Simplest way, that will take very low overhead for your program (such as might be needed for real-time work):
fid = fopen(OutputFileName, 'w'); %not 'wt'
for ...
fwrite(fid, YourBufferOfDoubles);
end
fclose(fid)
However, you have about 480 megabytes worth of raw data, and that is enough that unless you are writing to SSD, that the file overhead is going to add up.
Because of that, you might want to consider methods that compress data as you go. You would buffer some data and call compression routines sometimes, and write out the compressed data. One way to do that would be https://docs.oracle.com/javase/7/docs/api/java/util/zip/GZIPOutputStream.html
A third way... just store all the values in memory as you produce them (having pre-allocated the array), and afterwards save() them to a .mat file. .mat does compression.
