U need an AC unit. My recommendation is ESAB CADDY 2200 AC/DC or Murex Tradestig AC220 which is probably more popular in UK than ESAB , but is the same machine, just difference in color and brand http://www.murexwelding.co.uk/mrxcont/arc/pdf/Tradestig AC220 AC220A (ACDC).pdf. With argon/helium mix you can go up to 8 mm.