How to extract only alpha from sequences?

i am struggling to extract alpha channel from sequences and save as sequences.

tried every way but "export image sequence" menu never render exactly alpha channel only.

i set Color option as "Alpha" and unchecked "Premultiply alpha" option.

but the results are always showing RGB images which masked by alpha.

i just want to extract alpha only. why this simple process is so hard to get?

is am i miss some or bug?

any suggestion would be thanks