Observation of space-charge driven beam instabilities in a radio frequency photoinjector

Dowell, D.H.; Joly, Simon; Loulergue, Alexandre; Brion, J.P. De; Haouat, G.

doi:10.1063/1.872477

Cited by 31 publications

(22 citation statements)

References 11 publications

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“…The distortion in the profile at the high-charge value in Fig. 6 may be signahng the onset of sub-bunching similar to that observed by Dowell et al 8 Limited laser power precluded taking the charge to higher values.…”

Section: Resultssupporting

confidence: 49%

Streak Camera Enhanced Quadrupole Scan Technique

Gierman¹

2000

The Physics of High Brightness Beams

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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

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Section: Resultssupporting

confidence: 49%

Streak Camera Enhanced Quadrupole Scan Technique

Gierman¹

2000

The Physics of High Brightness Beams

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“…The longitudinal distribution can be described by three parameters -pulse width, correlated energy spread and uncorrelated energy spread -analogous to the three independent beam matrix parameters in transverse phase space. However, space charge and the sinusoidal dependence of the applied RF field tend to distort the longitudinal emittance ellipses so that it is necessary to introduce two additional parameters to fully characterize the longitudinal emittance exiting the RF gun [4]. The energy spread of the beam, ∆E, is given by Equation (1.1) as a function of the deviation from the centroid temporal position, ∆t.…”

Section: Measurement Techniquementioning

confidence: 99%

6D Phase Space Measurements at the SLAC Gun Test Facility

Schmerge¹

2003

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Proposed fourth generation light sources using SASE FELs to generate short pulse, coherent X-rays require demonstration of high brightness electron sources. The Gun Test Facility (GTF) at SLAC was built to test high brightness sources for the proposed Linac Coherent Light Source at SLAC. The GTF is composed of an Sband photocathode rf gun with a Cu cathode, emittance compensating solenoid, single 3 m SLAC linac section and e-beam diagnostic section with a UV drive laser system. The longitudinal emittance exiting the gun has been determined by measuring the energy spectrum downstream of the linac as a function of the linac phase. The e-beam pulse width, correlated and uncorrelated energy spread at the linac entrance have been fit to the measured energy spectra using a least square error fitting routine. The fit yields a pulse width of 2.9 ps FWHM for a 4.3 ps FWHM laser pulse width and 2% rms correlated energy spread with 0.07% rms uncorrelated energy spread. The correlated energy spread is enhanced in the linac to allow slice emittance measurements by conducting a quadrupole scan in a dispersive section. The normalized slice emittance has been measured to be as low as 2 mm-mrad for beams with peak currents up to 150 A (300 pC with a laser pulse length of 1.8 ps) while the full projected emittance is 3 mm-mrad.

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“…Photoinjectors generally operate below the space charge limit, which allows the electron pulse shape to duplicate the laser pulse shape. Attempting to exceed the space charge limit will cause the onset of virtual cathode oscillations that cause the electron pulse shape to be very different than the laser pulse shape [17,18]. Gridded electron guns are generally designed to operate at the space charge limit; even if the gun as a whole is not operated at the space charge limit, the grid-cathode region will be, as it is this mechanism which allows the grid voltage to control the beam current [2].…”

Section: Generation Of Density-modulated Beamsmentioning

confidence: 99%

Longitudinal density modulation and energy conversion in intense beams

et al. 2007

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Density modulation of charged particle beams may occur as a consequence of deliberate action, or may occur inadvertently because of imperfections in the particle source or acceleration method. In the case of intense beams, where space charge and external focusing govern the beam dynamics, density modulation may under some circumstances be converted to velocity modulation, with a corresponding conversion of potential energy to kinetic energy. Whether this will occur depends on the properties of the beam and the initial modulation. This paper describes the evolution of discrete and continuous density modulations on intense beams, and discusses three recent experiments related to the dynamics of density-modulated electron beams. PACS

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Observation of space-charge driven beam instabilities in a radio frequency photoinjector

Cited by 31 publications

References 11 publications

Streak Camera Enhanced Quadrupole Scan Technique

Streak Camera Enhanced Quadrupole Scan Technique

6D Phase Space Measurements at the SLAC Gun Test Facility

Longitudinal density modulation and energy conversion in intense beams

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