Microinstabilities often result in turbulence that influences energy confinement in tokamak discharges. One such microinstability, of particular importance to the design of next-generation spherical tokamaks (STs) such as STEP [1], is the microtearing mode (MTM), a tearing-parity mode centred on high-order rational surfaces. MTMs are short-wavelength ion scale (low kyρs) electromagnetic instabilities that are primarily driven unstable by the electron temperature gra- dient. In plasmas where βe (the ratio of electron thermal pressure to magnetic pressure) is suffi- ciently high, MTMs can become the dominant instability contributing to electron transport in the plasma core, and local linear gyrokinetic (GK) simulations find that this is likely to be the case for STEP [2]. Thus, to predict the evolution of next-generation ST discharges, we must improve our capability to predict electron thermal transport driven by MTMs. A key focus for STEP is designing a reduced model for use in whole device predictive modelling codes to assist in mak- ing intelligent design choices. The first focus of this contribution will explore one promising existing model [3] and our attempts at both (i) benchmarking this model using GK simulations and (ii) generating new tools for modelling turbulence in next-generation STs. Whilst GK simu- lations have thus far proven to be a very accurate tool in modelling turbulent transport, obtaining saturated nonlinear MTM simulations has proven computationally challenging. Local simula- tions suffer from the so-called high β runaway [4], where turbulent amplitudes and transport levels grow to tremendous values once a certain βe is exceeded. It has been a source of consid- erable confusion as to whether this runaway effect resulted from shortcomings of the available GK codes or due to the failure of local GK at low kyρs. In recent studies, global physics has been found to play a prominent role in obtaining saturated MTM simulations (e.g., [5]). In the second thrust of this contribution, we will also exploit the unique additional capabilities of the gyrokinetic code GENE [6] in our attempts to obtain the first saturated nonlinear simulations relevant to a conceptual design of a burning ST plasma.

References
[1] H.R.Wilsonetal,CommercialisingFusionEnergy2053–2563(Bristol:IOPPublishing)pp8–18(2020)
[2] B.S.Patel,D.Dickinson,C.M.Roach,andH.RWilson,NuclearFusion62,016009(2022)
[3] T.Rafiq,J.Weiland,A.H.Kritz,L.Luo,andA.Y.Pankin,PhysicsofPlasmas23,062507(2016) [4] M.J.Pueschel,P.W.Terry,andD.R.Hatch,PhysicsofPlasmas21,055901(2014).
[5] J.L.Larakersetal,PhysicalReviewLetters126,225001(2021).
[6] F.Jenko,W.Dorland,M.Kotschenreuther,andB.N.Rogers,PhysicsofPlasmas7,1904(2000)