Experiments

Laser-driven electron accelerators (laser linacs) offer the potential for enabling much more economical and compact devices with very high acceleration gradients.However, the development of practical laser linacs requires accelerating a large ensemble of electrons together (“trapping”) while keeping their energy spread small (“monoenergetic”).This has never been realized before for any laser acceleration system until the STELLA experiment.We have demonstrated for the first time efficient, monoenergetic trapping and acceleration of electrons via laser acceleration.

The basic concept utilized by STELLA is to first group the electrons within the electron-beam (e-beam) into microbunches (see Principle of Electron Microbunch Formation for more details).Briefly, the initial e-beam energy is distributed uniformly over all phases of the accelerating electromagnetic wave (i.e., laser beam optical field).A sinusoidal energy modulation is imparted onto the e-beam using the intense electric field of the laser beam.This accelerates some of the electrons and decelerates others.The fast electrons are allowed to catch up with the slow ones resulting in grouping (“bunching”) of the electrons into tiny clusters (“microbunches”). These microbunches can then be efficiently trapped and accelerated by a second laser acceleration device while maintaining a narrow energy spread.This process of trapping and acceleration by a second device is fundamental to staging multiple laser acceleration sections, which is necessary for enabling high net energy gains.Microbunching and staging were first demonstrated in an earlier precursor experiment [1]-[2].

The basic STELLA concept can be applied to many different laser acceleration schemes.For experimental convenience, inverse free electron lasers (IFEL) were chosen as the laser acceleration mechanism.An IFEL is a free electron laser operating in reverse.The laser beam co-propagates with the e-beam within the gap between a pair of parallel-facing magnet arrays called an undulator.Depending on the sign of the laser field seen by an electron, it will be accelerated or decelerated.

Figure 1 shows a schematic layout of the experiment.It consists of two IFELs driven by a single laser beam from the ATF CO₂ laser.The first IFEL (IFEL1) is called the buncher and causes the sinusoidal energy modulation.The second IFEL (IFEL2) is referred to as the accelerator. This IFEL traps and accelerates the microbunches.A bunch compressor or chicane is located between IFEL1 and IFEL2. The chicane has both a fixed magnetic field (i.e., permanent magnets) and a variable magnetic field (i.e., electromagnets).The fixed field forces the electrons to travel through a “V-shaped” trajectory in which the faster electrons generated by IFEL1 traverse a shorter path than the slower ones, thereby causing the electrons to bunch at the entrance to IFEL2. (See Principle of Electron Microbunch Formation for more details.) This creates a train of microbunches separated by 10.6 mm with individual bunch lengths of »1 mm (equivalent to »3 fs). Control of the phase delay between the microbunches and the laser field in IFEL2 is achieved by adjusting the variable field of the chicane. This enables resynchronizing the microbunches with the accelerating portion of the laser field by slightly delaying when the electrons exit the chicane. Usage of the chicane also makes the entire system more compact with a total length of 1.2 m from the entrance of IFEL1 to the exit of IFEL2. At the end of the experiment is an energy spectrometer for measuring the electron energy spectrum. See Photographic Tour of STELLA Experiment for more details of the experimental hardware.

In IFEL1 the gap separation is uniform along the undulator. IFEL2 uses a tapered undulator where the gap separation decreases by 11% towards the end of the undulator. Tapering is important for achieving high trapping efficiency and sufficient energy gain to separate the accelerated electrons from the background electrons.

Figure 2 shows an example of the energy spectrum of the e-beam only (red curve) along with the model prediction (gray histogram). Plotted on the abscissa is the energy shift from the initial e-beam energy (»45 MeV). The ordinate represents the number of electrons where the areas under the data and model curves have been normalized to each other. The intrinsic energy spread of the e-beam is very small (0.03%), thus the energy spread shown in Fig. 2(b) is actually due to the finite spectrometer resolution. The model includes the finite spectrometer resolution and demonstrates very good agreement with the data. (For more details of the model see Ref. 2.]

Figure 2(a). Comparison of e-beam-only spectrum (red curve) with model.

Figure 2(b). Enlarged view of spectrum shown in (a).

With the laser beam driving both IFELs, there is a dramatic change in the e-beam spectrum. Figure 3 gives an example of high trapping efficiency by IFEL2. The data (red curve) shows a large number of electrons gaining 7-9 MeV; in fact, an integration of the data curve indicates that »80% of the electrons have been trapped and accelerated with an energy spread of »1.2% (1s). The model prediction (gray histogram) agrees well with the data showing a similar amount of trapping efficiency.

Figure 3. Example of high-trapping efficiency data and comparison with model.

Figure 4 is an example of very narrow energy spread. The accelerated electrons (red curve) still gained >7 MeV, but with an energy width of »0.36% (1s) and a smaller trapping efficiency of »14%. The lower trapping efficiency appears to be due to nonsymmetric focusing of the e-beam within IFEL2 and a slight transverse offset between the e-beam and laser beam inside IFEL2, which causes some of the electrons to be partially accelerated. This is reflected in the model simulation (gray histogram), which included e-beam misfocusing and beam offset.

Figure 4. Example of narrow energy spread data and comparison with model.

1) W. D. Kimura, A. van Steenbergen, M. Babzien, I. Ben-Zvi, L. P. Campbell, C. E. Dilley, D. B. Cline, J. C. Gallardo, S. C. Gottschalk, P. He, K. P. Kusche, Y. Liu, R. H. Pantell, I. V. Pogorelsky, D. C. Quimby, J. Skaritka, L.C. Steinhauer, and V. Yakimenko, “First Staging of Two Laser Accelerators,” Phys. Rev. Lett. 86, 4041-4043 (2001).

2) W. D. Kimura, L. P. Campbell, C. E. Dilley, S. C. Gottschalk, D. C. Quimby, A. van Steenbergen, M. Babzian, I. Ben-Zvi, J. C. Gallardo, K. P. Kusche, I. V. Pogorelsky, J. Skaritka, V. Yakimenko, D. B. Cline, P. He, Y. Liu, L. C. Steinhauer, and R. H. Pantell, “Detailed Experimental Results for Laser Acceleration Staging,” Phys. Rev. ST Accel. Beams 4, 101301 (2001).

Other References Related to STELLA Program:

W. D. Kimura, M. Babzien, I. Ben-Zvi, L. P. Campbell, D. B. Cline, C. E. Dilley, J. C. Gallardo, S. C. Gottschalk, K. P. Kusche, R. H. Pantell, I. V. Pogoresky, D. C. Quimby, J. Skaritka, L. C. Steinhauer, V. Yakimenko, and F. Zhou, “STELLA-II: Staged Monoenergetic Laser Acceleration – Experiment Update,” in Advanced Accelerator Concepts, Jun. 23-28, 2002, Mandalay Beach, CA, AIP Conference Proceedings No. 647, C. E. Clayton and P. Muggli, Eds., (American Institute of Physics, New York, 2002), p. 269-277.

L. C. Steinhauer, W. D. Kimura, N. E. Andreev, S. V. Kuznetsov, and A. A. Pogosova, “Analysis of Laser Wakefield Acceleration Using ATF CO2 Laser,” in Advanced Accelerator Concepts, Jun. 23-28, 2002, Mandalay Beach, CA, AIP Conference Proceedings No. 647, C. E. Clayton and P. Muggli, Eds., (American Institute of Physics, New York, 2002), p. 751-759.

L. P. Campbell, C. E. Dilley, S. C. Gottschalk, W. D. Kimura, L. C. Steinhauer, M. Babzien, I. Ben-Zvi, J. C. Gallardo, K. P. Kusche, I. V. Pogorelsky, J. Skaritka, A. van Steenbergen, V. Yakimenko, D. B. Cline, P. He, Y. Liu, and R. H. Pantell, “Inverse Cerenkov Acceleration and Inverse Free Electron Laser Experimental Results for Staged Electron Laser Acceleration,” IEEE Trans. Plasma Science 28, 1143-1151 (2000).

W. D. Kimura, G. H. Kim, R. D. Romea, L. C. Steinhauer, I. V. Pogorelsky, K. P. Kusche, R. C. Fernow, X. Wang, and Y. Liu, “Laser Acceleration of Relativistic Electrons Using the Inverse Cerenkov Effect,” Phys. Rev. Lett. 74, 546-549 (1995).