Results of a gas-ionization beam size monitor at the final focus test beam

A new type of gas-ionization monitor, built at Orsay, has been used to optimize and measure the transverse beam dimensions of the Final Focus Test Beam at SLAC. This monitor detects the ions created by the beam to probe its space charge field and to derive both transverse dimensions. The ultimate values measured with the monitor are 1.5 μm × 60 nm. Moreover, the high sensitivity of the monitor has been used to detect and cancel out detrimental “banana” shapes of the beam.