Two new, longer duration, modes of operation for Ludwieg Tunnels have been implemented in the Oxford High Density Tunnel (HDT): Extended Ludwieg Mode (ELM) and Plenum Augmented Ludwieg Mode (PALM). In standard Ludwieg Tunnel operation, the duration of the steady test time is limited by the return of the rarefaction wave to the facility nozzle throat. Operation in ELM extends the rarefaction wave such that the tail is generated as the head returns and results in a steady decrease in supply pressure. PALM takes advantage of the dual-throat arrangement of the HDT, which features a plenum region between the facility barrel and the nozzle throat, and tailors plug valve opening to produce a steady supply pressure to the facility nozzle at the expense of total pressure capability. This paper describes the theory of operation of these new modes, their implementation in the existing Oxford High Density Tunnel and presents transient quasi-1D numerical simulations confirming their principles of operation. It then presents experimental results from initial testing of the HDT in the new modes. These results demonstrate a successful implementation of ELM, producing a test flow that is over fifteen times longer, but of comparable steadiness to a Mach 7 Ludwieg Mode plateau in the Oxford HDT. PALM was implemented with partial success, resulting in a test flow that is both ten times the duration of, and significantly steadier (for supply pressure and unit Reynolds number), than a Ludwieg Mode plateau. Thus, the implementation of ELM and PALM significantly expand the capability of the facility to investigate long duration flow phenomena
