The key design and performance characteristics of a proposed high performance, long pulse tokamak (HLT) are described. The main parameters of the machine are: major radius, 3.4 m; minor radius, 1.2 m; elongation, 2; maximum toroidal field on axis, 8.3 T; maximum plasma current, 20 MA. The discharge flat top length is 40 s for a plasma with a 20 MA inductively driven current at a toroidal field of 8.0 T, and 500 s for a discharge with a 7 MA non-inductively driven current at a field of 3.5 T. With HLT, the main physics issues for a fusion reactor can be assessed: plasma physics and plasma operation in the ignited and sub-ignited regimes; burn stabilization; behaviour of divertor components under long-pulse thermal loads; and semi-continuous operation with non-inductive current drive. The crucial element in obtaining this level of performance in a machine of medium size is the use of active cooling with liquid nitrogen both for the toroidal field coils and the central solenoid. The toroidal field coils have a pure copper conductor in a stainless steel casing. The stress in the copper does not exceed a relatively conservative limit of 180 MPa.