[en] Molecular assisted recombination (MAR) via negative ions formation has been observed in a linear plasma simulator. A small amount of hydrogen gas puffed into a hydrogen plasma strongly reduced the heat flux to the target and rapidly increased the density of negative ions of hydrogen atom, n_H^-, in the circumference of the plasma, while the conventional radiative and three-body recombination processes disappeared. n_H^- is localized in the outer region where electrons exist (n_e∼5x10¹⁷ m^-3, T_e=3-5 eV). The peak value of n_H^- is 1.2x10¹⁷ m^-3 and the ratio of n_H^-/n_e in the outer region goes up to over 20%. These results can be well explained by taking the mutual neutralization between the negative and positive ions of MAR in the detached plasma into account