Increase number in text format for each row

I´m struggling with something here that makes me dizzy... =/

I want to fetch data from one sheet called 'Index Series' and take a value from cell A1 and put together with
an increasing number starting from 0000001 that is in B1 in the same sheet.

The result would be 'test0000001' when values are cell A1 = 'test' cell B1 = '0000001'. 'Cell0000001'.

Now I want to put them in 'Sheet1' in cell A2 BUT i want to increase 00000001 by 1 for each row being 00000002 on the second and so on.
I want to keep the value from cell A1 though so 'test' would never change unless i do it manually.

Also if cell cell B2 in 'Sheet1' is empty i don't want anything at all. Just empty.

I started with something like this: (doesnt work)

=IF(B2=0;"";'Index Series'!A1&""&'Index Series'!B1)

But that only gives me first row... =(

I don't want to fill index series in 'Index Series' by dragging down the value row by row so the series fill up.
Want to keep the file as small as possible.

Thanks, works like a charm though i need it to start at row 3. Now first index number is 0000003.

Never mind my stupid head I just had to add the A1 so it starts from there......

Thank you Pepe!