Harmonically resonant cavity as a bunch-length monitor

A compact, harmonically resonant cavity with fundamental resonant frequency 1497 MHz was used to evaluate the temporal characteristics of electron bunches produced by a 130 kV dc high voltage spin-polarized photoelectron source at the Continuous Electron Beam Accelerator Facility (CEBAF) photoinjector, delivered at 249.5 and 499 MHz repetition rates and ranging in width from 45 to 150 picoseconds (FWHM). A cavity antenna attached directly to a sampling oscilloscope detected the electron bunches as they passed through the cavity bore with a sensitivity of $\sim 1\mathrm{mV}/\mu \mathrm{A}$. The oscilloscope waveforms are a superposition of the harmonic modes excited by the beam, with each cavity mode representing a term of the Fourier series of the electron bunch train. Relatively straightforward post-processing of the waveforms provided a near-real time representation of the electron bunches revealing bunch-length and the relative phasing of interleaved beams. The noninvasive measurements from the harmonically resonant cavity were compared to measurements obtained using an invasive RF-deflector-cavity technique and to predictions from particle tracking simulations.

Figure 1

Top: cut-away view of the harmonically resonant cavity. Bottom: photograph of the cavity nested inside the bore of a 10” double-sided knife edge Conflat flange. Two additional 10” Conflat flanges attach to either side to form the UHV-compatible vacuum vessel.

Figure 2

The CEBAF photoinjector. Note the location of the chopping system, and the two measurement locations of the harmonically resonant cavity bunch-length monitor.

Figure 3

Electron bunch-length measurements using the RF-deflector cavity, for RF-bunched beam at 499 MHz using “laser 1” with 45 ps optical pulsewidth FWHM (top) and 249.5 MHz using “laser 2” with 60 ps optical pulsewidth FWHM (bottom), for average beam current ranging from 1 to $100\mu \mathrm{A}$. The insets provide more detail for the lowest current measurements which suffer the least amount of space charge induced bunch-length growth, and therefore most closely resemble the laser optical pulse shape.

Figure 4

(a) the raw oscilloscope trace for $10\mu \mathrm{A}$ beam through the harmonically resonant cavity at 499 MHz repetition rate (red) and the assumed bunch profile that provided a transfer function that produced transformed oscilloscope waveforms with the minimum signal outside the central bunch (blue), (b) the relative signal strength for each resonant mode of the harmonically resonant cavity: red, cavity as measured, and blue, the response of an ideal harmonic cavity.

Figure 5

Bunch-length plots using the harmonically resonant cavity for electron beams at 499 MHz using “laser 1” with 45 ps optical pulsewidth FWHM (top) and 249.5 MHz using “laser 2” with 60 ps optical pulsewidth FWHM (bottom), for different extracted beam currents. These plots were obtained by transforming the raw oscilloscope waveforms.

Figure 6

Simulation results showing the transverse (top) and longitudinal (bottom) rms sizes the beam as a function of distance from the photocathode for various beam currents using the laser at 249.5 MHz. The arrows denote the locations of the harmonically resonant cavity and the narrow aperture between the two RF-deflecting cavities.

Figure 7

Electron bunch-length as a function of bunch charge (average current varied from 1 to $100\mu \mathrm{A}$) with measurements obtained using the invasive RF-deflecting cavity technique (top) and the harmonically resonant cavity (bottom), together with simulation results using the particle tracking code ASTRA. Left) measurements made using “laser 1” at 499 MHz and, Right) measurements made using “laser 2” at 249.5 MHz.

Figure 8

Current from the zero-bias Schottky diode as one electron bunch train was passed across the other by varying the RF-phase of one drive laser. Both beams were produced using lasers with 249.5 MHz pulse repetition rate. The prominent dips corresponds to precise time intervals between bunches, $n\times 668\mathrm{ps}$. The dip with smallest magnitude corresponds to bunches coincident in time.

Figure 9

Top: Raw oscilloscope waveform showing a bandwidth-limited electron bunch-length measurement for $25\mu \mathrm{A}$ average current at 499 MHz repetition rate and 500 keV beam energy. Bottom: the spectral content of the same waveform indicating the harmonically resonant cavity’s operational bandwidth of $\sim 20\mathrm{GHz}$.