Schlieren Imaging for the Determination of the Radius of an Excitated Rubidium Column

AWAKE develops a new plasma wakefield accelerator using the CERN SPS proton bunch as a driver Muggli et al. (2017). The proton bunch propagates through a <math id="mml11" display="inline" overflow="scroll" altimg="si11.gif"><mn>10</mn><mspace width="0.16667em"/><mtext>m</mtext></math> long rubidium plasma, induced by an ionizing laser pulse. The co-propagation of the laser pulse with the proton bunch seeds the self modulation instability of the proton bunch that transforms the bunch to a train with hundreds of bunchlets which drive the wakefields. Therefore the plasma radius must exceed the proton bunch radius. Schlieren imaging is proposed to determine the plasma radius on both ends of the vapor source. We use Schlieren imaging to estimate the radius of a column of excited rubidium atoms. A tunable, narrow bandwidth laser is split into a beam for the excitation of the rubidium vapor and for the visualization using Schlieren imaging. With a laser wavelength very close to the D2 transition line of rubidium (<math id="mml12" display="inline" overflow="scroll" altimg="si12.gif"><mi>λ</mi><mo>≈</mo><mn>780</mn><mspace width="0.16667em"/><mtext>nm</mtext></math>), it is possible to excite a column of rubidium atoms in a small vapor source, to record a Schlieren signal of the excitation column and to estimate its radius. We describe the method and show the results of the measurement.