Jamison, S.P. Cliffe, M.J. Graham, D.M. Lake, D. Snedden, E.W. Walsh, D.A. JACoW Geneva, Switzerland 978-3-95450-177-9 10.18429/JACoW-IBIC2016-WEPG48 English 749-752 WEPG48 electron laser vacuum diagnostics acceleration Contribution to a conference proceedings 2017 2017-02 http://dx.doi.org/10.18429/JACoW-IBIC2016-WEPG48 http://jacow.org/ibic2016/papers/wepg48.pdf Progress towards a THz-driven transverse deflecting longitudinal profile diagnostic is presented. The deflector is driven with sub-picosecond quasi-single cycle THz fields generated by non-linear optical rectification. To utilize the large deflection field strength of the source for longitudinal diagnostics it is necessary to maintain the single-cycle field profile of the THz pulse throughout the interaction with the relativistic beam. Our scheme allows for the octave spanning bandwidth of the single-cycle pulses to propagate without dispersion at subluminal velocities matched to co-propagating relativistic electrons, by passing the pulse distortion and group-carrier walk-off limitations of dielectric loaded waveguide structure. The phase velocity is readily tuneable, both above and below the speed of light in a vacuum, and single-cycle propagation of deflecting fields at velocities down to 0.77c have been demonstrated.