\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
(Fig.~\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 $300\kHzt$
  are the maximum background rates for which the MDT chambers have been designed.
  $140\Hzcm$ and $33\kHzt$ as well as $280\Hzcm$ and $65\kHzt$ are the maximum 
  background rates expected in the BIS1-6 and in the BIS7/8 chambers (with active tube 
  length of $1640\mm$), respectively, 
  in the small sectors of the barrel inner (BI) layer of the ATLAS muon spectrometer 
  at the HL-LHC (see Fig.~\ref{MDT:limitations}). 
  The values of spatial resolution 
  and muon detection efficiency of the drift tubes given correspond to readout with 
  standard MDT readout electronics with nominal deadtime settings of $820\nm$ and $220\nm$
  for MDT and sMDT chambers, respectively (see Figs.~\ref{fig:resolution} and \ref{fig:efficiency}).
  For given tube lenght and background hit fluence, sMDT tubes experience half the 
  counting rate of MDT tubes.
  }
\label{tab:parameters}
\vspace{2mm}

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

\hline
Type & MDT & sMDT \\
\hline\hline
Tube material & Aluminum & Aluminum \\
              & 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
Wire positioning accuracy & $20\mum$ r.m.s. & $10\mum$ r.m.s. \\
\hline
Average tube spatial resolution & & \\
\hspace{5mm}without background irradiation& $83\mum$ & $106\mum$ \\
\hspace{5mm}at $140\Hzcm$ background rate & $107\mum$ & $107\mum$ \\
\hspace{5mm}at $280\Hzcm$ background rate & $115\mum$ & $108\mum$ \\
\hspace{5mm}at $500\Hzcm$ background rate & $130\mum$ & $110\mum$ \\
\hline
Drift tube muon efficiency  &  &  \\ 
\hspace{5mm}without background irradiation & $95\%$ & $94\%$ \\
\hspace{5mm}at $33\kHzt$ counting rate& $91\%$ & $93\%$ \\ 
\hspace{5mm}at $65\kHzt$ counting rate& $86\%$ & $92\%$ \\ 
\hspace{5mm}at $300\kHzt$ counting rate& $76\%$ & $84\%$ \\ 
\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{sMDTperf21,sMDTperf22, 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 the Gamma Irradiation Facility (GIF) at CERN using the existing 
ATLAS MDT readout electronics with bipolar
shaping~\cite{sMDTperf1, MDTperf01, MDTperf02, sMDTperf21, sMDTperf22}. 
The new MDT and sMDT amplifier-shaper-discriminator (ASD) chips for 
Phase II 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 $20\%$ (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{blr1, blr2}. 
  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 tubeswith
  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{blr1,blr2}. 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 (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 $7.5\mm$ (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}. Values relevant for the BIS layer 
at HL-LHC are given in Table~\ref{tab:parameters}.
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 (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 $220\ns$, 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
the 8-layer BIS sMDT chambers at the maximum background rates expected 
at the HL-LHC, resulting in a momentum resolution in the small barrel 
sectors which is as good as the present one. 

The rate capability of the MDT chambers is fully adequate 
for the background rates expected at HL-LHC in all regions of the muon 
spectrometer except for the EI layer where they will be replaced by the
detectors of the New Small Wheels. sMDT chambers with their smaller tube 
diameter are required for the upgrade of the BIS layer only to 
make space for the additional RPC chambers. Like the sMDT chambers, the 
MDT chambers can also be operated at the minimal adjustable deadtime 
of the ASD chip of $220\ns$ in order to improve the muon detection 
efficiency in the presence of background radiation, however at the cost
of higher required readout bandwidth.


\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 aluminum 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 (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 (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
(Fig.~\ref{fig:swaging_tensioning}). Finally, the wires are fixed in
copper crimp tubelets, inserted in the endplug central pins after
tensioning them to $350\pm 15\g$ (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 behavior 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 $5\mum$ (Fig.~\ref{fig:assembly}). The
tubes are glued to each other and to the spacer and support frame
using an automated glue dispenser (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 viscosity and long curing time so that it can
be deposited with the glue dispenser in the grooves between
neighboring 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.  The gaps are afterwards filled with the glue deposited for
the gluing of the next tube layer, which runs through the glue gaps
between neighboring tubes and fixes and encapsulates the ground
screws.  Accelerated aging tests and beam tests of the BME chambers
have shown 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).

\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: Cross section of the
  Faraday cage of the BIS chambers consisting of $0.5\mm$ thick
  chromatized aluminum walls, a $0.2\mm$ thick chromatized aluminum
  ground foil serving as back plane, and $0.5\mm$ thick chromatized
  aluminum shielding plates integrated between hedgehog and mezzanine
  cards on the readout side (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}


On the multilayers of the BIS chambers, $0.2\mm$ thick chromatized
aluminum ground foils with cutouts for the signal caps of the
endplugs will be mounted. They 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 for
the fabrications of the foils.  The concept has been successfully
tested mechanically and electrically on the BMG module-0 chamber.

After mounting the gas connections, ground pins connecting to the
readout and high-voltage distribution boards are screwed onto the
ground screws which are accessible in spite of the gas connectors, i.e.\ 
on all screws between the first and second and between the 
third and fourth tube layer of a multilayer (Fig.~\ref{fig:stacked_mezzanine_card}) providing good ground contacts
between tube and Faraday cage walls and the electronics. 
It is not necessary to have ground pins on all screw connections 
between tube walls and tube layers.

\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 (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 (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} (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}

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 $2\mum$ (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
(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 aluminum gas distribution
bars~\cite{sMDTprototype, BMGnote, BIS78note}
(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 neighboring signal or high-voltage
distribution boards, when touching each other because their dimensions
exceed the fabrication tolerances, may tilt the gas connectors on the
endplugs and create gas leaks.  For the BIS78 chamber electronics
production, the gaps between neighboring 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 (Fig.~\ref{fig:bis78_gassystem_photo}).


\subsection{sMDT chamber electronics}

\begin{figure}[tbp]
\centering
\ \hfill\includegraphics[height=4cm]{sMDT_HVHH_BMG.pdf}\hfill
\includegraphics[height=4cm]{sMDT_ROHH_BMG.pdf}\hfill\
\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 minimize leakage currents under
  high voltage.}
\label{fig:hedgehog}

\vspace{4mm}

\centering
\includegraphics[width=0.77\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{4mm}

\centering
\includegraphics[width=0.77\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}
	
After the installation of the gas distribution system, ground pins,
and Faraday cages, the high-voltage (HV) and signal distribution
boards (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 (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.


\subsection{BIS sMDT chamber design}

\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}

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 local chamber coordinate system is defined in the same
  way as for the ATLAS MDT chambers, with the $x$ axis pointing along
  the tubes, the $z$ axis perpendicular to the tubes inside the
  chamber plane, and the $y$ axis pointing out of the chamber plane,
  and away from the IP when the chamber is installed in ATLAS.}
\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}


\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}


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.


%\subsection{Alignment}
