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\begin{document}

\section{Barrel Inner Layer Upgrade}
\label{sec:BI_layer_upgrade}

\subsection{The 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 are used, with 0.4~mm wall thickness like for the MDT chambers.
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 mixture, gas pressure and gas gain
as the MDT chambers, however with high-voltage reduced from 3080~V for MDTs to 2730~V. Table~\ref{tab:parameters} shows a comparison of the MDT and sMDT operating parameters.
The drift time spectra are shown in the left-hand part of Figure~\ref{fig:spectrum_gasgain}. 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 close to linear space-to-drift time relationship with the standard MDT drift gas Ar:CO$_2$ (93:7) at 3 bar pressure
(see Figure~\ref{fig:rtrelation_resolr}, left).


\begin{figure}[htb]
\centering
\includegraphics[width=0.5\columnwidth]{tubes.pdf}
\caption{Picture of MDT (left) and sMDT (right) tubes.}
\label{fig:tubes}
\end{figure}

\begin{table}[ht]

\begin{center}
\caption{Material and operating parameters of ATLAS sMDT chambers compared to the 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 resistance/m & $44~\Omega /$m & $44~\Omega /$m \\
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 & $83~\mu$m & $106~\mu$m \\
without backgr.\ irradiation & & \\
Average 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}[htb]

\hspace{-6mm}
\begin{tabular}{lr}
\includegraphics[width=0.50\columnwidth]{spectrum.pdf} &

\includegraphics[width=0.50\columnwidth]{gasgain.pdf} \\

\end{tabular}

\caption{Left: Drift time spectra of MDT (green) and sMDT tubes (grey) 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 at CERN compared to predictions based on the Diethorn formula~\cite{sMDTperf3}.}
\label{fig:spectrum_gasgain}

\begin{tabular}{lr}
\includegraphics[width=0.54\columnwidth]{rtrelation.pdf} &

\hspace{-10mm}
\includegraphics[width=0.50\columnwidth]{res_with_TS_new.pdf} \\


\end{tabular}

\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}


\begin{figure}[htb]
\centering
\includegraphics[width=0.7\columnwidth]{resolution.pdf}
\caption{Average spatial resolution of MDT and sMDT drift tubes measured at the Gamma Irradiation Facility at CERN as a function of the $\gamma$ background rate 
using standard MDT readout electronics with bipolar shaping. The same front-end electronics scheme and parameters will be used for MDT and sMDT chambers at HL-LHC.
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} which is not needed and will not be implemented for phase-2 operation.}
\label{fig:resolution}
\end{figure}

\begin{figure}[htb]
\centering
\includegraphics[width=0.7\columnwidth]{efficiency.pdf}
\caption{Muon detection efficiencies of  MDT and sMDT drift tubes with corresponding electronics deadtime settings (see text) measured at the Gamma Irradiation facility at CERN
as a function of the $\gamma$ background counting rate using standard MDT readout electronics with bipolar shaping as planned also for operation at HL-LHC.   
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. 
Further improvement of the muon efficiency of the sMDT drift tubes at high background counting rates are possible by employing fast baseline restoration (BLR) 
in order to suppress signal pile-up effects (blue dashed curve)~\cite{blr} which is not needed and will not be implemented for phase-2 operation.}
\label{fig:efficiency}
\end{figure}

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

\vspace{2mm}

\begin{tabular}{lr}
	\includegraphics[width=0.48\textwidth]{wire_crimping_and_tube_swaging_tool.pdf} &

	\includegraphics[width=0.48\textwidth]{wire_tensioning_and_crimping_tool.pdf} \\

\end{tabular}	

\caption{Automated tools for the mechanical swaging of the aluminium tubes onto the O-rings on the endplugs (see Figure~\ref{fig:endplug1})
        for fixation and gas sealing (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 the feeding of the wire through the tube by air flow, without manual interference, and after automated tensioning of the wire  
	using a stepping motor controlled by a strain gauge (right).} 	
	\label{fig:swaging_tensioning}
	
\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,BMEop} 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. 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 muon spectrometer~\cite{BMGproposal,BMGproduction}. 

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  
the barrel inner layer in the small sectors in LS2 in order to improve the trigger efficiency and the rate capability of the chambers in the transition regions 
between barrel and endcaps~\cite{BMGproduction,BIS78proposal}. This Phase~I upgrade serves as pilot project for the complete replacement of the BIS~1--6 MDT chambers in LS3
by sMDT-RPC chamber modules of very similar design as the BIS7/8 chambers. 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 under the high
irradiation rates at HL-LHC. The installation of the new triple thin-gap RPCs of only $5\cm$ thickness is possible only due to the replacement of the BIS MDT chmabers
by sMDT chambers of about half the height. In addition, the sMDT chambers provide about an order of magnitude higher rate capability than the MDTs.


\subsection{Performance of the sMDT chambers}
%
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{sMDTperf1, MDTperf, 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~Hz/cm$^2$ and 300~kHz per tube.  
At background rates above 500~Hz/cm$^2$, the gas gain drops by more than $20\%$  
(see Figure~\ref{fig:spectrum_gasgain}, right) 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 with respect to the tube wall of 2730~V in sMDT tubes), 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:spectrum_gasgain}, right). 
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_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 the right-hand part of Figure~\ref{fig:rtrelation_resolr}~\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 $106\pm 2~\mu$m 
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 rate is shown in Figure~\ref{fig:resolution}. The spatial resolution deteriorates quickly
with increasing background flux for the MDTs while it is affected only little by space charge effects up to very high irradiation rates for the sMDTs. 

At the same background rate, the small-diameter drift tubes experience 8 times lower occupancy than the 30~mm diameter MDT tubes because of the 4 times shorter maximum drift time 
(see Figure~\ref{fig:spectrum_gasgain})
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 are 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 tube design and fabrication procedures are the same as used for the construction of the BMG sMDT chambers in 2016~\cite{BMGconstr, BMGproduction}.

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. Afterwards the endplugs are fixed and and the tubes gas sealed 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),
corresponding to a gravitational sag of only $17\pm 1~\mu$m (absolute tolerances) for 1~m long tubes, 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. 



\begin{figure}[ht]
\vspace{-18mm}
	\includegraphics[width=1.15\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}[th]
	\centering
	\includegraphics[width=\textwidth]{assembly_station.pdf}
	\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}

\vspace{2mm}

%\begin{figure}[th]
%	\centering
	\includegraphics[width=\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}[ht]
	\centering
	\includegraphics[angle=90,width=\textwidth]{bis78_jig_drawing.pdf}
	\caption{Sketch of the precision jigging and illustration of the steps of the BIS~7/8 and BIS~1-6 chamber assembly procedure. 
	The brackets for the precise mounting of the global alignment sensor platforms are an integral part of the jigging design.  
	The sensors for alignment with respect to the NSWs are mounted 
	to the bottom of the first multilayer in the first step and are only present for BIS~7/8. The axial, praxial and CCC alignment sensors
	are mounted, together with the in-plane alignment system on the outside of the top multilayer during chamber assembly.}	
	\label{fig:jig_drawing}
\end{figure}

\begin{figure}[ht]
	\centering
	\includegraphics[width=\textwidth]{bis78_comb_drawing.pdf}
	\caption{Drawing of the assembly jig for the BIS~1--8 chambers with the precision machined hole grids for the insertion of the endplugs of the drift tubes.}	
	\label{fig:comb_drawing}
%
\hspace{3mm}

%\begin{figure}[th]
%	\centering
	\includegraphics[width=0.6\textwidth]{bis7_jig_photo.pdf}
	\caption{Photograph of the assembly jig for the BIS~1--8 chambers with the precision machined hole grids for the insertion of the endplugs of the drift tubes.} 
	\label{fig:bis7_jig_photo}
%
\hspace{3mm}

	\includegraphics[width=0.54\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 mounted on the gold-plated signal and ground pins of the drift tubes of a BMG chamber. 
	The signal hedgehog boards contain also 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 ensure high-voltage stability.}	
	\label{fig:stacked_mezzanine_card}
\end{figure}

\begin{figure}[ht]
\centering
	\includegraphics[angle=90,width=\textwidth]{BIS78_3D_sMDT.pdf}
	\caption{Drawings of a BIS~7/8 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}[th]
	\centering
	\includegraphics[width=0.8\textwidth]{CMM_measurement.pdf}
	\caption{Measurement of the sense wire and alignment platform positions of a sMDT chamber (BMG with cutouts for the global alignment rays) 
	with a Coordinate Measuring Machine (CMM). Individual wire positions are 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}

\hspace{3mm}

	\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 $2~\mu$m~\cite{BMGconstr}.}  	
	\label{fig:residuals}
\end{figure}

\begin{figure}[ht]

\begin{tabular}{lr}

\includegraphics[width=0.48\textwidth]{Gassystem_BIS78_2.pdf} &

	\includegraphics[width=0.48\textwidth]{Gassystem_BIS78_1.pdf} \\
	
\end{tabular}	

	\caption{Schematics of the gas distribution system of the BIS~7/8 and BIS~1--6 sMDT chambers (left: 3D model, right: cross section). 
	The principle 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 Figure~\ref{fig:endplug1}), to connect the tubes in columns perpendiculat to the chamber plane to the 
        aluminium gas bars mounted along each multilayer on the HV and RO side.}  
	\label{fig:bis78_gassystem}

\vspace{2mm}
\centering
	\includegraphics[width=0.6\textwidth]{bis78_gassystem_photo.pdf}
	\caption{Test module of the BIS sMDT gas distribution system.} 
	\label{fig:bis78_gassystem_photo}

\vspace{2mm}
\begin{tabular}{lr}
	\includegraphics[width=0.49\textwidth]{sMDT_HVHH_BMG.pdf} &
	
	\includegraphics[width=0.47\textwidth]{sMDT_ROHH_BMG.pdf} \\
	
\end{tabular}	
	\caption{sMDT high-voltage (left) and signal (right) distribution (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 ensure HV stability.}
	\label{fig:hedgehog}

\end{figure}

\begin{figure}[ht]
\centering
	\includegraphics[width=0.7\textwidth]{HV_side.pdf}
	\caption{HV distribution boards~\cite{sMDTprototype} for 6 x 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{2mm}

	\includegraphics[width=0.7\textwidth]{RO_side.pdf}
	\caption{Signal distribution and active readout electronics (mezzanine) boards~\cite{BMGproduction} with 6 x 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 Figure~\ref{fig:stacked_mezzanine_card}).}
	\label{fig:RO_side}

\vspace{2mm}

	\includegraphics[width=0.7\textwidth]{BMG_cables.pdf}
	\caption{Readout side of sMDT chambers (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}[th]

\hspace{-12mm}
	\includegraphics[width=1.15\textwidth]{BIS_sMDT_layout_2.pdf}
	\caption{Layout example of the BIS sMDT layer with two neighbouring BIS~1 and BIS~2 chambers.} 
	\label{fig:BIS_sMDT_layout}
		
\end{figure}


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 (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 accumulation on the sense wire of 9~C/cm, 15 times the requirement for the ATLAS MDT chambers, 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 failure rate of about $4\%$, which is
mostly due to occasional failures of the assembly devices~\cite{BMGnote}.
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{BMGnote}, mostly due to too high dark currents
under high voltage.


\subsection{sMDT chamber construction and test}
%
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 complete 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, which will also be used for the BIS~1-6 chambers, is shown in Figures~\ref{fig:jig_drawing}, \ref{fig:comb_drawing} and 
\ref{fig:bis7_jig_photo}. The same two-component expoxy glues as for the MDT chamber construction are used, Araldite~2014 between the tube layers and DP~490 between multilayers and spacer 
and support structures.
After the glueing of each new tube layer, ground connection screws are inserted into the triangular gaps between adjacent tube layers through holes in the jig, scratching the chromatised tube walls.
The gaps are filled with glue during the assembly of the next layer, fixing and encapsulating the ground screws. Conducting glue may be added in order to improve the conductivity 
of the ground connection if necessary.  
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 $20~\mu$m 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 (CMM) with a precision of about $2~\mu$m (see Figure~\ref{fig:CMM}). The measurement was performed within 1-2 hours for every BME and BMG chamber and is 
planned as regular spot check during the BIS chamber serial production. 
In particular, the positions of the alignment sensor platforms with respect to the wire grid can be measured with a few micron accurracy.
Sense wire positioning accuracies of better than $10~\mu$m (rms) have been routinely achieved during BME and BMG chamber construction~\cite{BMEop,BMGconstr}. 
An ultimate wire positioning accuracy of $5~\mu$m (rms) has been achieved in the BMG chamber construction, which comes close to the precision of the 
assembly jigs (see Figure~\ref{fig:residuals}). All BMG sMDT chambers have a wire positioning accuracy of better than $10~\mu$m with an average of $7~\mu$m~\cite{BMGnote}.
After the CMM measurement, the individual wire positions are known for tracking even with $2~\mu$m accuracy.
 
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 chromatised aluminium gas distribution bars  
(see Figures~\ref{fig:endplug1} and \ref{fig:bis78_gassystem}).  
Gas leak rates at 3~bar pressure below the limit of $2n \times 10^{-8}$~bar~l/s required for a chamber with n tubes 
have been achieved for all BMG chambers~\cite{BMGconstr}. 
Minor modifications of the gas connectores and gas bars had to be implemented due to the small available space in the BIS layer.
The new injection molded connectors have been produced and successfully tested (see Figure~\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 Figure~\ref{fig:hedgehog}) as well as the active readout (RO) electronics (mezzanine) cards with $6\times 4$ channels matching the transverse 
cross section of the quadruple-multilayers are mounted at the HV and RO ends of the chambers, 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 MDT signal cables. 
as well as a CERN HPTDC chip~\cite{HPTDC_manual} and a FPGA for JTAG programming, respectively. 
They provide the same functionality as the standard MDT readout electronics.
The stacked sMDT readout boards which will be used for the BMG and BIS~7/8 chambers until the end of LHC Phase~I 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, but with new versions of the ASD and TDC chips in 130~nm CMOS technology, will be used for the sMDT chambers in LHC phase~2.     

\subsection{BIS sMDT chamber design}
%

\begin{table}[ht]
\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)    & 1081 &  901 \\
Chamber length in x (mm) & 1839 & 1839 \\
Aluminium 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 & 71 & 59 \\
Tubes/chamber & 568 & 472 \\
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 \\
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}


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

\caption{BIS 1--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 & 37248 \\
Total tube and wire length & 63.1~km \\ 
Chamber active area & 145~m$^2$ \\
Gas volume & 12.3~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 $2\times 4$ 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 LS2 is a pilot project for the complete replacement of the BIS MDT chambers by integrated sMDT and RPC chambers for HL-LHC. 
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 drawings of a BIS~7/8 sMDT chamber are shown in Figure~\ref{fig:BIS78_3D_sMDT}.  
The design of the remaining new BIS~1-6 sMDT and RPC chambers will be very similar to the BIS~7/8 design, 
with the simplifications of rectangular shape and only two chamber types differing in their width in z direction.  
The parameters of the BIS~1-6 sMDT chambers are summarised in Tables~\ref{tab:bis16_parameters} and \ref{tab:bis16_numbers}. A possible new BIS sMDT chamber layout is shown 
in Figure~\ref{fig:BIS_sMDT_layout}.  
The same assembly procedure and precision tooling will be used both for the BIS~7/8 and for the BIS~1--6 sMDT chambers.

The common challenge for the mechanical design is the small available space to be shared with the new RPC chambers 
and the control of gravitational deformations of the thin chambers with minimal space for support structure. 
The RPC chambers are mounted on their own support frame and will be installed on the existing rails in the BIS layer independently from the sMDTs to prevent distortion of the BIS sMDT chambers. 
The BIS~1-6 sMDT chambers are each reduced in width by one drift tube per layer compared to the maximum possible to provide a gap of 20~mm between adjacent chambers which allows for the passage
of the RPC rail supports.
The chamber readout service modules (CSMs) will be mounted on the toroid coils as close as possible to each chamber such that the signal cable length between mezzanine 
cards and CSMs will not be longer than 4~m which is still acceptable for realiable signal transmission. 


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\end{thebibliography}
\end{document}