Thecollectiveresponseofelectronsinanultrathinfoiltargetirradiatedbyanultraintense(6×1020Wcm-2)laserpulseisinvestigatedexperimentallyandvia3Dparticle-in-cellsimulations.Itisshownthatifthetargetissufficientlythinthatthelaserinducessignificantradiationpressure,butnotthinenoughtobecomerelativisticallytransparenttothelaserlight,theresultingrelativisticelectronbeamiselliptical,withthemajoraxisoftheellipsedirectedalongthelaserpolarizationaxis.Whenthetargetthicknessisdecreasedsuchthatitbecomesrelativisticallytransparentearlyintheinteractionwiththelaserpulse,diffractionofthetransmittedlaserlightoccursthroughasocalled‘relativisticplasmaaperture’,inducingstructureinthespatial-intensityprofileofthebeamofenergeticelectrons.Itisshownthattheelectronbeamprofilecanbemodifiedbyvariationofthetargetthicknessanddegreeofellipticityinthelaserpolarization.
