from 27 February 2017 to 3 March 2017
Budker Institute of Nuclear Physics
Asia/Novosibirsk timezone
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Contribution Contributed Oral

Micropattern gas detectors

Discharge and stability studies for the new readout chambers of the upgraded ALICE TPC

Speakers

  • Mr. Alexander DEISTING

Primary authors

  • Mr. Alexander DEISTING (GSI Helmholtzzentrum für Schwerionenforschung GmbH; Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg)

Co-authors

  • Dr. Piotr GASIK (Technische Universität München, Arcisstraße 21, 80333 München, Germany)

Content

ALICE (A Large Ion Collider Experiment), taking data at the CERN Large Hadron Collider (LHC), uses a Time Projection Chamber (TPC) to provide tracking and particle identification of charged particles in the central barrel. This TPC is the largest TPC built so far (almost $90textrm{m}^3$ volume) and - while operated in a 0.5T magnetic field - provides a momentum resolution of $textrm{d}p/p=1%$ (at the multiple scattering limit) as well as a $textrm{d}E/textrm{dx}$ resolution of $5$-$7%$. The readout chambers are Multi Wire Proportional Chambers (MWPCs), employing a gating grid to prevent ions, which are produced during the gas amplification, from moving into the drift volume. Hence there is a maximal readout rate of about 3kHz - given by the closing time of the grid ($sim 250textrm{ms}$) and the electron drift time through the whole TPC ($sim 90textrm{ms}$).

After the long shut-down 2 (from 2021 onwards) the LHC will provide lead-lead collisions at interaction rates of 50kHz. In order to cope with these rates the TPC needs to be upgraded with new readout chambers, which allow for continuous read-out and preserve the energy and momentum resolution of the current MWPCs. Therefore the amount of ions in the drift volume can only be as big as $1%$ of the ions produced during the gas amplification. Hence the ion back flow from the chambers has to be small. It was found that chambers with a stack of four Gas Electron Multipliers (GEMs) fulfil these requirements, if the voltages applied to all the GEMs are tuned properly. In addition the chambers must be stable while being operated at the LHC. Hence studies of the discharge behaviour with small prototypes (equipped with only one or two GEMs) as well as stability studies with full readout chambers have been performed. During these studies the phenomenon of "secondary discharges" was observed. This special kind of discharge occurs after an initial discharge in a time between 0 to several $10textrm{ms}$, if the electric field above or below the GEM is high enough.

In this talk we will give an overview of the ALICE TPC upgrade and of the current design status of the GEM based readout chambers. We will focus on the studies of the chamber stability and present our current knowledge on the observed "secondary discharges" and our measures on how to avoid them. This includes our considerations of the high voltage supply schema. In addition we'll give a short outlook on the challenges of the mass production of the new readout chambers.

Summary

In this talk we will give an short overview of the ALICE TPC upgrade and of the current design status of the GEM based readout chambers. This upgrade is done in order to enable the TPC to take data at interaction rates of 50kHz (lead-lead collisions), which the LHC will provide from 2021 onwards. The goal of the upgrade is to replace the current MWPCs of the TPC by chambers with quadruple GEM stacks, allowing for continuous readout while providing the same performance as the current chambers. We will focus on the studies of the chamber stability, which were done with small prototypes as well as studies with full size readout chamber prototypes. Furthermore we will report on the phenomenon of "secondary discharges". This special kind of discharge occurs after an initial discharge in a time between $0$ to several $10textrm{ms}$, if the electric field above or below a GEM is high enough. Our current knowledge on these discharges and our measures on how to avoid them will be presented. In addition we'll give a short outlook on the challenges of the mass production of the new readout chambers.