CSM~2 (outer ML2) & 12 & 10 \\
CSMs/chamber & 2 & 2 \\
T sensors/chamber             & 10 & 10 \\
B-field sensors/chamber       &  2 &  2 \\
In-plane alignment systems/chamber &  4 &  4 \\
Axial alignment sensors/chamber &  2 &  4 \\
Praxial alignment sensors/chamber &  4 &  8 \\
CCC alignment sensors/chamber &  0 &  2 \\
Survey targets/chamber        &  4 &  4 \\
\hline
  
\end{tabular}

\vspace{1mm}

$^\ast$ Numbers in brackets refer to sectors 2 and 16.

\end{center}
%\end{table}

\vspace{1mm}

%\begin{table}[tb]
\begin{center}

\caption{BIS1--6 sMDT chambers in numbers}

\vspace{2mm}
\label{tab:bis16_numbers}

\begin{tabular}{|l|c|}
\hline
Parameter & Value \\
\hline\hline
Number of chambers & 96 \\
Number of tubes & 46976 \\
Total tube and wire length & $78\km$ \\ 
Chamber active area & $145\msq$ \\
Gas volume & $12.4\mcu$ \\
Chamber weight & $8.5\tons$ \\
Number of mezzanine cards & 1984 \\
Number of hedgehog boards & 3968 \\
Number of CSMs & 192 \\
\hline 

\end{tabular}

\end{center}
\end{table}



The challenges for the mechanical design of the BIS sMDT chambers are
the small available space that needs to be shared with the new RPC
chambers, and minimizing the gravitational deformations of the thin
chambers with minimal space available for support structures.  The RPC
chambers are mounted on their own support frame and will be installed
on the existing rails in the BIS layer independent of the sMDTs, in
order to prevent distorting the BIS sMDT chambers.  The BIS1-6 sMDT
chambers are each reduced in width by one drift tube per layer
compared to the size of the original BIS MDT chambers, to provide a
gap of $20\mm$ between adjacent chambers for the RPC supports.  The
resulting tube layout of a sector of the new BIS sMDT, including the
BIS78 chambers, is shown in Fig.~\ref{fig:BIS_layout}.

The chamber readout service modules (CSMs) will be mounted on the
toroid coils as close as possible to the chambers such that the signal
cable length between mezzanine cards and CSMs does not exceed $4\m$
which is required for reliable signal transmission.

\begin{figure}[p]
\centering
\includegraphics[angle=90,width=0.94\textwidth]{BIS_layout.pdf}
\caption{Layout of a complete BIS sMDT sector. Left: view from the
  readout electronics side; right: view from the inside of the ATLAS
  detector towards the rail supports (shown only for the BIS78
  chamber).}
\label{fig:BIS_layout}
\end{figure}


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\end{thebibliography}
\end{document}