N. Barov

Initial measurements of the UCLA rf photoinjector

The 1.5 cell standing wave rf photoinjector has been operated for the past several months using a copper cathode. The photoinjector drive laser produces sub 2 ps pulses of UV (A = 266 nm) light with up to 200 p~J/pulse which generates up to 3 nC of charge. The emittance of the photoinjector was measured as a function of charge, rf launching phase, and peak accelerating field. Also, the quantum efficiency and pulse lengths of the laser beam and the electron beam were measured.

Pulse compression in radio frequency photoinjectors-applications to advanced accelerators

While RF photoinjectors are an excellent source of high brightness electron beams, there are constraints to tying together the expected emittance and peak current performance of a given photoinjector system. These constraints, which arise from the complicated dynamics of the electrons due to the interplay of RF and space-charge forces within the photoinjector, tend to favor lower peak current operation. For some ultimate uses of photoinjector beams, such as linear collider test beams, wakefield accelerators, and free-electron lasers (FELs), one may desire much higher peak currents. In this case, an inexpensive and reliable method for producing extremely short high-current electron bunches is to use magnetic compression. We examine this scheme analytically and by computer simulation. Many applications are illustrated, including the TESLA Test Facility/FEL injector, ultra-high current beams for plasma wakefields and generation of femtosecond electron pulses for injection into short wavelength laser-based accelerators. It is shown that the injection timing jitter associated with the laser can be nearly eliminated using this scheme, making it an indispensable component in many of the advanced accelerator injectors we consider.

Towards a plasma wake-field acceleration-based linear collider

Abstract A proposal for a linear collider based on an advanced accelerator scheme, plasma wake-field acceleration in the extremely nonlinear regime, is discussed. In this regime, many of the drawbacks associated with preservation of beam quality during acceleration in plasma are mitigated. The scaling of all beam and wake parameters with respect to plasma wavelength is examined. Experimental progress towards high-gradient acceleration in this scheme is reviewed. We then examine a linear collider based on staging of many modules of plasma wake-field accelerator, all driven by a high average current, pulse compressed, RF photoinjector-fed linac. Issue of beam loading, efficiency, optimized stage length, and power efficiency are discussed. A proof-of-principle experimental test of the staging concept at the Fermilab test facility is discussed.

Emittance measurements of the 4.5 MeV UCLA RF photoinjector

The 1.5 cell RF photoinjector has been operated for the past several months using a copper cathode illuminated by 4 ps long pulses of UV (246 nm light, with a variable energy of between 0 to 300 /spl mu/J. This typically produces up to 3 nC of charge per bunch. Because space charge forces dominate the electron beam transport a pepper pot measurement system is used to measure the emittance. The emittance is measured as a function of charge, peak accelerating field, laser spot size and initial phase with respect to the RF field. This is accomplished with an automated control and data acquisition system which can measure single shot emittances at a rate of 5 Hz developed at UCLA. The experimental results obtained are then compared with theory and simulations.<<ETX>>

Trapping of background plasma electrons in a beam-driven plasma wake field using a downward density transition

Trapping of background plasma electrons by a beam-driven plasma wake field is studied as a new self-injection method. In this scheme, a short electron beam pulse is sent through an underdense plasma with a downward density transition and some background plasma electrons are trapped by the strong wake field due to the sudden increase of the wake wave wavelength at the density transition. Two-dimensional PIC (Particle-In-Cell) simulations show that a significant amount of plasma electrons are trapped and accelerated to a higher energy than the driving beam energy. Furthermore, the trapped-beam quality is fairly good. In this paper, the 2-D simulation results, dynamics of the trapped beam and the driving beam, and the proposed experiment for the UCLA Neptune Laboratory are described.

Initial operation and beam characteristics of the UCLA S-band RF photo-injector

The UCLA RF photo-injector system has been commissioned. All of the sub-components such as the high power RF, pico-second laser, RF photo-injector cavity, diagnostics, and supporting hardware have been tested and are operational. We briefly discuss the performance of the various components since the details of each subsystem are very lengthy. The laser delivers a sub 4 ps pulse containing 0-300 /spl mu/J of energy per pulse. The photo-injector produces 0-3 nC per bunch with an RF induced emittance of 1.5 /spl pi/(mm-mrad).<<ETX>>

The UCLA IR FEL project

Abstract A 10.6 μm free electron laser (FEL) operating in the high gain regime is under construction of UCLA. FEL physics significant to future short wavelength operation will be emphasized including optical guiding, superradiance, saturation and self-amplified spontaneous emission (SASE). A 5 MeV rf photocathode gun illuminated by a UV laser will supply a high brightness electron beam which will be injected into a plane wave transformer (PWT) linac for acceleration to 20 MeV. Recent measurements of the gun emittance as well as quantum efficiency are presented. The undulator is of modified hybrid design producing ∼7.5 kG peak field on axis with 5 mm gap spacing and 1.5 cm pole period. Simulation results which include three-dimensional effects are furnished. The present status and future plans of the project are summarized.

Use on an inverse free electron laser in a linear collider B factory

We examine the possibility of using an IFEL as an accelerator in a linear collider B Factory. An IFEL is able to utilize a sizable fraction of the energy of the laser pulse used to accelerate the beams. It is also able to meet the stringent requirements imposed on the energy spread and luminosity at the interaction point. Two separate examples are considered, differing in the way the laser pulse energy is coupled to the electron beam. The first maximizes the slippage between the beam bunch and the radiation, in order to decrease the peak laser power. In the second example the slippage is minimized. This results in uniform beam loading and may in principle be run at higher efficiency and lower average power. We also address the laser required to drive this accelerator. The power and frequency requirements suggest the use of a FEL drive laser. Our design for this system includes the use of superconducting cavities to accelerate the drive beam, which is then propagated through an initially constant period undulator that is tapered after saturation.

Photoelectron beams from the UCLA rf gun

A high brightness, low emittance photocathode rf gun is starting operation at UCLA as an injector to a 20 MeV linac. This linac will initially be used to drive FELs, plasma wakefield accelerators, and to test plasma lenses. The gun is a 1‐1/2 cell π‐mode standing wave structure running at 2.856 GHz, and has a copper photocathode. In the initial commissioning of the gun, photoelectron beams of up to 2.5 nC at 4.5 MeV have been produced. We report on the current status of the system, experimental data taken with 50 ps UV laser pulses, and plans for the future.

ENERGY LOSS OF A HIGH CHARGE BUNCHED ELECTRON BEAM IN PLASMA: SIMULATIONS, SCALING, AND ACCELERATING WAKE-FIELDS

The energy loss and gain of a beam in the nonlinear, “blowout” regime of the plasma wakefield accelerator (PWFA), which features ultra-high accelerating fields, linear transverse focusing forces, and nonlinear plasma motion, has been asserted, through previous observations in simulations, to scale linearly with beam charge. In a new analysis that is the companion to this article, it has been shown that for an infinitesimally short beam, the energy loss is indeed predicted to scale linearly with beam charge for arbitrarily large beam charge. This scaling holds despite the onset of a relativistic, nonlinear response by the plasma, when the number of beam particles occupying a cubic plasma skin-depth exceeds that of plasma electrons within the same volume. This paper is intended to explore the deviations from linear energy loss using 2D particle-in-cell (PIC) simulations that arise in the case of finite length beams. The peak accelerating field in the plasma wave excited behind the finite-length beam is also examined, with the artifact of wave spiking adding to the apparent persistence of linear scaling of the peak field amplitude well into the nonlinear regime. At large enough normalized charge, the linear scaling of both decelerating and accelerating fields collapses, with serious consequences for plasma wave excitation efficiency. Using the results of parametic PIC studies, the implications of these results for observing the collapse of linear scaling in planned experiments are discussed.