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Current enhancement update

Description: Net current enhancement to levels in excess of the beam current has been observed in gases at pressures excess of 50 torr. We delineate the regimes where enhancement is observed. The experimental results fall into two very distinct classes; current enhancement at injection where the beam is only slightly displaced and current enhancement clearly associated with the high amplitude hose instability. A careful theoretical and experimental study of the diagnostics revealed no fundamental flaws although there are several complex and unlikely scenarios which could introduce fictitious current enhancement. Theoretical efforts indicate several mechanisms for generating enhancement but none of the theories can account for the detailed observations. 4 references, 4 figures.
Date: June 14, 1984
Creator: Chambers, F.W.; Clark, J.C.; Struve, K.W. & Yu, S.S.
Partner: UNT Libraries Government Documents Department

Beam front accelerators

Description: An intense relativistic electron beam cannot propagate in a metal drift tube when the current exceeds the space charge limit. Very high charge density and electric field gradients (10/sup 2/ to 10/sup 3/ MV/m) develop at the beam front and the electrons are reflected. When a neutral gas or a plasma is present, collective acceleration of positive ions occur, and the resulting charge neutralization enables the beam to propagate. Experimental results, theoretical understanding, and schemes to achieve high ion energies by external control of the beam front velocity will be reviewed.
Date: January 1, 1982
Creator: Reiser, M.
Partner: UNT Libraries Government Documents Department

Computer models in the design of FXR

Description: Lawrence Livermore National Laboratory is developing a 15 to 20 MeV electron accelerator with a beam current goal of 4 kA. This accelerator will be used for flash radiography and has a requirement of high reliability. Components being developed include spark gaps, Marx generators, water Blumleins and oil insulation systems. A SCEPTRE model was developed that takes into consideration the non-linearity of the ferrite and the time dependency of the emission from a field emitter cathode. This model was used to predict an optimum charge time to obtain maximum magnetic flux change from the ferrite. This model and its application will be discussed. JASON was used extensively to determine optimum locations and shapes of supports and insulators. It was also used to determine stress within bubbles adjacent to walls in oil. Computer results will be shown and bubble breakdown will be related to bubble size.
Date: January 1, 1980
Creator: Vogtlin, G. & Kuenning, R.
Partner: UNT Libraries Government Documents Department

Brightness limitations in multi-kiloampere electron beam sources

Description: Heuristic relationships such as the Lawson-Penner criterion, used to scale Free Electron Laser (FEL) amplifier gain and efficiency over orders of magnitude in beam current and brightness, have no fundamental basis. The brightness of a given source is set by practical design choices such as peak voltage, cathode type, gun electrode geometry, and focusing field topology. The design of low emittance, high current electron guns has received considerable attention at Livermore over the past few years. The measured brightnesses of the Experimental Test Accelerator (ETA) and Advanced Test Accelerator (ATA) guns are less than predicted with the EBQ gun design code; this discrepancy is due to plasma effects from the present cold, plasma cathode in the code. The EBQ code is well suited to exploring the current limits of gridless relativistic Pierce columns with moderate current density (<50 A/cm/sup 2/) at the cathode. As EBQ uses a steady-state calculation it is not amenable for study of transient phenomena at the beam head. For this purpose, a Darwin approximation code, DPC, has been written. The main component in our experimental cathode development effort is a readily modified electron gun that will allow us to test many candidate cathode materials, types and electrode geometries at field stresses up to 1 MV/cm. 6 references, 6 figures.
Date: August 24, 1984
Creator: Barletta, W.A.; Boyd, J.K.; Paul, A.C. & Prono, D.S.
Partner: UNT Libraries Government Documents Department

Initial measurements of beam breakup instability in the advanced test accelerator

Description: This paper reports the measurements of beam breakup (BBU) instability performed on the Advanced Test Accelerator (ATA) up to the end of February, 1984. The main objective was to produce a high current usable electron beam at the ATA output. A well-known instability is BBU which arises from the accelerator cavity modes interacting with the electron beam. The dominant mode is TM/sub 130/ at a frequency of approximately 785 MHz. It couples most strongly to the beam motion and has been observed to grow in the Experimental Test Accelerator (ETA) which has only eight accelerator cavities. ATA has one hundred and seventy cavities and, therefore, the growth of BBU is expected to be more severe. In this paper, BBU measurements are reported for ATA with beam currents of 4 to 7 kA. Analysis showed that the growth of the instability with propagation distance was as expected for the lower currents. However, the high-current data showed an apparent higher growth rate than expected. An explanation for this anomaly is given in terms of a ''corkscrew'' excitation. The injector BBU noise level for a field emission brush cathode was found to be an order of magnitude lower than for a cold plasma discharge cathode. These injector rf amplitudes agree very well with values obtained using the method of differenced B sub solar loops.
Date: May 13, 1985
Creator: Chong, Y.P.; Caporaso, G.J. & Struve, K.W.
Partner: UNT Libraries Government Documents Department

Pulsed power for particle beam accelerators in military applications

Description: Techniques useful for generating and conditioning power for high energy pulsed accelerators with potential weapon applications are described. Pulsed electron accelerators are exemplified by ETA and ATA at Lawrence Livermore Laboratories and RADLAC at Sandia Laboratories Albuquerque. Pulse-power techniques used in other applications are briefly mentioned, including some that may be useful for collective ion accelerators. The limitations of pulse-power and the general directions of desirable development are illustrated. The main needs are to increase repetition rate and to decrease size.
Date: June 20, 1980
Creator: Smith, I.D.
Partner: UNT Libraries Government Documents Department

Simulation of power flow in magnetically insulated convolutes for pulsed modular accelerators

Description: Two distinct simulation approaches for magnetic insulation are developed which can be used to address the question of nonsimultaneity. First, a two-dimensional model for a two-module system is simulated using a fully electromagnetic, two-dimensional, time-dependent particle code. Next, a nonlinear equivalent circuit approach is used to compare with the direct simulation for the two module case. The latter approach is then extended to a more interesting three-dimensional geometry with several MITL modules.
Date: January 1, 1980
Creator: Seidel, D. B.; Goplen, B. C. & VanDevender, J. P.
Partner: UNT Libraries Government Documents Department

TV-acquired optical diagnostics systems on ATA

Description: The purpose of this paper is to report on optical system developments on the ATA and their applications to ATA beam characterization. Television (TV)-acquired optical diagnostics data provide spatial and temporal properties of the ATA beam that complements recorded information from other types of sensors, such as, beam-wall current monitors, x-ray probes, and rf probes. The ATA beam operates: (1) in the normal mode at 50-MeV, 10-kA at a 1-Hz rate; and (2) in the 1-KHz burst mode (for 10-pulses) at a 0.5 Hz rate. The beam has a 70-ns pulse width in vacuum propagation; however, beam-head erosion will occur in atmospheric propagation, thus limiting the pulse width to less than 50-ns. Various optical systems are used for ATA diagnostics. Optical-imaging provides a convenient measurement in a single pulse of the 2-dimensional profile of the beam intensity. It can also provide multiple 2-D framing in a single pulse. In some studies it may be desirable to study optical events with temporal resolution less than 100-ps with 1-dimensional streak cameras. Spatially integrated data from phototube cameras can also be used for background measurement applications as well as for single pixel monitoring. The optical line-of-sight (LOS) configurations have been made versatile to accommodate a large number of options for the various optical systems.
Date: June 1, 1984
Creator: Kalibjian, R.; Chong, Y.P.; Cornish, J.P.; Jackson, C.H. & Fessenden, T.J.
Partner: UNT Libraries Government Documents Department

Computational studies of beam dynamics in the ETA gun

Description: A new general purpose computer code call EBQ, has been written to simulate the beam dynamics of the ETA, find its beam emittance and evaluate effects of changes in the electrode positions and external magnetic fields. The original calculations of the ETA were made with EGUN and yielded considerable insight into the operation of the device in the non-relativistic regime. The EBQ code was written specifically to attend to the special problems associated with high current relativistic beam propagation in axially symmetric machines possessing external 2-dimensional electric and magnetic fields. The coherent electric and magnetic self-fields of the beam must be calculated accurately. Special care has been used in the relativistic regime where a high degree of cancellation occurs between the self-magnetic and self electric forces of the beam. Additionally, EBQ can handle equally well non-relativistic problems involving multiple ion species where the space charge from each must be included in its mutual effect on the others. Such problems arise in the design of ion sources where different charge and mass states are present.
Date: March 1, 1979
Creator: Paul, A.C. & Neil, V.K.
Partner: UNT Libraries Government Documents Department

Database requirements for the Advanced Test Accelerator project

Description: The database requirements for the Advanced Test Accelerator (ATA) project are outlined. ATA is a state-of-the-art electron accelerator capable of producing energetic (50 million electron volt), high current (10,000 ampere), short pulse (70 billionths of a second) beams of electrons for a wide variety of applications. Databasing is required for two applications. First, the description of the configuration of facility itself requires an extended database. Second, experimental data gathered from the facility must be organized and managed to insure its full utilization. The two applications are intimately related since the acquisition and analysis of experimental data requires knowledge of the system configuration. This report reviews the needs of the ATA program and current implementation, intentions, and desires. These database applications have several unique aspects which are of interest and will be highlighted. The features desired in an ultimate database system are outlined. 3 references, 5 figures.
Date: November 5, 1984
Creator: Chambers, F.W.
Partner: UNT Libraries Government Documents Department

Measurements of the emittance and current density profile of the beam produced by the ATA injector

Description: Measurements of the normalized emittance of the beam produced by the Advanced Test Accelerator (ATA) injector yielded values near 0.4 Radian-Centimeters at currents up to 10 kAmps. The instrument was also used to obtain beam-current-density profiles in two dimensions at the entrance mask of the instrument.
Date: June 27, 1983
Creator: Fessenden, T.J.
Partner: UNT Libraries Government Documents Department

Resistance-driven bunching mode of an accelerated ion pulse

Description: Amplification of a longitudinal perturbation of an ion pulse in a linear induction accelerator is calculated. The simplified accelerator model consists only of an applied field (E/sub a/), distributed gap impedance per meter (R) and beam-pipe capacity per meter (C). The beam is treated as a cold, one-dimensional fluid. It is found that normal mode frequencies are nearly real, with only a very small damping rate proportional to R. This result is valid for a general current profile and is not restricted to small R. However, the mode structure exhibits spatial amplification from pulse head to tail by the factor exp(RCLv/sub o//2), where L is pulse length and v/sub 0/ is drift velocity. This factor is very large for typical HIF parameters. An initially small disturbance, when expanded in terms of the normal modes, is found to oscillate with maximum amplitude proportional to the amplification factor.
Date: October 16, 1981
Creator: Lee, E.P.
Partner: UNT Libraries Government Documents Department

Preliminary experiments with a carbon fiber tuft cathode

Description: This work reports initial tests of a carbon brush or tuft cathode intended for use by the Beam Research Program. It was found that electric fields of approximately 100 kV/cm were required to produce current densities above 20 A/sq cm. The beam extracted from the cathode consisted of many beamlets - one for each tuft. The beamlets were found to be quite uniform in peak current density and the cathode operation was microscopically repeatable. The turn-on time was estimated to be 200 ns.
Date: January 11, 1984
Creator: Fessenden, T.J.
Partner: UNT Libraries Government Documents Department

Design/cost of an induction linac for heavy ions for pellet-fusion

Description: The physics of the pellet implosion sets stringent conditions on the accelerator driver. The beam energy should be > 1 MJ, the beam power > 100 TW (implying a pulse length approx. = 10 ns), and the specific energy deposition in the pellet > 20 MJ/g. Thus, considerable current amplification is required, e.g. from some 10 amps at the source to perhaps 10 kiloamps at the pellet. Most of this amplification can be accomplished continuously along the accelerator and the remainder achieved at the end by bunching in the final transport lines to the target chamber. A conceptual schematic of an Induction Linac Fusion Driver is shown, which includes an injector, an accelerator-buncher, and a final transport system. Here only the accelerator portion of the driver is discussed.
Date: March 1, 1979
Creator: Faltens, A.; Hoyer, E.; Keefe, D. & Laslett, L.J.
Partner: UNT Libraries Government Documents Department

Operation of repetitively pulsed 300 kV, 20 kA electron beam diode

Description: Repetitively pulsed diodes will be required for most of the proposed inertially confined fusion systems, yet little information is available on the operation of vacuum beam diodes under repetitive conditions. Most data are relevant to laser exciter/sustainer diodes operating at low current density (< 1 A/cm/sup 2/) for long pulses. As such they are of marginal value for estimating the properties of repetitive high current density diodes such as will be required for particle beam and some gas laser fusion drivers.
Date: January 1, 1979
Creator: Buttram, M.T.
Partner: UNT Libraries Government Documents Department

IBEX - annular beam propagation experiment

Description: IBEX is a 4-MV, 100-kA, 20-ns cylindrical isolated Blumlein accelerator. In the experiments reported here, the accelerator is fitted with a specially designed foilless diode which is completely immersed in a uniform magnetic field. Several diode geometries have been studied as a function of magnetic field strength. The beam propagates a distance of 50 cm (approx. 10 cyclotron wavelengths) in vacuum before either striking a beam stop or being extracted through a thin foil. The extracted beam was successfully transported 60 cm downstream into a drift pipe filled either with 80 or 640 torr air. The main objectives of this experiment were to establish the proper parameters for the most quiescent 4 MV, 20 to 40 kA annular beam, and to compare the results with available theory and numerical code simulations.
Date: January 1, 1983
Creator: Mazarakis, M G; Miller, R B; Shope, S L; Poukey, J W; Ramirez, J J; Ekdahl, C A et al.
Partner: UNT Libraries Government Documents Department

Experimental test accelerator: description and results of initial experiments

Description: The ETA is a high current (10,000 Amp) linear induction accelerator that produces short (30 ns) pulses of electrons at 5 MeV twice per second or in bursts of 5 pulses separated by as little as one millisecond. At this time the machine has operated at 65% of its design current and 90% of the design voltage. This report contains a description of the accelerator and its diagnostics; the results of the initial year of operation; a comparison of design codes with experiments on beam transport; and a discussion of some of the special problems and their status.
Date: June 2, 1980
Creator: Fessenden, T.; Birx, D. & Briggs, R.
Partner: UNT Libraries Government Documents Department

Collective ion acceleration. Progress report, October 1977--September 1978

Description: Numerical and analytic studies of collective ion acceleration in intense relativistic electron beams are presented. Investigation of autoresonant acceleration has shown that radial beam inhomogeneities distort the slow cyclotron wave into a surface-localized mode. Variation of linear wave fields in inhomogeneous magnetic fields is strongly affected by this. Numerical studies of self-consistent cyclotron waves show the persistence of linear characteristics even at large amplitudes. Propagation of large amplitude waves has been observed over moderate distances in simulations without significant attenuation or nonlinear disruption. Ion acceleration in virtual cathodes has also been studied. Insights into both formation and late time dynamics have been gained. Increased virtual cathode understanding is being pursued toward defining optimal configurations. Future plans are outlined.
Date: July 1, 1979
Creator: Faehl, R.J. & Godfrey, B.B. Shanahan, W.R.
Partner: UNT Libraries Government Documents Department

Preliminary simulation studies of accelerator cavity loading

Description: Two-dimensional simulations of loading effects in a 350 MHz accelerator cavity have been performed. Electron currents of 1-10 kA have been accelerated in 5 MV/m fields. Higher order cavity modes induced by the beam may lead to emittance growth. Operation in an autoaccelerator mode has been studied.
Date: June 1, 1980
Creator: Faehl, R.J.
Partner: UNT Libraries Government Documents Department

PHERMEX as an injector to a modified betatron

Description: The PHERMEX accelerator is a pulsed three-cavity, 50-MHz, standing-wave rf linear accelerator. It is used to produce a 30-MeV, 200-ns envelope of electrons for flash radiography and electron beam experiments. The 200-ns electron pulse contains 10 micropulses. The FWHM of a single micropulse is 3.3 ns. Peak micropulse current varies from 350 to 850 A with widths of 3 and 5 ns, respectively. We propose to inject this beam into a solenoidal field with a neutralizing background gas and stack the PHERMEX micropulses in a 28-cm-dia ring to obtain a 3-ns multi-kiloampere beam. Coupled to the background magnetic field will be a ramped field driven by a magnetic flux compression generator. Beam kinetic energy approaching 100 MeV is theoretically possible. Simulations of ring stability with and without the accelerating field are presented. 9 refs., 4 figs.
Date: January 1, 1985
Creator: Moir, D.C. & Gisler, G.R.
Partner: UNT Libraries Government Documents Department

Simulations of intense relativistic electron beam generation by foilless diodes

Description: Foilless diodes used to produce intense annular relativistic electron beams have been simulated using the time-dependent, two-dimensional particle-in-cell code CCUBE. Current densities exceeding 200 kA/cm/sup 2/ have been obtained in the simulations for a 5 MeV, 35 ..cap omega.. diode. Many applications, including microwave generation, collective ion acceleration and high-density plasma heating require a laminar electron flow in the beams. The simulation results indicate that foilless diodes immersed in a strong external magnetic field can achieve such a flow. Diodes using technologically achievable magnetic field strengths (approx. 100 kG) and proper electrode shaping appear to be able to produce beams with an angular scatter of less than 35 mrad at the current densities and energies mentioned above. Scaling of the impedance and temperature of the beam as a function of geometry, magnetic field strength and voltage is presented.
Date: January 1, 1979
Creator: Jones, M.E. & Thode, L.E.
Partner: UNT Libraries Government Documents Department

Experimental results of beam brightness experiments at the Advanced Test Accelerator (ATA)

Description: Experimental data show improved brightness of Lawrence Livermore National Laboratory's Advanced Test Accelerator both at the injector and at the high-energy output. The effects of matching onto a laser-produced ion channel have been demonstrated, and an improved matching technique is now being used.
Date: March 1, 1987
Creator: Weir, J.T.; Boyd, J.K.; Caporaso, G.J. & Orzechowski, T.J.
Partner: UNT Libraries Government Documents Department

Emittance measurements on ETA and ATA

Description: Emittance measurements on beams produced by the ETA and ATA accelerators are discussed. Emittance and brightness are defined. The significance of emittance for a beam in an accelerator and in gas is discussed. Various measurement techniques and results are presented and contrasted. Implicit calculations of emittance are also reported. Finally, the measurement of the time variation of emittance is discussed and the techniques to be used on the upcoming ATA experiments are outlined.
Date: June 14, 1984
Creator: Chambers, F.W.; Prosnitz, D. & Fessenden, T.J.
Partner: UNT Libraries Government Documents Department

Accelerator power concepts using iolated transmission lines

Description: The rationale and the advantages of using multiple transmission-line sections isolated by transit time and inductance in accelerating high-current (approx. = 10 kA), short-pulse (less than or equal to 100 ns) particle beams to the multimegavolt level is outlined. The main advantages of this system include reducing the number of output switches required per output pulse by nearly an order of magnitude over conventional systems and increasing the system capability for repetition-rate operation. The isolated transmission-line concept is developed and possible modes of operation outlined. In addition, a point design of a 10-kA short-pulse accelerator is presented.
Date: January 1, 1981
Creator: Nunnally, W.C.
Partner: UNT Libraries Government Documents Department