To investigate the transition from the low density mode to the high density mode in an electron cyclotron resonance (ECR) discharge, a Langmuir probe and anEfield probe were used to measure ion density andEfield intensity as functions of axial position and power. The experiments were performed in argon at 0.13 Pa in a 7.9 cm diam cyclindrical source chamber propagating TE11mode 2.45 GHz microwave power. Low mode was characterized by a standing wave throughout the plasma chamber and minimal power absorption. High mode exhibited nearly complete power absorption and no standing wave past the ECR zone. A sliding short (ss) was used to determine if the position of anEfield null in the source chamber affected the transition between these two modes for various magnetic field configurations. The ss position had little effect on mode transition, relative power absorption or ion density when positioned downstream from a broad, large volume resonance zone (resonance near the mirror midplane). However, the plasma could not be ignited if the short was placed at or upstream from the large volume resonance zone.If the magnetic field was adjusted to yield a sharp, small volume resonance zone (resonance midway between the midplane and the throat), then positioning the ss to force anEfield null at the resonance zone would prevent plasma ignition, even at 800 W forward power. The ion density exhibited a hysteresis (i.e., a direct jump from no plasma to a plasma density observed at other sliding short positions for that same forward power, thereafter following the ion density versus forward power dependence observed at nearby ss positions) on power cycling when the sliding short was within ∼1 cm of the broad resonance zone or within ∼1 cm of forcing a null in the sharp resonance zone caes. The ion density versus power curve for the broad volume case exhibited a change in slope over the transition region (Pforward≊40 W) when the ion density at the resonance zone was ≊1×1011cm−3. For the narrow zone cases, a region of bistability (i.e., rapid plasma density fluctuations between low and high mode with little or no change in external input power) was observed for which the ion density fluctuated between a value near 5×1010cm−3in low mode and 5×1011cm−3in high mode. Similar transitions were observed in a 14.6 cm diam ECR source with a TM01mode, indicating that the effect is not solely dependent on the microwave field structure in the ECR chamber.