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The Negative Ion Drift (NID) gas SF$_6$ has favourable properties for trackreconstruction in directional Dark Matter (DM) searches utilising low pressuregaseous Time Projection Chambers (TPCs). However, the electronegative nature ofthe gas means that it is more difficult to achieve significant gas gains withregular Thick Gaseous Electron Multipliers (ThGEMs). Typically, the maximumattainable gas gain in SF$_6$ and other Negative Ion (NI) gas mixtures,previously achieved with an $^{55}$Fe X-ray source or electron beam, is on theorder of $10^3$; whereas electron drift gases like CF$_4$ and similar mixturesare readily capable of reaching gas gains on the order of $10^4$ or greater. Inthis paper, a novel two stage Multi-Mesh ThGEM (MMThGEM) structure ispresented. The MMThGEM was used to amplify charge liberated by an $^{55}$FeX-ray source in 40 Torr of SF$_6$. By expanding on previously demonstratedresults, the device was pushed to its sparking limit and stable gas gains up to$\sim$50000 were observed. The device was further optimised by varying thefield strengths of both the collection and transfer regions in isolation.Following this optimisation procedure, the device was able to produce a maximumstable gas gain of $\sim$90000. These results demonstrate an order of magnitudeimprovement in gain with the NID gas over previously reported values andultimately benefits the sensitivity of a NITPC to low energy recoils in thecontext of a directional DM search.

Gas Gains Over 10$^4$ and Optimisation using $^{55}$Fe X-rays in Low  Pressure SF$_6$ with a Novel Multi-Mesh ThGEM for Directional Dark Matter  Searches