Single-wall carbon nanohorn (SWNH) aggregates can be produced by CO2 laser vaporization of carbon, and a single aggregate can take either a "dahlia-like" or "bud-like" form. We found that "dahlia-like" SWNH aggregates were produced with a yield of 95% when Ar at 760 Torr was used as the buffer gas while "bud-like" SWNH aggregates were formed with a yield of 70 or 80% when either He or N-2 at 760 Torr was used. The internal structures of both aggregates were studied by partially burning them in an O-2 atmosphere. We were then able to examine the mechanism for the formation of SWNH aggregates.