DFQ--Double Frequency RFQ
  RFQ with a harmonic higher order quadrupole mode is studied. Assuming that we superpose a higher order mode with twice the frequency of the fundamental mode, a sawtooth waveform is approximated. In such a case, the bunching function is enhanced while the transverse stability is modified. Second order longitudinal harmonic component is also required to enable the effect. 
Y. Iwashita