Storage ring two-color free-electron laser

We report a systematic experimental study of a storage ring two-color free-electron laser (FEL) operating simultaneously in the infrared (IR) and ultraviolet (UV) wavelength regions. The two-color FEL lasing has been realized using a pair of dual-band high-reflectivity FEL mirrors with two different undulator configurations. We have demonstrated independent wavelength tuning in a wide range for each lasing color, as well as harmonically locked wavelength tuning when the UV lasing occurs at the second harmonic of the IR lasing. Precise power control of two-color lasing with good power stability has also been achieved. In addition, the impact of the degradation of FEL mirrors on the two-color FEL operation is reported. Furthermore, we have investigated the temporal structures of the two-color FEL beams, showing simultaneous two-color micropulses with their intensity modulations displayed as FEL macropulses.

Figure 1

Two different undulator configurations for the two-color operation. (a) Three-undulator configuration with the helical OK-5A undulator lasing in IR and the OK-4 optical klystron lasing in UV; (b) Four-undulator configuration with two OK-5 undulators (OK-5A and OK-5D) working as an optical klystron to replace the single OK-5A undulator in (a); (c) A schematic layout of a set of typical optical diagnostics for the two-color FEL operation.

Figure 2

Measured round-trip cavity losses around the high-reflectivity bands in the (a) IR and (b) UV regions of the FEL mirrors over a 25-month period. Loss curves L1 present the cavity losses after initial conditioning of the FEL mirrors. Loss curves L2 and L3 show the continued degradation of the mirrors’ reflectivity after they have been exposed to substantial accumulated radiation, estimated to be 9.1 and 11.2 ampere-hours respectively with electron beam energy between 280 and 560 MeV. The ranges enclosed by the dashed lines represent the ranges of single wavelength tuning, and the shaded regions on the curves represent the tuning ranges of the harmonic lasing. (see Sec. 3).

Figure 3

Tuning of the (a) IR and (b) UV wavelength of the two-color lasing with the four-undulator configuration and loss curves L2 (Run 3). In (a), the IR lasing (${\lambda }_1$) is shown to be tuned from 734.97 nm to 674.93 nm ($\mathrm{\Delta }{\lambda }_1=60.04\mathrm{nm}$) while fixing the UV lasing (${\lambda }_2$) around $360.69\pm 0.08\mathrm{nm}$. In (b), ${\lambda }_2$ is tuned from 374.02 nm to 358.10 nm ($\mathrm{\Delta }{\lambda }_2=15.92\mathrm{nm}$) while keeping ${\lambda }_1$ around $720.06\pm 0.07\mathrm{nm}$. The beam current is maintained between 12.03 mA and 12.78 mA in both (a) and (b).

Figure 4

Wavelength tuning with the UV wavelength set to the second harmonic of the IR wavelength using the four-undulator configuration and loss curves L2. ${\lambda }_1$ is tuned from 716.15 nm to 744.08 nm and ${\lambda }_2$ is varied from 358.25 nm to 371.99 nm simultaneously. During the measurement, the beam current was maintained between 12.35 mA and 12.94 mA.

Figure 5

Relative spectral widths (rms) in the wavelength tuning of the harmonic lasing in two runs, Run 1 and Run 3. Run 1 curves (starred and circled) represent the relative spectral widths in the measurement shown in Fig. 3 of Ref. [47]. In this measurement, the three-undulator configuration was used with loss curves L1 and beam current about 20 mA. Run 3 curves (squared and triangled) show the relative spectral widths in Fig. 4, where four undulators are used with loss curves L2, and the beam current is set to about 12 mA.

Figure 6

Electron beam bunch length measurements in the two-color operation with the four-undulator configuration and loss curves L2 (Run 3). The data points marked by “star” and “plus” represent the measured longitudinal profiles of the electron bunch with the lasing turned on and off, respectively. The data are fitted using Gaussian distribution (solid curves). The measured dissector resolution ${\sigma }_{b,\mathrm{res}}=69\mathrm{ps}$. The FEL is operated at ${\lambda }_1=712.31\mathrm{nm}$ and ${\lambda }_2=367.01\mathrm{nm}$. The beam current is 12.34 mA.

Figure 7

Two-color FEL power control by tuning buncher B3 with the four-undulator configuration and loss curves L2. The beam current is kept between 12.01 mA and 12.79 mA. (a) The measured spectra. (b) The FEL power of IR lasing (red solid), UV lasing (blue dashed), and the sum of the two powers (black).

Figure 8

Two-color FEL power control at two levels of cavity loss by tuning buncher B1 with the three-undulator configuration. (a) A revisualization of the data shown in Fig. 4(b) of Ref. [47] (loss curves L1, Run 1). $N_{B1}$ is tuned from 0 to 11.3. All data points are projected into $\mathrm{\Delta }{N}_{B1}\sim [0,1]$. ${\lambda }_1$ varies between 714.78 nm and 720.58 nm, while ${\lambda }_2$ is fixed at $360.13\pm 0.05\mathrm{nm}$. The beam current is maintained between 15.60 mA and 16.41 mA. (b) At a larger cavity loss (loss curves L3, Run 4). $N_{B1}$ is varied from 0 to 11.2. ${\lambda }_2$ is fixed at $368.03\pm 0.02\mathrm{nm}$ and ${\lambda }_1$ is shifted between 692.11 nm and 698.08 nm. The beam current is kept between 20.01 mA and 20.93 mA.

Figure 9

Long-term FEL power measurements with the three-undulator configuration and loss curves L1 (Run 2). (a) Spectra and (b) FEL powers of two-color lasing. The narrow shaded regions in (b) represent the injection periods in which the data points are eliminated in the calculation of rms power variations. The beam current is maintained between 15.78 mA and 16.54 mA with top-off injection.

Figure 10

Long-term FEL power measurements with the three-undulator configuration but different cavity losses. The data points and error bars represent the mean values and the rms variations of FEL power for the six 20-min measurements. The powers of each measurement are normalized to its sum of IR and UV power. Measurements no. 1 and no. 2 were taken with loss curves L1 and measurements no. 3 to no. 6 were taken with loss curves L3.

Figure 11

Two-color lasing power in phace space with the four-undulator configuration and loss curves L3. $N_{\mathrm{rf}}$ (y-axis) is the rf detune expressed in length normalized to ${\lambda }_1=695\mathrm{nm}$. (a) $N_{\mathrm{rf}}$ vs $N_{B1}$, while fixing B3 setting ($N_{B3}=0.60$ for 695 nm). The mean wavelength of IR and UV lasings are 697.52 nm and 368.40 nm, respectively. (b) $N_{\mathrm{rf}}$ vs $N_{B3}$, while fixing B1 setting ($N_{B1}=0.25$ for 695 nm). The mean wavelength of IR and UV lasing are 694.32 nm and 367.97 nm, respectively. In both cases, the rf detune is scanned from $-0.5\mathrm{Hz}$ ($N_{\mathrm{rf}}=-27.69$) to 0.5 Hz ($N_{\mathrm{rf}}=27.69$). $K_{\mathrm{OK}-4}=3.45$, $K_{\mathrm{OK}-5}=3.26$ and $N_{B2}=0.77$ for 368 nm. The beam current is maintained between 12.3 mA and 13.0 mA.

Figure 12

Two-color lasing phace spaces with the three-undulator configuration and loss curves L3. (a) $N_{B2}=0.47$ for 368 nm. (b) $N_{B2}=0.45$ for 368 nm. (c) Fine tuning of $N_{\mathrm{rf}}$ with $N_{B2}=0.45$ for 368 nm and more preferable optical axis alignment for IR lasing. (d) Fine tuning of $N_{\mathrm{rf}}$ with $N_{B2}=0.45$ for 368 nm and less preferable optical axis alignment for IR lasing. In (a) and (b), the rf detune is scanned from $-0.5\mathrm{Hz}$ ($N_{\mathrm{rf}}=-27.69$) to 0.5 Hz ($N_{\mathrm{rf}}=27.69$). In (c) and (d), the rf detune is scanned from $-0.29\mathrm{Hz}$ ($N_{\mathrm{rf}}=-16.06$) to -0.09 Hz ($N_{\mathrm{rf}}=-4.98$). In all cases, the IR and UV lasing wavelengths are about 695 nm and 368 nm, respectively, and $N_{B3}=0$. The beam current is maintained between 19.99 mA and 21.13 mA.

Figure 13

Short-term (16 milliseconds) power data of the two-color FEL beams in the three-undulator configuration (Run 2). The sampling rate is $1\mathrm{GS}/\mathrm{s}$. Two micropulses with the separation of about 358 ns (equal to the electron revolution period) are shown in the inset. In the main figure, the macropulse structure is shown by plotting the peaks of all the micropulses. The horizontal dashed lines show the mean power of each color. The rf detune is $-0.4\mathrm{Hz}$, and the beam current is about 16 mA.

Figure 14

Longer-term (50 seconds) power data of two-color lasing in the three-undulator configuration (Run 2). (a) Raw power data with a sampling rate of $100\mathrm{kS}/\mathrm{s}$. The inset is a zoom-in plot showing the details of FEL macropulses. (b) Amplitude of the Fourier transform of the same data plotted for the low-frequency range from 0 to 305 Hz. The rf detune is $-0.4\mathrm{Hz}$, and the beam current is about 16 mA.

Figure 15

Filtered 60-Hz ac component of the power data shown in Fig. 14 after applying a numerical bandpass filter (59.54 Hz–60.54 Hz). The slopes in the inset represent the period of the ac components (0.01666 s, corresponding to 60.01 Hz), and the intercept difference between two linear fittings (4.7 milliseconds) represents the phase difference ($0.56\pi \mathrm{rad}$) between this ac component in the two-color power signals.