We intend to carry out a series of plasma lens experiments at the Final Focus Test Beam facility at SLAC. These experiments will be the first to study the focusing of particle beams by plasma focusing devices in the parameter regime of interest for high energy colliders, and is expected to lead to plasma lens designs capable of unprecedented spot sizes. Plasma focusing of positron beams will be attempted for the first time. We will study the effects of lens aberrations due to various lens imperfections. Several approaches will be applied to create the plasma required including laser ionization and beam ionization of a working gas. At an increased bunch population of 2.5 X 10(10), tunneling ionization of a gas target by an electron beam - an effect which has never been observed before - should he significant. The compactness of our device should prove to be of interest for applications at the SLC and the next generation linear colliders.
We discuss an electron-positron linear collider B-Factory using Inverse Free Electron Lasers (IFEL) to accelerate the beams. The requirements on luminosity, larger than 10/sup 33/ cm/sup -2/ s/sup -1/, and energy spread of a B Factory introduce stringent conditions on the accelerator and the interaction region. We study the longitudinal dynamics through the IFEL, the efficiency of the acceleration process, and the ratio of particles which become accelerated, and fall within the resonance. The device is found to perform well in the presence of large variations in the laser field intensity over the beam. We also discuss the laser system powering the IFEL, and some of the system tolerances. (7 References).
Our program for an experimental plasma wake field accelerator (PWFA) to take place at the Argonne Wakefield Accelerator (AWA) facility, in the recently proposed blow-out regime relies on the propagation of an intense electron beam through an underdense plasma with a minimum of degradation. This paper presents a near-equilibrium model of beam propagation using the Maxwell-Vlasov equations governing the beam?s transverse behavior. Numerical results are presented which use this model simultaneously with the plasma electron cold fluid equations. A solenoidal magnetic field, which is necessary for high density plasma containment, also provides an initial beam equilibrium to begin the calculation. We compare the equilibrium model with a discrete beam particle simulation, which verifies the basic conclusions of the equilibrium model, and shows the collisionless damping approach to equilibrium in the beam head. The initial matching requirements for the beam?s entry into the plasma are examined. We also discuss the possibility of performing an adiabatic lens experiment.
A 4.5 MeV photocathode RF gun has been commissioned at UCLA. A photo-injector drive laser produces sub 2 ps pulses of UV ( lambda =266 nm) light with up to 200 mu J/pulse, and illuminates a copper cathode. The photoelectrons are accelerated to an energy of 3.5 MeV within the gun. The electron beam charge is measured as a function of laser energy using an integrating current transformer (ICT). We present measurements of quantum efficiency as a function of laser polarization for injection angles of 2 degrees and 70 degrees with respect to the cathode normal. At 70 degrees incidence a 50% enhancement in quantum efficiency (>10(-4)) is observed for p-polarized light over s-polarized light. (7 References).
RF voltage modulation at a finite number of discrete frequencies is described in a Hamiltonian framework. The theory is applied to the problem of parasitic extraction of protons from a circulating beam in a high energy hadron collider, using a bent crystal as a thin ?septum? extraction element. Three modes of employing discrete resonances are discussed: a strong, single drive resonance which may be used to excite protons to hit, deep within the crystal; a single resonance ramped in such a manner that the island can carry trapped particles from low to high amplitudes; and overlapping resonances to create a chaotic band for separating the moving island and the large amplitude island. Simulations are used to confirm the expected dynamics, and finally a prototypical extraction scheme is described.
The magnetic self-focusing of a relativistic electron beam propagating through a plasma is demonstrated. The plasma which is produced by an RF discharge in a glass tube with no externally applied magnetic field focuses a 3.5 MeV, 25 ps (FWHM) long electron beam from an initial size of 2.5 mm (FWHM) to about 0.5 mm (FWHM) at a focal length of 18 cm. (7 References).
Short laser pulses (sub 2 ps) of UV ( lambda =266 nm) light with 200 mu J/pulse are used to produce electrons from a copper cathode in an RF gun. The electron bunch length is measured by streaking the Cerenkov radiation ( lambda =530 nm) from a thin (250 mu m) fused silica etalon. Streaks for both 0 degrees and 70 degrees laser incidence angles with respect to the cathode normal are presented with a temporal resolution of 3.6 ps. The shortest electron bunch length measured was 9 ps. (3 References).
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 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. (4 References).
We intend to carry out a series of plasma lens experiments at the Final Focus Test Beam facility at SLAC. These experiments will be the first to study the focusing of particle beams by plasma focusing devices in the parameter regime of interest for high energy colliders, and is expected to lead to plasma lens designs capable of unprecedented spot sizes. Plasma focusing of positron beams will be attempted for the first time. We will study the effects of lens aberrations due to various lens imperfections. Several approaches will be applied to create the plasma required including laser ionization and beam induced tunneling ionization of a working gas - the latter which has never been observed before. The compactness of our device should prove to be of interest for applications at the SLC and the next generation linear colliders.
The longitudinal beam-beam interaction, which can lead to incoherent heating, synchrobctatron coupling, and coherent longitudinal instabilities in circular colliders, is examined. This analysis discusses two types of energy kicks, those due to the transverse particle motion coupling to the electric portion of the transverse kick, and those derived from the inductive electric field induced near the interaction, which is obtained from the transverse kick through use of a generalization of the Panofsky-Wenzel Theorem. Implications for low energy e+e- colliders (phi & B factories) with beams crossing head-on, and at finite angles, with and without crab crossing, are discussed.
A 4 nm free electron laser (FEL) operating in Self Amplified Spontaneous Emission (SASE), and using the SLAC linac as a driver has been extensively studied using the FRED3D and TDA3D codes. Using a 7 GeV beam with a normalized RMS emittance of 3 mm-mrad and a peak current of 2500 A, obtained by longitudinal bunch compression, the FEL can provide about 20 GWatt of peak power, in a subpicosecond pulse. The FEL saturation length is about 60 m. Strong focusing in both planes is provided throughout the undulator by a FODO quadrupole system. We have studied the system gain, its optimization and FEL tolerance to beam parameter changes, wiggler errors and misalignments. (14 References).
A 4.5 MeV RF gun has been in operation at UCLA as a part of a 20 MeV linac. To improve the photoelectron beam parameters without changing the major characteristics of the driving laser and RF systems, a revised and extended version of the present RF gun has been investigated. The new gun consists of 6 full cells terminated at either end by one half cell each. The gun operates in pi -mode at 2.856 GHz. Accelerating fields and mode structures have been studied, and based on this, particle dynamics has been simulated. An aluminum prototype has been built for cold tests. Description of the gun is presented along with initial computational and experimental results. (10 References).
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 mu J of energy per pulse. The photo-injector produces 0-3 nC per bunch with an RF induced emittance of 1.5 pi (mm-mrad). (5 References).
A high gradient standing wave linear accelerator provides axisymmetric transverse focusing due to the presence of strong alternating gradient transverse electromagnetic fields arising from the backward rf wave. This effect is second order in both the field amplitude and in gamma(-1), so it is of importance only for high gradient, relatively low energy beams. The purpose of the present analysis is to examine the effect of this focusing on multi-bunch beam breakup in a superconducting linear collider, which has both a high accelerating gradient and long bunch train. As an interesting test case, we discuss the beam breakup
problem in the TESLA test bed at DESY.
The electron injector for the SLAC Linear Coherent Light Source[l] (LCLS) must produce a low jitter, high brightness beam. This beam must be accelerated and longitudinally compressed to yield a sub-picosecond beam which radiates a burst of self-amplified spontaneous emission xrays upon passing through a long undulator. As the gain of this amplifier is very sensitive to the emittance, energy spread and peak current the electron source and subsequent transport, acceleration, and compression systems must reproducibly give a very high quality beam. The conceptual design of an rf photocathode gun which satisfies the requirements of the LCLS is presented here. While the design peak current emitted by this gun is above 200 A, the requirements of gain length in the undulator are such that the beam must undergo longitudinal compression to raise the peak current by an order of magnitude. This compression is very much dependent on both the phase of the accelerating rf wave and the longitudinal wake-fields in the linac, and so the final bunch length will not be reproducible if the time of beam emission (laser beam injection) and/or the charge per bunch have large variations. Because of concerns on reliability and feasibility of the photoinjector, a design which takes maximum advantage of proven technology, and recent experience in photoinjector development has been explored, and is outlined below.
One of the attractive features of the superconducting approach to linear collider design is that the transverse emittances demanded are much larger than in normal conducting schemes. For TESLA design parameters, the damping rings appear to be relatively large and expensive, and it is therefore of some interest to look into alternative sources. For electrons, a promising source candidate is an RF photocathode. In this paper, we present conceptual design work towards development of an asymmetric emittance RF photocathode source which can operate at the TESLA repetition rates and duty cycle, and is capable of emitting beams with the required emittances and charge per pulse. (7 References).
Variable momentum compaction lattices have been proposed for electron-positron colliders and synchrotron radiation sources to control synchrotron tune and bunch length. To address questions of single particle stability limits, a study has been initiated to change the SPEAR lattice into a variable momentum compaction configuration for experimental investigation of the beam dynamics. In this paper, we describe a model-based method used to transform SPEAR from the injection lattice to the low momentum compaction configuration. Experimental observations of the process are reviewed. (12 References).
The present trend towards short wavelength operation with long undulators places tight requirements on the electron beam quality and hence the need to maintain a well focused beam. This paper examines the performance of alternating gradient (AG) sextupole focusing in planar undulators. Numerical simulation results of free electron laser performance using AG sextupole focusing are compared to results using only natural focusing and to those using quadrupole focusing. (10 References).
We describe the use of the SLAC linac to drive a unique, powerful, short wavelength Linac Coherent Light Source (LCLS). Operating as an FEL, lasing would be achieved in a single pass of a high peak current electron beam through a long undulator by self-amplified spontaneous emission. The main components are a high-brightness rf photocathode electron gun; pulse compressors; about 1/5 of the SLAC linac; and a long undulator with a FODO quadrupole focusing system. Using electrons below 8 GeV, the system would operate at wavelengths down to about 3 nm, producing [right angle bracket]or=10 GW of peak power in sub-ps pulses. At a 120 Hz rate the average power is approximately=1 W. (6 References).
A plane wave transformer linac (PWT), offering advantages of high efficiency, compactness, fabrication simplicity and cost, is being developed at UCLA. The PWT prototype at UCLA is an 8-cell, pi -mode, S-band standing-wave linac. To fully understand its physical properties, numerical modeling of the PWT prototype has been carried out by using the 3-D code MAFIA. A microwave test-stand with a network analyzer has also been set up to test these properties. In this paper. We present the important physical features, such as mode structures, dispersion curves, wake field, from the computation and/or the experiment. The measurements show good agreement with the numerical computation. (7 References).
In this paper we describe design, fabrication, and measurement aspects of a pure permanent magnet (PM) insertion device designed to operate as an FEL at a 1st harmonic energy of 300 eV and an electron energy of 7 GeV in the self-amplified spontaneous emission regime. (13 References).