Accelerator Test Facility

We report the first results of high intensity x-ray generation using Inverse Laser Compton scattering. This experiment was carried out by a US-Japan collaboration at the Brookhaven National Laboratory (BNL) Accelerator Test Facility (ATF) in September 1999.The ATF is an accelerator and beam physics user facility. The 3.5 ps x-ray pulse at 6.5 keV, containing ~10⁷ x-ray photons was generated by interacting 60 MeV, 0.5 nC electron bunches and CO₂ laser pulses with the peak power of 600 MW.

1 Introduction

High intensity, short pulse and compact x-ray sources are required in various fields of the scientific, industrial and medical research. To meet these demands, R&D on the next generation light sources has been initiated in several laboratories [1]. One of the most promising approaches to ultra-bright pulsed x-ray sources is the Laser Synchrotron Source (LSS). It is based on inverse Compton scattering via interaction between pulsed high power laser beams with pico-second relativistic electron bunches.

One of the attractive features of the laser Compton scattering is easy control of polarization of the produced high energy photons that duplicates polarization of the applied laser beam. This method have been proposed to generate circularly polarized g -rays in the prospective polarized positron source for Japan Liner Collider [2].

The US-Japan collaborative experiment on ultra-bright x-ray generation using inverse Compton scattering was performed at the Brookhaven Accelerator Test Facility (BNL-ATF). This experiment takes advantage of the availability of a high-brightness 60 MeV electron RF linac and high peak power CO₂ laser. The CO₂ laser, with it’s long wavelength (ten times longer than solid state lasers), is a good choice for the LSS driver because it can generate large numbers of x-ray photons for a given laser energy. In this communication, we report the first results of this experiment.

2 Experiment Description

A principle design of the vacuum interaction chamber for the CO₂ laser Compton scattering experiment is shown in Fig. 1 and described in [3].

Figure 1. Top view of the Compton chamber

The CO₂ laser pulse and the electron bunch propagate along the same axis in the opposite directions and collide at focal point. The laser beam is focused and re-collimated inside the chamber with two off-axis parabolic Cu mirrors with focal length 150 mm and 5 mm diameter hole drilled along the beam axis. These holes are necessary for the propagation of the electron beam and backscattered x-rays. To bypass the holes, the laser beam is transformed from the Gaussian spatial profile to a "donut"-shaped profile using an axicon telescope located outside the chamber. The 600 MW, 200 ps pulsed CO₂ laser beam is introduced through the ZnSe windows and focused at the center of the chamber. After interaction, laser beam is reflected on another parabolic mirror again and is extracted from the chamber.

The 3.5-10 ps, 0.5-1.0 nC electron bunches produced at the photocathode RF gun and accelerated to 60 MeV (g =120) by the RF linac are magnetically focused in the middle point of the interaction cell to the s =40 m m spot. The timing jitter between the CO₂ laser and electron bunches is negligible in comparison with the electron pulse width since a mode-locked Nd:YAG laser is used for both processes of the CO₂ pulse optical switching and a photo cathode illumination of the RF gun.

To measure the electron beam size and align the CO₂ laser and the electron beam at the collision point precisely, we used a double target that has a phosphor screen and a glass fiber cross. The cross allows correlation between the e-beam and the CO₂ focal spot positions.

The electron beam and the CO₂ laser beam parameters are given in Table 1.

Electron Bunch

Beam energy

Bunch charge

Bunch length (FWHM)

Beam size at focal point (sx/sy)

60 MeV

0.5 nC

3.5 ps

40/40 mm

CO₂ laser

Wave length

Energy/pulse

Pulse length (FWHM)

Beam size at focal point (sx/sy)

10.6 mm

200 mJ

180-ps

40/40 mm

Table 1. Electron beam and CO₂ laser parameters

A dipole magnet separated the electron bunches and the scattered x-ray beam after the interaction point. The total number of the back-scattered photons was measured using a silicon (Si) diode detector (f=25 mm), which placed 1.4 m downstream from the interaction region. Low energy photons were attenuated by the 250 mm thick beryllium (Be) output window and 20 cm of air.

3. Experimental Results

Thanks to the matched focusing and exact alignment and synchronization of the laser and electron beams, we were able to observe strong Compton x-ray signal on the Si detector, much above the background level (defined by high energy x-rays due to the 60 MeV electron beam bremsstrahlung). A typical signal to noise ratio was up to 100 (see Fig. 2). The strong signal permitted a confident characterizing of the observed effect and a study of the Compton yield dependence upon various experimental parameters as is reported below.

Figure 2. Typical scope traces of the Si diode output show the Compton x-ray signal with the "laser on" (top trace, 100mV/div scale) and the bremsstrahlung "laser off" signal (bottom trace, 50mV/div).

The measured maximum x-ray signal from the Si detector was about 2 V. This signal corresponds to 5´ 10⁶ photons/pulse within energy range from approximately 5 keV to 6.5 keV. In the backscattering configuration, the x-ray pulse duration is equal to the electron bunch length (3.5 ps). This brings us to 2´ 10¹⁸ photons/second.

For the current experiment conditions, the maximum scattered photon energy that results from the 0.113 eV CO₂ laser photon upshift by the 60 MeV electrons was 6.5 keV. The detectable minimum photon energy was 5 keV. The minimum energy threshold is due to the combined effect of the angle acceptance of the Si detector and x-ray absorption in the Be window and air. The energy bandwidth is confirmed by using metal foil filters and, indirectly, by a comparison of the theoretical and experimental dependence of the x-ray flux upon the opening solid angle on the detector. To obtain the angular distribution of scattered x-ray photons, we used a variable iris diaphragm placed in front of the Si detector. Fig.3 shows the output signal of the Si detector versus the iris diameter and the calculation results that take into account an attenuation at the Be window and 20 cm of air.

Figure 3. Compton x-ray transmission through the iris diaphragm as a function of the diameter. Bars represent the experimental results compared with the theoretically calculated curve.

Figure 4. CO₂ laser longitudinal profile

A variable delay was used in the Nd:YAG laser line used to slice the CO₂ laser pulse to adjust the timing between the electron bunch and CO₂ laser pulse. Incidentally, this provides a tool to measure the CO₂ laser longitudinal profile. Fig. 4 Shows the CO₂ laser pulse envelope obtained by measuring the x-ray signal while scanning the Nd:YAG laser delay. On the basis of these observations we conclude that the laser pulse has a non-Gaussian asymmetric shape and is about 180 ps FWHM. The 600 MW peak power is obtained by time integrating of the plot in Fig.4 and normalizing it to the typical 200 mJ energy in the pulse.

Similarly, the transverse of the CO₂ laser beam is measured by observing the x-ray signal as a function of the transverse steering of the electron beam. The results of these measurements are shown in Fig. 5 and yield a s =40 m m spot at the interaction point. For the purpose of this measurement the electron beam size on the phosphor screen was characterized.

Figure 5. Transverse scan of the laser focus with the electron beam.

4. SUMMARY

We report results of the intense x-ray generation using inverse Compton scattering of CO₂ laser pulses from relativistic electron bunches. The generated number of x-ray photons was 5´ 10⁶ photons/pulse and 2´ 10¹⁸ photons/second. We believe that this is the strongest x-ray yield observed so far in the proof-of-principle LSS experiments. This is achieved due to the availability of a combination of the high-brightness picosecond electron beam, the high mid-IR photon flux CO₂ laser at the BNL ATF and the use of a backscattering configuration. Upon completion of the ongoing ATF CO₂ laser upgrade to the terawatt power and proposed electron bunch compression to femtoseconds we plan to demonstrate LSS with the x-ray yield of the order of 10¹⁰ photons/pulse and flux up to 10²³ photon/sec.

ACKNOWLEDGMENTS

The authors wish to thank the BNL scientific and technical personnel for their help in the preparation and conducting of this experiment. We would like to acknowledge P. Siddons, C. Kao, S. Khalid and D. Lott for their help with the Si detector calibration. Support of this study is provided by the U.S. Dept. of Energy and by the US-Japan Collaboration in High Energy Physics.

REFERENCES

[1] W. Leemans et al., Proc. of Int. Conf. on PAC 95,

[2] T. Okugi et al., Jpn. J. Appl. Phys. 35 (1996) 3667

[3] A. Tsunemi et al., Proc. of Int. Conf. On PAC 99, New York USA

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