Streak Camera Enhanced Quadrupole Scan Technique

Abstract

SummaryTo (Claerbout, 1985). The phase-shift migration (Gazdag, 1978) (Stoffa et al., 1990 Huang and Fehler, 1998) (Huang et al., 1999b), the extended local Rytov Fourier (Huang et al., 1999a), and the quasi-Born Fourier (Huang and Fehler, 2000) methods, are more accurate than the SSF method, but they are less accurate than the FFD method for large lateral velocity contrasts. Another version of FFD (Xie and Wu, 1998) is based on the approximation of the square-root operator using the first order Pad6 approximation which is the same as Claerbout's 45°or Muir's Rz apaproximations (Claerbout, 1985 followed by a finitedifference scheme to accurately image structures with large lateral velocity contrasts. Some other Fourier transform based methods, such as the extended local Born Fourier Expansionof Square-RootOperatorThe one-way wave equation in the frequency-space domain iswhere P is the pressure and the operator Q is defined bywhere w is the circular frequency, v is the velocity, and R is the square-root operator given bywith We expand the square-root operator R in the form A streak camera is used in combination with the quadruple scan technique to characterize the temporal dependence of the trace-space distribution of a bunched electron beam. The quadruple scan procedure is used to determine the ellipse that represents the beam on the two-dimensional trace space (the xx' plane), and the streak camera diagnostic shows how electrons in axial slices at different locations within a single, average bunch contribute to the trace-space distribution and emittance of the full bunch. For purposes of analysis, the 15-20 pe FWHM electron bunch is partitioned into seven contiguous 4-ps slices. The quadruple scans are performed over a range of focusing strengths in the solenoid lens that is wrapped around the photocathode cell of an 1l-cell 1300 MHz rf photoinjector. The radial focusing of the solenoid is intended to reverse a space-charge-driven emittance blow-up that occurs in the low-energy end of the photoinjector and that is correlated with the axially nonuniform transverse space-charge field. In the parameter space investigated in this experiment, the solenoid does not appear to perform that function. Simulations of the experiment with the particle code PARMELA indicate that by keeping the transverse dimension of the beam small, the focusing does circumvent an rf-driven emittance growth, but otherwise does little to reverse the emittance blow-up that occurs in the first few cells. R=l-ax2

End-point energy measurements of field emission current in a continuous-wave normal-conducting rf injector

“…The applied rf field at the time of photoemission needs to be sufficiently high to overcome the image charge. Otherwise, the electron bunch will exhibit temporal distortions characteristic of space-charge limited emission [17]. For a given injection phase, bunch charge, and photoemission radius, the following inequality must be satisfied:…”