
\section{sMDT chambers}
%
The ATLAS Monitored drift tube (MDT) chambers provide reliable muon tracking with excellent spatial resolution and high tracking efficiency independent of the track incident angle.
Small-diameter muon drift tube (sMDT) chambers~\cite{sMDTperf1} with a tube diameter of 15~mm, i.e.\ half of the tube diameter of the MDT chambers, 
have been developed since 2008 to cope with the higher background irradiation rates at HL-LHC and to allow for more tube layers or additional trigger chambers
under the tight space constraints in the muon spectrometer. Standard aluminium tubes with 0.4~mm wall thickness like for the MDT chambers are used.
Figure~\ref{fig:tubes} shows a comparison of the MDT and the sMDT drift tubes. The sMDT chambers are operated in ATLAS with the same gas, gas pressure and gas gain
as the MDT chambers. Table~\ref{tab:parameters} shows a comparison of the MDT and sMDT operating paremters.
The drift time spectra are shown in Figure~\ref{fig:spectrum}. The maximum drift time
of the sMDT tubes is only 175~ns compared to about 720~ns of the MDT chambers leading, together with the twice smaller cross section exposed to the radiation, 
to about 8 times lower occupancy and a linear space-to-drift time relationship with the standard MDT drift gas Ar:CO$_2$ (93:7) at 3 bar pressure.

%
\begin{figure}[h]
\centering
\includegraphics[width=0.6\columnwidth]{tubes}
\caption{Picture of MDT (left) and sMDT (right) tubes.}
\label{fig:tubes}
\end{figure}


\begin{table}[h]

\begin{center}

\caption{Materials and operating parameters of ATLAS sMDT chambers compared to MDT chambers. 
500 Hz/cm$^2$ and 200 kHz/tube are the maximum background rates expected in the barrel inner layer of the ATLAS muon spectrometer at 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 chromatisation \\
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 & $50~\mu$m & $50~\mu$m \\
with gold plating thickness & 3\% & 3\% \\        
Wire pitch & 30.035~mm & 15.099~mm \\
\hline
Gas mixture & Ar:CO$_2$ (93:7) &  Ar:CO$_2$ (93:7) \\
Gas pressure & 3 bar (abs.) & 3 bar (abs.) \\
Gas gain & $2\cdot 10^4$ & $2\cdot 10^4$ \\
Wire potential & 3080~V & 2730~V \\
Maximum drift time & 720~ns & 175~ns \\
\hline
Av. tube spatial resolution & $83~\mu$m & $106~\mu$m \\
without backgr.\ irradiation & & \\
Av. tube spatial resolution & $160~\mu$m & 110~$\mu$m \\
at 500 Hz/cm$^2$ backgr.\ rate & & \\
Drift tube muon efficiency  & 95\% & 94\% \\ 
without backgr.\ irradiation & & \\
Drift tube muon efficiency  & 80\% & 90\% \\ 
at 200 kHZ/tube backgr.\ rate & & \\
Wire positioning accuracy & $20~\mu$m rms & $20~\mu$m rms \\
\hline

\end{tabular}

\end{center}

\end{table}

\begin{figure}[tb]
\centering
\includegraphics[width=0.6\columnwidth]{spectrum}
\caption{Drift time spectra of MDT (green) and sMDT tubes (grey) together with the prediction for sMDT tubes from the GARFIELD simulation (red line)~\cite{sMDTperf1}.}
\label{fig:spectrum}
\end{figure}

\begin{figure}[tb]
\centering
\includegraphics[width=0.6\columnwidth]{gasgain}
\caption{Measurements of the relative gas gain of MDT and sMDT as a function of the background $\gamma$ flux with respect to the nominal gas gain of 20000 in each case compared to 
predictions based on the Diethorn formula~\cite{sMDTperf3}.}
\label{fig:gasgain}
\end{figure}

\begin{figure}[tb]
\centering
\includegraphics[width=0.6\columnwidth]{rtrelation}
\caption{Space-to-drift time relationship of MDT drift tubes. The part for drift radii r below 7.5~mm, relevant for the sMDT tubes, is linear to good approximation.}
\label{fig:rtrelation}
\end{figure}

\begin{figure}[tb]
\centering
\includegraphics[width=0.8\columnwidth]{res_with_TS_new.pdf}
\caption{Spatial resolution with 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 without background irradiation~\cite{sMDTperf2,sMDTperf3}.
As expected, there is good agreement between 15 and 30~mm diameter tubes for drift radii below 7.5~mm.}
\label{fig: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} have been installed in the elevator regions of the barrel muon spectrometer,
each with two new RPC chambers, and are in operation since the start of LHC Run~2~\cite{BMEop}. In January 2017, 12 BMG sMDT chambers~\cite{BMGconstr},
have been installed in the detector feet in sectors 12 and 14 of the barrel muon spectrometer~\cite{BMEop,BMGproposal}. 

The construction of 16 BIS~7/8 sMDT chambers with integrated triple thin-gap RPC chambers has started. They will be installed at the ends of the BIS layers 
in the barrel inner layer in LS2 2019-20 in order to improve the trigger efficiency and the rate capability of the chambers in the transition regions 
between barrel and endcaps~\cite{BIS78proposal}. This phase~1 upgrade serves as pilot project for the complete replacement of the BIS~1-6 MDT chambers 
by sMDT-RPC chamber modules of very similar design as the BIS7/8 chambers in LS3. Together with the installation of additional thin-gap RPCs 
on the BIL MDT chambers, the robustness of the barrel muon trigger system will be greatly enhanced over the full lifetime of the experiment at the high
irradiation rates at HL-LHC. The installation of new BIS triple thin-gap RPCs of only 5~cm thickness becomes possible only by replacing the BIS MDT chmabers
by sMDT chambers which have about half the height. In addition, the sMDT chambers provide about an order of magnitude higher rate capability than the MDTs.



\subsection{Performance of sMDT chambers}
%
\begin{figure}[tb]
\centering
\includegraphics[width=0.8\columnwidth]{resolution}
\caption{Average spatial resolution of MDT and sMDT drift tubes as a function of the $\gamma$ background fluence rate measured at the Gamma Irradiation Facility at CERN.}
\label{fig:resolution}
\end{figure}

\begin{figure}[tb]
\centering
\includegraphics[width=0.8\columnwidth]{efficiency}
\caption{Muon detection efficiencies of  MDT and sMDT drift tubes with corresponding electronics deadtime settings (see text) as a function of the $\gamma$ background counting rate per tube.  
The efficiency is defined as the probablity 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.} 
\label{fig:efficiency}
\end{figure}

The performance of MDT and sMDT chambers has been extensively studied at the Gamma Irradiation Facility at CERN using the existing ATLAS MDT readout electronics with 
bipolar shaping~\cite{MDTperf, sMDTperf1, sMDTperf2}. For the (s)MDT amplifier-shaper-discriminator (ASD) chips at HL-LHC the same specifications will be used 
as for the  present system. The MDT chambers can be operated up to background rates of 500~kHz/cm$^2$ and 300~kHz per tube.  
At background fluxes above 500~Hz/cm$^2$, the gas gain drops by more than $20\%$  
(see Figure~\ref{fig:gasgain}) leading together with the effect of space charge fluctuations to rapid deterioration of the spatial resolution with increasing background flux. 
The limitations of the MDT chambers are overcome by using drift tubes with half the diameter of the ATLAS MDT tubes while leaving the operating parameters, 
Ar:CO$_2$ (93:7) gas mixture at 3 bar pressure and nominal gas gain of 20000 (for a wire potential of 2730~V with respect to the tube wall), unchanged~\cite{sMDTperf1}. 

As the space charge density inside the drift tubes is proportional to the third power of the tube radius, 15~mm diameter drift tubes show a significant gain drop only at 8 times higher 
background rates compared to 30~mm diameter drift tubes (see Figure~\ref{fig:gasgain}). 
At the same time, the deteriorating effect of space charge fluctuations on the spatial resolution is eliminated because the drift gas is linear to good approximation for drift radii below 7.5~\mm{} 
(see Figure~\ref{fig:rtrelation}). Without irradiation and associated space charge effects, the average sMDT drift tube resolution is $106\pm 2~\mu$m (see Figure~\ref{fig:resolr})
compared to $83\pm 2~\mu$m for the MDTs~\cite{sMDTperf3}.
The dependence of the average spatial resolution of MDT and sMDT drift tubes on the $\gamma$ background flux is shown in Figure~\ref{fig:resolution}. The spatial resolution deteriorates quickly
with increasing background flux for the MDTs while it is almost unaffected by space charge effects up to very high irradiation rates for the sMDTs. 

For the same background flux, the small-diameter drift tubes have 8 times lower occupancy than the 30~mm diameter tubes because of the 4 times shorter maximum drift time (see Figure~\ref{fig:spectrum})
and the twice smaller tube cross section exposed to the radiation. Because of the much shorter maximum drift time, the dead time 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 adjustable 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 ($3\sigma$ efficiency). Figure~\ref{fig:efficiency} shows the improvement of the $3\sigma$ efficiency of sMDT tubes at high 
background counting rates compared to the MDT tubes. 
Muon track segment reconstruction efficiencies of almost $100\%$ and a spatial resolution of better than $30~\mu$m can be achieved with 8-layer sMDT chambers at the maximum background rates 
expected at HL-LHC.


\subsection{Drift tube design and fabrication}
%
The sMDT chamber design and construction procedures have been optimized for mass production while they provide highest 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 chromatised on the in- and outside for the cleaning purposes 
and reliable electrical ground contact. The ground pins are screwed into the holes between adjacent tube triplets during the glueing of the tube layers (see Figure~\ref{fig:endplug1}).

The drift tubes are assembled using a semi-automated wiring station in a temperature-controlled clean room (see Figure~\ref{fig:wiring}). 
The endplugs are inserted into the tubes and the sense wires fed through the tubes and the endplugs (see Figs.~\ref{fig:endplug1} and \ref{fig:endplug2}) 
by means of air flow without manual contact (see Fig.~\ref{fig:insertion}). Afterwards the endplugs are fixed and and the tubes gas sealed by swaging (see Fig.~\ref{fig:swaging}). 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:tensioning})
corresponding to a gravitational sag of only $17\pm 1~\mu$m (absolute tolerances) including overtensioning to 430~g for 10~s.
The wires are positioned at each tube end with a few micron precision with respect to a cylindrical external reference surface
on the central brass insert of the endplug which also holds the spiral shaped wire locator on the inside of the tube. 
The drift tubes are sealed with the endplugs using two O-rings per endplug and mechanical swaging of the tube walls. For the injection molded endplug insulators 
and gas connectors for the individual tubes, plastic materials with minimum outgassing have been selected which are also immune against cracking. 
 
Only materials already certified for the ATLAS MDT chambers are used for sMDT drift tubes and their gas connections in order to prevent ageing.
No outgassing of the plastic materials of endplugs and gas connectors (Pocan) 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 accumulation on the sense wire of 9~C/cm without any sign of ageing~\cite{sMDTperf3,aging}.

\begin{figure}[th]
	\centering
	\includegraphics[width=\textwidth]{wiring_station}
	\caption{Semi-automated drift-tube assembly station in a climatised clean room.}
	\label{fig:wiring}
%\end{figure}
\vspace{5mm}
%\begin{figure}[th]
%	\centering
	\includegraphics[width=0.7\textwidth]{wire_and_endplug_insertion}
	\caption{Insertion of wires via air flow and of endplugs into tubes without manual interference.} 
	\label{fig:insertion}
\end{figure}

\begin{figure}[th]
	\centering
	\includegraphics[width=0.7\textwidth]{wire_crimping_and_tube_swaging_tool}
	\caption{Automated tools for mechanical swaging of the aluminium tubes on the O-rings in the endplugs for fixation and gas sealing and for crimping of the sense wire
	in heat-treated copper crimp tubelets inserted into the central brass pins of the endplugs.} 
	\label{fig:swaging}
%\end{figure}
\vspace{5mm}
%\begin{figure}[th]
%	\centering
	\includegraphics[width=0.7\textwidth]{wire_tensioning_and_crimping_tool}
	\caption{Tensioning of the sense wire by a stepping motor controlled by a strain gauge.} 
	\label{fig:tensioning}
\end{figure}

\begin{figure}[th]
	\centering
	\includegraphics[width=1.1\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}

\begin{figure}[th]
	\centering
	\includegraphics[angle=-90,width=0.6\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}[th]
	\centering
	\includegraphics[width=\textwidth]{assembly_station}
	\caption{Assembly of a BMG sMDT chamber. The first multilayer has been assembled, the alignment sensor platforms have been mounted on the inside of the multilayer 
	and the spacer frame is positioned on top, waiting for the assembly of the second multilayer on top.
	The wire grids at each chamber end are defined by the precisely machined grids of holes  
        into which the cylindrical endplug reference surfaces are inserted. The tube and corresponding jig layers are stacked, including the spacing between the two multilayers.}  
	\label{fig:assembly}
\end{figure}

\begin{figure}[th]
	\centering
	\includegraphics[width=\textwidth]{glueing}
	\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}[th]
	\centering
	\includegraphics[angle=-90,width=\textwidth]{bis78_jig_drawing}
	\caption{Sketch of the precision jigging and the steps for the assembly of the BIS~7/8 and BIS~1-6 chambers. 
	The brackets for the precise mounting of the global alignment sensor platforms are an integral part of the jigging design. The sensors for alignment to the NSWs mounted in the first step 
	to the bottom of the first multilayer are only present for BIS~7/8.} 
	
	\label{fig:jig_drawing}
\end{figure}

\begin{figure}[th]
	\centering
	\includegraphics[width=\textwidth]{bis78_comb_drawing}
	\caption{Drawing of the assembly jig for the BIS~1-7 chambers with the precision machined hole grids for the insertion of the endplugs of the drift tubes.}	
	\label{fig:comb_drawing}
\end{figure}

\begin{figure}[th]
	\centering
	\includegraphics[width=0.8\textwidth]{bis7_jig_photo}
	\caption{Photograph of the assembly jig for the BIS~1-7 chambers with the precision machined hole grids for the insertion of the endplugs of the drift tubes.} 
	\label{fig:bis7_jig_photo}
\end{figure}
%

\begin{figure}[th]
\centering
	\includegraphics[width=0.5\textwidth]{stacked_mezzanine_card} \includegraphics[width=0.5\textwidth]{sMDT_mezzanine_card_BMG}
	\caption{Left: sMDT signal distribution board with stacked mezzanine card carrying three ASD chips (middle layer) and the HPTDC with programming FPGA (top layer).
	The coupling capacitors to the signal pins of the drift tubes are encapsulated in injection molded white plastic containers to ensure high-voltage stability. 	
	The gold-plated signal pins and ground pins are inserted into receptacles on the signal distribution boards which also contain the protection network for the ASD chips.
	The passive signal distribution (hedgehog) board with the plastic containers is shielded from the mezzanine cards by an aluminum plate which is electrically connected to the
	outer Faraday cage. Right: sMDT signal distribution board with stacked mezzanine card mounted on the signal and ground pins of the drift tubes of a BMG chamber.}	
	\label{fig:stacked_mezzanine_card}
\end{figure}

\begin{figure}[th]
\centering
	\includegraphics[angle=-90,width=\textwidth]{BIS78_3D_sMDT.pdf}
	\caption{Drawings of a BIS~7/8 sMDT chamber with integrated triple thin-gap RPC and the }
	\label{fig:BIS78_3D_sMDT} 
\end{figure}

\begin{figure}[th]
	\centering
	\includegraphics[width=0.8\textwidth]{CMM_measurement}
	\caption{Measurement of the sense wire and alignment platform positions of a sMDT chamber with a Coordinate Measuring Machine (CMM). Individual wire positions can be measured with an accuracy 
	of $2~\mu$m. 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 deformations 
	like torsion between the two ends.} 
	\label{fig:CMM}
\end{figure}

\begin{figure}[th]
\centering
	\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 measureing machine of about 2~\mum{}~\cite{BMGconstr}.}  	
	\label{fig:residuals}
\end{figure}

\begin{figure}[th]
\centering
	\includegraphics[width=0.8\textwidth]{bis78_gassystem.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 measureing machine of about 2~\mum{}~\cite{BMGconstr}.}  	
	\label{fig:bis78_gassystem}
\end{figure}

A typical production rate of 100 tubes per day has been achieved with a failure rate of about $4\%$, mostly due to mechanical failures of the assembly devices which are documented in the data base.
During the production of the 4300 BMG drift tubes, the failure rate of the standard drift tube quality tests of wire tension ($350\pm 15$~g), gas leak rate ($<10^{-8}$~bar~l/s) and 
leakage current ($< 2$~nA/m) at the nominal operating voltage of 2730~V was only $2\%$~\cite{BMGconstr}, mostly due to HV problems.


\subsection{Chamber construction and test}
%
\begin{figure}[th]
\centering
	\includegraphics[width=0.9\textwidth]{gassystem}
	\caption{sMDT gas distribution system.}
	\label{fig:gassystem}
%\end{figure}
\vspace{2mm}
%\begin{figure}[th]
%\centering
	\includegraphics[width=0.5\textwidth]{sMDT_HVHH_BMG}\includegraphics[width=0.5\textwidth]{sMDT_ROHH_BMG}
	\caption{sMDT high-voltage (left) and signal (right) distribution (hedgehog) boards.}
	\label{fig:hedgehog}

\end{figure}

\begin{figure}[th]
\centering
	\includegraphics[width=\textwidth]{HV_side}
	\caption{HV distribution boards~\cite{sMDTprototype} for 6 x 4 channels mounted on the signal and ground pins on the high-voltage side of a BMG chamber inside the Faraday cage. 
	The termination resistors are encapsulated in injection molded white plastic containers to ensure HV stability.}
	\label{fig:HV_side}
%\end{figure}
\vspace{2mm}
%\begin{figure}[th]
%\centering
	\includegraphics[width=\textwidth]{RO_side}
	\caption{Signal distribution and active readout electronics (mezzanine) boards~\cite{BMGproduction} for 6 x 4 channels mounted on the signal and ground pins 
	on the readout side of a BMG chamber inside the Faraday cage (see also Figure~\ref{fig:stacked_mezzanine_card}).}
	\label{fig:RO_side}
\end{figure}

\begin{figure}[th]
\centering
	\includegraphics[width=\textwidth]{BMG_cables}
	\caption{BMG chambers during commissioning at CERN in January 2017 with view of the readout side with closed Faraday cages for top and bottom multilayer and flat blue signal cables.}
	\label{fig:BMG_cables}
\end{figure}

After passing the quality assurance tests, the drift tubes are assembled to chambers in a climatised clean room by inserting the endplug reference surfaces into a grid of fitting bores in the assembly jigs 
at each chamber end which define the wire positions with an accuracy of better than 5 micron (see Figure~\ref{fig:assembly}) and glueing them together and to the spacer and support frame 
using an automated glue dispenser (see Figure~\ref{fig:glueing}). A chamber can be assembled within two working days, including the precise mounting of the global alignment sensor platforms.   
The precisely machined assembly jig used for the BIS~7/8 chambers and afterwards for the BIS~1-6 chambers is shown in Figures~\ref{fig:jig_drawing}, \ref{fig:comb_drawing} and \ref{fig:bis7_jig_photo}.
After the glueing of each new tube layer, ground connection screws are inserted in the triangular gaps between the adjacent tube layers through holes in the jig and fixed with glue.   
After mounting of the gas connections, ground pins connecting to the readout and high-voltage distribution boards are screwed onto the ground screws  
(see Figure~\ref{fig:stacked_mezzanine_card}).

Like the BME sMDT chambers, but in contrast to the BMG chambers, the BIS7/8 and BIS~1-6 sMDT chambers will have in-plane alignment monitoring systems. The longitudinal sag monitors of the 
in-plane alignment system of the BIS~7/8 chambers is rotated by 180$^\circ$ with respect to the standard orientation parallel to the tube direction in the MDT and also the BME and BIS~1-6 chambers 
in order to properly monitor potential deformations of the complex shaped chambers transverse to the tubes. Two diagonal straightness monitors measure torsions between the readout and 
high-voltage ends in all types of chambers. In addition to the axial, praxial and CCC alignment sensors which are mounted on the outer multilayer of all new BIS sMDT chambers in the same positions 
as on the present BIS MDT chambers, the BIS~7/8 chambers carry barrel-endcap alignment sensors on the inner multilayer connecting to the NSWs (see Figure~\ref{fig:BIS78_3D_sMDT}). The global alignment 
sensors are mounted with 20~\mum{} positioning accuracy with respect to the sense wires on the outer tube layers using the precision assembly jigging (see Figure~\ref{fig:jig_drawing}).   

After the glueing 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 with a precision of about 2~\mum{} (see Figure~\ref{fig:CMM}). The measurement can be performed within a few hours for every chamber or only used 
as a spot check. In particular, the positions of the alignment sensor platforms with respect to the wire grid can be measured in this way with a few micron accurracy.
Sense wire positioning accuracies of better than 10~\mum{} (rms) have been routinely achieved during BME and BMG chamber construction~\cite{BMEop,BMGconstr}. 
An ultimate wire positioning accuracy of 5~\mum{} (rms) has been achieved in BMG chamber construction which reaches the precision of the 
assembly jigs (see Fig.~\ref{fig:residuals}). After the measurement, the individual wire positions are known with 2~\mum{} accuracy.
 
After the wire position measurement, the parallel gas distribution system consisting of modular injection molded gas connectors (see Figures~\ref{fig:endplug1} and \ref{fig:bis78_gassystem}) 
is mounted (see Figure~\ref{fig:gassystem}). Gas leak rates at 3 bar pressure of better than the required 0.2 mbar/h have been achieved for all BMG chambers~\cite{BMGconstr}.


\subsection{Readout electronics}
%
After the installation of the gas distribution system and ground pins, the Faraday cages and the high-voltage distribution boards and the 
signal distribution boards (see Figure~\ref{fig:hedgehog}) as well as the active readout electronics boards (mezzanine cards) with 6 x 4 readout channels matching the transverse 
cross section of the quadruple-multilayers are mounted on the HV and the RO side, respectively 
(see Figures~\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 digitises 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 
(blue cables in Figure~\ref{fig:BMG_cables}) which fulfill the same specifications as the round MDT signal 
cables which are not available anymore. 
as well as a CERN HPTDC chip~\cite{HPTDC_manual} and a FPGA for the JTAG programming, respectively. 
They provide the same functionality as the standard MDT readout electronics.
The stacked sMDT readout boards used for the BMG and, until the end of LHC Phase 1, for the BIS~7/8 chambers are shown in Figure~\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 MDT mezzanine cards. 
The same board layout with new versions of ASD and TDC chips in 130~nm CMOS technology will be used for the sMDT chambers in phase 2.     

\section{BIS chamber design}
%
\begin{figure}[th]
\centering
	\includegraphics[angle=90,width=1.1\textwidth]{BIS78_3D_sMDT_RPC.pdf}
	\caption{Mechanical drawings of a BIS~7/8 sMDT chamber with integrated triple thin-gap RPC.}
	\label{fig:BIS78_3D_sMDT_RPC} 
\end{figure}
%
\begin{figure}[th]
\centering
	\includegraphics[angle=-90,width=\textwidth]{BIS78_RPC.pdf}
	\caption{Mechanical drawing of a BIS triple thin-gap RPC chamber with only 50 mm thickness fitting into the BIS layer together with the BIS sMDTs.}
	\label{fig:BIS78_RPC} 
\end{figure}
%
\begin{figure}[th]
\centering
	\includegraphics[width=\textwidth]{BIS78_3D_RPC_support}
	\caption{Mechanical drawings of a BIS~7/8 RPC chamber with rail supports.}
	\label{fig:BIS78_3D_RPC_support} 
\end{figure}
%
\begin{figure}[th]
\centering
	\includegraphics[angle=-90,width=1.1\textwidth]{BIS_layout.pdf}
	\caption{Layout of a BIS sMDT layer for phase 2 operation of the ATLAS muon spectrometer.}
	\label{fig:BIS_layout} 
\end{figure}


\begin{table}
\begin{center}

\caption{Geometrical parameters of the BIS~1-6 sMDT chambers}
\label{tab:bis16_parameters}
\vspace{2mm}

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

\hline
Type & BIS~1 & BIS~2-6 \\
\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)    & 1087 &  906 \\
Chamber length in x (mm) & 1839 & 1839 \\
Al tube length (mm)        & 1660 & 1660 \\
Assembled tube length (mm) & 1669 & 1669 \\
Active tube length (mm) & 1640 & 1640 \\ 
\hline
Tube layers & 2 x 4 & 2 x 4 \\
Tubes/layer & 72 & 60 \\
Tubes/chamber & 576 & 480 \\
Spacer height (mm)  & 45.6 & 45.6 \\
Tubes height (mm)   & 139 & 139 \\
Chamber height (mm) & 249 & 249 \\
\hline
Gas volume/chamber (l) & 151 & 126 \\
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 \\
T-sensors/chamber             & 10 & 10 \\
B-field sensors/chamber       &  2 &  2 \\
Praxial alignment sensors/chamber &  4 &  4 \\
Survey targets/chamber        &  4 &  4 \\
\hline
  
\end{tabular}
\vspace{1mm}

$^\ast$ Numbers in brackets refer to sectors 2 and 16.
\vspace{5mm}

\caption{BIS 1-6 sMDT chambers in numbers}
\label{tab:bis16_numbers}

\vspace{2mm}

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

\hline
Parameter & Value \\
\hline\hline
Number of chambers & 96 \\
Number of tubes & 38016 \\
Total tube and wire length & 63.1~km \\ 
Chamber active area & 147~m$^2$ \\
Gas volume & 12.5~m$^3$ \\
Chamber weight & 8.5 t \\
Number of mezzanine cards & 1984 \\
Number of hedgehog boards & 3968 \\
Number of CSMs & 192 \\
\hline 

\end{tabular}

\end{center}
\end{table}



The design and construction of new BIS78 sMDT chambers with 2 x 4 tube layers and integrated thin-gap RPCs with three gas gaps for the replacement of the current BIS~7 and BIS~8 MDT chambers 
in the 2019-20 LHC shutdown is a pilot project for the complete replacement of the BIS MDT chambers by sMDT + RPC chambers for HL-LHC. The design of the remaining new BIS sMDT and RPC chambers is 
be very similar to the BIS~7/8 design. The common challenge for the mechanical design is the small available space and the control of gravitational deformations 
with a minimum  of support structure. The detailed mechanical design drawings of a BIS~7/8 sMDT and RPC chamber are shown in Figures~\ref{fig:BIS78_3D_sMDT}, \ref{fig:BIS78_3D_sMDT_RPC}
and \ref{fig:BIS78_RPC}. The RPC chambers are supported and installed on the rails on an independent support structure from the sMDTs (see Figure~\ref{fig:BIS78_3D_RPC_support}). 


There are 12 different types of BIS~78 sMDT chambers with complex shapes maximising the coverage in the overlap region with the New Small Wheels~\cite{sMDT_parameterbook}. 
The design and construction of the remaining new BIS~1-6 sMDT chambers will be simpler as they are all of rectangular shape and of only two different widths in z. The geometrical parameters of the 
BIS~1-6 sMDT chambers are summarised in Tables~\ref{tab:bis16_parameters} and \ref{tab:bis16_numbers}. The layout of a new BIS sMDT layer for phase 2 is shown in Figure~\ref{fig:BIS_layout}.
