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 ion of hydrogen atom, n(H-), in the circumference of the plasma, while the conventional radiative and three-body recombination processes disappeared. nH- is localized in the outer region where electrons exist (n(e) similar to 5 x 10(17) m(-3), T-e = 3-5 eV). The peak value of n(H-) is 1.2 x 10(17) 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. (C) 2003 Elsevier Science B.V. All rights reserved.