A momentum flux measuring instrument (MFMI) has been used to take thrust measurements of the helicon double layer thruster (HDLT) prototype immersed in a vacuum chamber. The MFMI recorded a sharp increase in force when the plasma transitioned into a low-magnetic field, high-density mode. The HDLT uses a helicon antenna to generate an ion beam-containing plasma in a diverging magnetic field. The presence of a low-magnetic field mode in the HDLT prototype was demonstrated and its properties were explored at 0.3 mTorr argon at various applied rf powers, locations within the source tube and downstream vacuum chamber, and applied magnetic field strengths. The low-magnetic field mode occurs at solenoid currents that correspond to peak magnetic fields between 20 and 30 G in the source tube and Langmuir probe measurements show that the mode is associated with an increase in ion density of up to six times the density at applied magnetic fields just outside the mode. This rise in ion density correlates with a peak in plasma resistance. Furthermore, the transition into the mode is accompanied by the appearance of a higher current ion beam in the downstream region of the expanding plasma and a rise in plasma potential of approximately 30% at 250 W rf power as measured by retarding field energy analyzers. This mode is of interest for the HDLT because it generates an ion beam-containing high-density (≈1017 m-3) expanding plasma at low-magnetic fields and rf powers as low as 130 W.