The mechanism underlying the electron capture acceleration (Phys. Rev. E 58
(1998) 6575) is studied. It is found that the longitudinal electric held,
resulting from the transverse spatial gradient of the laser fields, is resp
onsible for the violent electron acceleration while the transverse fields p
lay the role of confining the electron inside the laser beam. (C) 2001 Else
vier Science B.V. All rights reserved.