Accelerator Test Facility

Nonlinear harmonic radiation was observed using the VISA self-amplified, spontaneous emission (SASE) free-electron laser (FEL) at saturation. The gain lengths, spectra, and energies of the three lowest SASE FEL modes were experimentally characterized. The measured nonlinear harmonic gain lengths and center spectral wavelengths decrease with harmonic number, n, which is consistent with nonlinear harmonic theory. Both the second and third nonlinear harmonics energies are about 1% of the fundamental energy. These experimental results demonstrate for the first time the feasibility of using nonlinear harmonic SASE FEL radiation to produce coherent, femtosecond x rays.

Three dimensional test particle simulations are applied to optimization of the plasma-channeled laser wakefield accelerator (LWFA) operating in a weakly nonlinear regime. Electron beam energy spread, emittance, and luminosity depend upon the proportion of the electron bunch size to the plasma wavelength. This proportion tends to improve with the laser wavelength increase. We simulate a prospective two-stage $\ensuremath{\sim}1\mathrm{GeV}$ LWFA with controlled energy spread and emittance. The input parameters correspond to realistic capabilities of the BNL Accelerator Test Facility that features a picosecond-terawatt ${\mathrm{CO}}_{2}$ laser and a high-brightness electron gun.

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Here we discuss how expanding the scope of relativistic plasma research to wavelengths longer than λ/≈0.8₋1.1μm covered by conventional mode-locked solid-state lasers would offer attractive opportunities due to the quadratic scaling of the ponderomotive electron energy and critical plasma density with λ. Answering this quest, a next-generation mid-IR laser project is being advanced at the BNL ATF as a part of the user facility upgrade. We discuss the technical approach to this conceptually new 100 TW, 100 fs, λ=9₋11 μm CO₂ laser BESTIA (Brookhaven Experimental Supra-Terawatt Infrared at ATF) that encompasses several innovations applied for the first time to molecular gas lasers. BESTIA will enable new regimes of laser plasma accelerators. One for example is shock-wave ion acceleration from gas jets. We review ongoing efforts to achieve stable, monoenergetic proton acceleration by dynamically shaping the plasma density profile from a hydrogen gas target with laser-produced blast waves. At its full power, 100 TW BESTIA promises to achieve proton beams at energy exceeding 200 MeV. In addition to ion acceleration in over-critical plasma, the ultra-intense mid-IR laser BESTIA will open new opportunities in driving wakefields in tenuous plasmas, expanding the landscape of Laser Wake Field Accelerator (LWFA) studies into unexplored long-wavelength spectral domain. Simple wavelength scaling suggests that a 100-TW CO2 laser beam will be capable to efficiently generate plasma “bubbles” thousand times bigger in volume compared to a near-IR solid state laser of an equivalent power. Combined with a femtosecond electron linac available at the ATF, this wavelength scaling will facilitate study of external seeding and staging of LWFA.

We utilized a nonlinear post-compression technique to generate 675-fs, 9.2-μm CO 2 laser pulses with a peak power of 1.6 TW. This achievement represents the highest peak power ever attained in the femtosecond pulse format within the long-wave infrared (LWIR) spectral range. The successful implementation of this post-compression technique opens avenues for the development of few-cycle, multi-terawatt 9–10 μm lasers, crucial for applications currently relying on near-infrared solid-state lasers, and which stand to benefit from the scaling of laser wavelengths into the long-wave infrared region.

Electron beam bunch compression directly from photocathode RF gun injector was experimentally observed at the Brookhaven Accelerator Test Facility (ATF). The analysis is presented in this report shows that, the configuration of transverse space-charge emittance compensation photoinjector can also be operated in bunch length compression mode for modest amount charge (<1.0 nC), i.e., longitudinal emittance compensation. For a constant laser energy, the electron beam bunch length almost linearly decrease with the RF gun phase, and the compression ratio as large as factor of 30 was experimentally observed for a 40 pC charge. We also discuss the effect of electron beam bunching inside the RF gun on the transverse emittance, and compared with experimental results.

We report on the measurement of filamented transport of laser-generated fast electron beams in near-critical density plasma. A relativistic intensity long-wave-infrared laser irradiated a hydrodynamically shaped helium gas flow at an electron density n_{e}≃10^{25} m^{-3}, generating a large flux of fast electrons that propagated beyond the critical surface. The beam-to-background electron density ratio was sufficiently high to drive growth of Weibel-like filamentation, which was measured by optical probing to extend up to 800 μm with radii ∼10 μm. Particle-in-cell simulations reproduce the main features of the filamentation generation, suggesting that collisionless processes are dominant in these interactions. Expansion of the filaments after formation infers a fast electron heated plasma temperature ∼400 eV in the overcritical density plasma.

The first terawatt picosecond (TWps) CO2 laser is under construction at the BNL Accelerator Test Facility (ATF). TWps-CO2 lasers, having an order of magnitude longer wavelength than solid state lasers, offer new opportunities for strong-field physics research. For laser wakefield accelerators (LWFA) the advantage of the new class of lasers is due to a gain of two orders of magnitude in the ponderomotive potential. The demonstrated large average power of CO2 lasers is important for the generation of hard radiation through Compton back-scattering of the laser off energetic electron beams. We discuss applications of TWps-CO2 lasers for LWFA modules of a tentative electron-positron collider, for a γ-γ (or γ-lepton) collider, for a possible “table-top” source of high-intensity x-rays and gamma rays, and the generation of polarized positron beams.

A picosecond CO2 laser was used successfully in a number of experiments exploring advanced methods of particle acceleration [1]. Proton acceleration from gas-jet plasma exemplifies another advantage of employing the increase in laser wavelength from the optical to the mid-IR region. Recent theoretical- and experimental-studies of ion acceleration from laser-generated plasma point to better ways to control the ion beam’s energy when plasma approaches the critical density. Studying this regime with solid-state lasers is problematic due to the dearth of plasma sources at the critical electron density ∼1021 cm−3, corresponding to laser wavelength λ = 1 μm. CO2 laser offers a solution. The CO2 laser’s 10 μm wavelength shifts the critical plasma density to 1019 cm−3, a value attainable with gas jets. Capitalizing on this approach, we focused a circular polarized 1-TW CO2 laser beam onto a hydrogen gas jet and observed a monoenergetic proton beam in the 1–2 MeV range. Simultaneously, we optically probed the laser/plasma interaction region with visible light, revealing holes bored by radiation pressure, as well as quasi-stationary soliton-like plasma formations. Our findings from 2D PIC simulations agree with experimental results and aid in their interpretation.

Demonstration of ultra-high acceleration gradients in the SM LWFA experiments put a next objective for the laser accelerator development: to achieve a low-emittance monochromatic acceleration over extended interaction distances. The emerging picosecond terawatt (ps-TW) CO2 laser technology helps to meet this strategic goal. Among the considered example are: the staged electron laser accelerator (STELLA) experiment, which is being conducted at the Brookhaven ATF, and the plasma-channeled LWFA. The long-wavelength and high average power capabilities of CO2 lasers may be utilized also for generation of intense x-ray and gamma radiation through Compton back-scattering of the laser beams off relativistic electrons. We discuss applications of ps-TW CO2 lasers for a tentative γ-γ (or γ-lepton) collider and generation of polarized positron beams.

Dependence of the LWFA performance upon the laser wavelength is applied to optimization of the plasma-channeled standard LWFA operating in a linear regime. Electron beam energy spread, emittance and luminosity depend upon the proportion of the electron bunch size to the plasma wavelength. This proportion tends to improve with the laser wavelength increase. We propose the two-stage ∼1 GeV LWFA with the controlled energy spread and emittance based on realistic capabilities of the BNL ATF that features: picosecond terawall CO2 laser and a high-brightness electron gun.

Mid-infrared gas laser technology promises to become a unique tool for research in strong-field relativistic physics. The degree to which physics is relativistic is determined by a ponderomotive potential. At a given intensity, a 10 μm wavelength CO2 laser reaches a 100 times higher ponderomotive potential than the 1 μm wavelength solid state lasers. Thus, we can expect a proportional increase in the throughput of such processes as laser acceleration, x-ray production, etc. These arguments have been confirmed in proof-of-principle Thomson scattering and laser acceleration experiments conducted at BNL and UCLA where the first terawatt-class CO2 lasers are in operation. Further more, proposals for the 100 TW, 100 fs CO2 lasers based on frequency-chirped pulse amplification have been conceived. Such lasers can produce physical effects equivalent to a hypothetical multi-petawatt solid state laser. Ultra-fast mid-infrared lasers will open new routes to the next generation electron and ion accelerators, ultra-bright monochromatic femtosecond x-ray and gamma sources, allow to attempt the study of Hawking-Unruh radiation, and explore relativistic aspects of laser-matter interactions. We review the present status and experiments with terawatt-class CO2 lasers, sub-petawatt projects, and prospective applications in strong-field science.

The progress of various high-brightness electron source technologies, specially laser based electron sources, such as photocathode RF gun, DC and pulsed photo-gun, and laser plasma electron source, will be reviewed. The physics processes of those sources to realize best performance are clarified according to three basic ideas. They are transverse space charge emittance compensation, space charge compensation and longitudinal emittance compensation. The major progresses and issues of photoinjector are briefly reviewed. The performance of photocathode RF gun at the Brookhaven Accelerator Test Facility (ATF) is presented.

We present an approach for generating collider-quality electron bunches using a plasma photoinjector. The approach leverages recently developed techniques for the spatiotemporal control of laser pulses to produce a moving ionization front in a nonlinear plasma wave. The moving ionization front generates an electron bunch with a current profile that balances the longitudinal electric field of an electron beam-driven plasma wave, creating a uniform accelerating field across the bunch. Particle-in-cell (PIC) simulations of the ionization stage show the formation of an electron bunch with 220 pC charge and low emittance ( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:msub> <a:mi>ɛ</a:mi> <a:mi>x</a:mi> </a:msub> <a:mo>=</a:mo> <a:mn>171</a:mn> </a:mrow> </a:math> nm rad, <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:msub> <b:mi>ɛ</b:mi> <b:mi>y</b:mi> </b:msub> <b:mo>=</b:mo> <b:mn>76</b:mn> </b:mrow> </b:math> nm rad). Quasistatic PIC simulations of the acceleration stage show that the bunch is efficiently accelerated to 24 GeV over 2 m with a final energy spread of less than 1% and emittances of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:msub> <c:mi>ɛ</c:mi> <c:mi>x</c:mi> </c:msub> <c:mo>=</c:mo> <c:mn>189</c:mn> </c:mrow> </c:math> nm rad and <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mrow> <d:msub> <d:mi>ɛ</d:mi> <d:mi>y</d:mi> </d:msub> <d:mo>=</d:mo> <d:mn>80</d:mn> </d:mrow> </d:math> nm rad. This high-quality electron bunch meets the requirements outlined by the Snowmass process for intermediate-energy colliders and compares favorably to the beam quality of proposed and existing accelerator facilities. The results establish the feasibility of plasma photoinjectors for future collider applications making a significant step toward the realization of high-luminosity, compact accelerators for particle physics research.

The exact solution of the classical nonlinear equation of motion for a relativistic electron in the field of two electromagnetic (EM) waves is obtained. For the particular case of the linearly polarized standing EM wave in the planar optical cavity, the intensity of the nonlinear Compton scattering, the time of flight, and the momentum variation after the relativistic electron passes along the cavity axis are calculated in weak and strong field limits. The extent of these effects depends on the initial phase of the EM wave when the electron enters the cavity. This can be used for the production, diagnosis, and acceleration of relativistic electron (positron) microbunches.

Schemes of double electron beams (e-beams) have wide applications in laser wakefield and plasma accelerations. At the ATF the seeded self-modulated laser wakefield acceleration (seeded SM-LWFA) is being conducted, which requires the first e-beam to initiate small plasma wakefield, whose amplitude is significantly amplified by the CO2 laser, and a second e-beam traveling after the first one to probe the accelerated electrons. To create and preserve the significant amount of wakefield in the seeded SM-LWFA experiment, the first e-beam must be ultra-small in spot size and ultra-short in bunch length within the interaction region. To probe the wakefield the separation between both e-beams is required ∼ 10-ps and the second beam must have a smaller intrinsic energy spread and ultra-small beam spot size. Design of the double beams within one RF-period (2856 MHz) to meet these strict requirements at the ATF is presented. Generation and delivery of the double beams through the ATF transport including a bunch compressor are experimentally investigated.