Status of the R&D activities for the upgrade of the ALICE TPC

In order to cope with the high interaction rates provided by the LHC after the long shut-down 2, the ALICE TPC needs to be upgraded. After the upgrade the TPC will run in a continuous mode, without any degradation of the momentum and $\textrm{d}E/\textrm{d}x$ resolution compared to the performance of the present TPC. Since readout by MWPCs is no longer feasible with these requirements, new technologies have to be employed. In the new readout the electron amplification is provided by a stack of four GEM foils. Here foils with a standard hole pitch of 140um as well as large pitch foils (280um) are used. Their high voltage settings and orientation have been optimised to provide an energy resolution of $\sigma_{{E}}/{E}\leq12\%$ at the photopeak of $^{55}\textrm{Fe}$. At the same settings the Ion BackFlow into the drift volume is less than 1% of the effective number of ions produced during gas amplification and the primary ionisations. This is necessary to prevent the accumulation of space charge, which eventually will distort the field in the drift volume. To ensure stable operation during the high loads during LHC run 3 the chambers have to be robust against discharges, too. With the selected configuration in a quadruple GEM stack the discharge probability is kept at the level of $10^{-12}$ . An overview of the ALICE TPC upgrade activities will be given in these proceedings and the optimised settings foreseen for the GEM stacks of the future readout chambers are introduced. Furthermore the outcome of two beam time campaigns at SPS and PS (at CERN) in the end of 2014 is shown. At this campaigns the stability against discharges and the $\textrm{d}E/\textrm{d}x$ performance of a full size readout chamber prototype was tested. In addition it is report on charging-up studies of 4GEM stacks and on tests of electromagnetic sagging of large GEM foils.