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In a stationary high density d. c. arc, the electric power input is balanced essentially by heat conduction losses in radial direction. These losses increase greatly with temperature and thus they limit the axial temperatures attainable with reasonable power input.An experiment is described in which considerably higher plasma temperatures have been obtained by reducing the coefficient of heat conduction with a superimposed axial magnetic field. At arc currents of about 2 kA and a magnetic field of 10 kG temperatures in the middle of the arc of the order of 10 eV were readied.The measured temperature, pressure and power input of the hydrogen plasma are compared with calculated values. In particular, the coefficient of heat conduction perpendicular to a magnetic field has been determined by measuring the radial temperature profile and the electric field strength. The results agree with theoretical predictions.Radiale Wärmeleitungsverluste begrenzen im allgemeinen weitgehend die in einem Lichtbogen hohen Druckes erreichbaren Temperaturen. 'Uberlagert man jedoch der Lichtbogensäule ein achsenparalleles Magnetfeld, so kann man die Wärmeleitfähigkeit des Plasmas senkrecht zum Magnetfeld, d. h. in radialer Richtung, dann stark reduzieren, wenn die freie Weglänge der geladenen Teilchen -insbesondere der Elektronen -vergleichbar oder größer als ihr Gyrationsradius wird.

In the four years of operation of the PLT tokamak , plasma parameters and machine operating conditions have been importantly affected by changes in the choice of limiter material and by the mode of vacuum vessel conditioning. Tungsten, stainless stuel, and graphite limiters have been used. The fractional power lost by radiation and the source distribution of the radis.tion vary strongly with limiter material, wall *UKAEA CuliVitn Laboratory, Abingdon, UK. **Exxon Co., USA. ***On leave from Max-Planck Institut fur Plasmaphysik, Garching, 7RG. When the injected gas is changed from D_ to H,, the time constant for disappearance of neutron emission can be slow (35 pulses for a reduction by a factor of three) or fast '4 pulses for a similar factor), depending on previous history of wall treatment.