Ultrashort electron bunch generation by an energy chirping cell attached rf gun

We present a new design for a rf electron gun to be used in ultrashort ($\sim 1\mathrm{ps}$) electron bunch generation. Using both simulation and measurement we evaluated the principle of this new type rf gun and were able to confirm an ultrashort bunch generation. During simulation, a bunch length of less than 100 fs(rms) with a $100\mathrm{pC}/\text{bunch}$ charge was confirmed at the optimum operating condition. The principle is to produce a linearly distributed longitudinal phase space by using an attached output cell specially designed for energy chirping. Such phase space distribution can be rotated by the velocity difference in the bunch. We already fabricated an energy chirping cell attached rf gun and successfully observed 0.2 THz coherent synchrotron radiation, which corresponds to less than 500 fs bunch. Such an electron gun can be used as a compact THz light source and a new electron injector with an ultrashort bunch.

Figure 1

Schematic of rf electron gun.

Figure 2

Bunch acceleration by rf electric field.

Figure 3

Evolution of $\gamma$ in the rf gun for the cases of ${\varphi }_0$, $\pm 2.5°$, and $\pm 5°$.

Figure 4

Phase space distribution at the end of part A (red), end of part B (blue), and at 2.33 m away from the cathode.

Figure 5

Profile of a conventional 1.6-cell rf gun. Designed to obtain a minimum energy spread.

Figure 6

Profile of an ECC-rf gun. ECC is added to a conventional rf gun shape. The parts, half cell (HC), full cell (FC), ECC, iris 1, and iris 2, are described.

Figure 7

Phase space distribution at the output of ECC-rf gun (top) and most compressed point (bottom) calculated by GPT.

Figure 8

Evolution of the bunch length by GPT. Three of laser injection rf phase are shown. The optimum phase was 20°, and a 88.4 fs (rms) ultrashort bunch was achieved at $z=2.3\mathrm{m}$.

Figure 9

Photograph of ECC-rf gun cavity. Cathode insertion port (near side), beam port (far side), rf waveguide (top), and vacuum pumping port (bottom). Water cooling pipes and rf tuners can be seen around the cavity.

Figure 10

Expected bunch length as a function of injection phase at $z=3.0\mathrm{m}$. The shortest bunch was 180 fs (rms) at the phase of 19°.

Figure 11

Experimental setup for the ECC-rf gun.

Figure 12

Results of charge and energy measurement as a function of injection phase. The dots and lines show measurement results and simulation results, respectively. Measurement and simulation show good agreement.

Figure 13

Calculated synchrotron radiation spectrum by a 3.7 MeV electron bent by a 0.14 T magnetic field. Coherent enhancement of various bunch lengths are plotted on the incoherent spectrum.

Figure 14

Waveform of SR detected by 0.1 THz SBD (pink) and bunch charge by FCT (blue). The pulse height of SBD waveform was 67.6 mV from about $100\mathrm{pC}/\text{bunch}$ electrons.

Figure 15

0.2 THz intensity as a function of bunch charge. The dotted line is the best fit for the experimental data. SR intensity was proportional to the square of bunch charge, i.e. coherent enhancement was observed.

Figure 16

CSR intensity as a function of the injection phase detected by 0.05 THz SBD (green), 0.1 THz SBD (blue), and 0.2 THz $\mathrm{BPF}+\mathrm{SBD}$ (red). Bunch charge is also plotted as a reference. The peak of THz intensity was around 20° and the profile became narrower as the radiation frequency grew higher.