Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session CI2: Accelerators, Beam Dynamics and Radiation |
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Chair: Joe Kwan, Lawrence Berkeley National Laboratory Room: Centennial I |
Monday, November 2, 2009 2:00PM - 2:30PM |
CI2.00001: Beam dynamics of NDCX-II, a novel pulse-compressing ion accelerator Invited Speaker: The near-term mission of the Heavy Ion Fusion Science Virtual National Laboratory (HIFS-VNL, a collaboration of LBNL, LLNL, and PPPL) is to study Warm Dense Matter (WDM) at $\sim $1 eV in thin foils heated volumetrically by ion beams. An emerging mission is ion-direct-drive target physics for inertial fusion energy. These goals (especially the WDM mission) require rapid target heating. Beam bunch compression factors exceeding 50 are routinely achieved on the Neutralized Drift Compression Experiment (NDCX) at LBNL. The next facility for this research program, NDCX-II, will employ a unique approach to ion beam acceleration and pulse compression. Using modified induction cells from the decommissioned Advanced Test Accelerator at LLNL, NDCX-II will compress pulses of singly-charged Lithium ions from $\sim $500 ns to $\sim $1 ns as they are accelerated to 3-4 MeV. The required $\sim $sixfold speed-up and $\sim $hundredfold spatial compression are to be accomplished in $\sim $15 m. The beam dynamics employs the strong longitudinal space charge field to halt, and then reverse, an initially imposed pulse compression. This initial compression enables efficient use of the Volt-seconds in the downstream induction cells. Those cells impose further acceleration and the head-to-tail velocity gradient that enables a final neutralized drift compression and focus onto the target. Discrete-particle simulations (1-D, 2-D, and 3-D) have been used to develop the ``physics design'' for NDCX-II. We present the elements of the design, and our progress toward building this machine at LBNL. [Preview Abstract] |
Monday, November 2, 2009 2:30PM - 3:00PM |
CI2.00002: Physics of neutralization of intense charged particle beam pulses by a background plasma Invited Speaker: Neutralization and focusing of intense charged particle beam pulses by a background plasma forms the basis for a wide range of applications to high energy accelerators and colliders, heavy ion fusion, and astrophysics. From the practical perspective of designing advanced plasma sources for beam neutralization, a robust theory should be able to predict the self-electric and self-magnetic fields during beam propagation through the background plasma. The major scaling relations for the self-electric and self-magnetic fields of intense ion charge bunches propagating through background plasma have been determined taking into account the effects of transients during beam entry into the plasma, the excitation of collective plasma waves, the effects of gas ionization, finite electron temperature, and applied solenoidal and dipole magnetic fields. Accounting for plasma production by gas ionization yields a larger self-magnetic field of the ion beam compared to the case without ionization, and a wake of current density and self-magnetic field perturbations is generated behind the beam pulse. A solenoidal magnetic field can be applied for controlling beam propagation. Making use of theoretical models and advanced numerical simulations, it is shown that even a small applied magnetic field of about 100G can strongly affect beam neutralization. It has also been demonstrated that in the presence of an applied magnetic field the ion beam pulse can excite large-amplitude whistler waves, thereby producing a complex wake structure of plasma density perturbations in the region of the ion beam pulse. The presence of an applied solenoidal magnetic field may also cause a strong enhancement of the radial self-electric field of the beam pulse propagating through the background plasma. If controlled, this physical effect can be used for optimized beam transport over long distances. [Preview Abstract] |
Monday, November 2, 2009 3:00PM - 3:30PM |
CI2.00003: Studies of Emittance Growth and Halo Particle Production in Intense Charged Particle Beams Using the Paul Trap Simulator Experiment Invited Speaker: The Paul Trap Simulator Experiment (PTSX) is a compact laboratory experiment that places the physicist in the frame-of-reference of a long, charged-particle bunch coasting through a kilometers-long magnetic alternating-gradient (AG) transport system. The transverse dynamics of particles in both systems are described by the same set of equations, including nonlinear space-charge effects. The time-dependent voltages applied to the PTSX quadrupole electrodes are equivalent to the spatially-periodic magnetic fields applied in the AG system. The transverse emittance of the charge bunch, which is the area in the transverse phase space that the beam distribution occupies, is an important metric of beam quality. Maintaining low emittance is an important goal when defining AG system tolerances and when designing AG systems to perform beam manipulations such as transverse beam compression. Results will be presented from experiments in which white noise and colored noise of various amplitudes and durations has been applied to the PTSX electrodes. This noise is observed to drive continuous emittance growth over hundreds of lattice periods. Additional results will be presented from experiments that determine the conditions necessary to adiabatically reduce the charge bunch's transverse size. During adiabatic transitions, there is no change in the transverse emittance. The transverse compression can be achieved either by a gradual change in the PTSX voltage waveform amplitude or frequency. [Preview Abstract] |
Monday, November 2, 2009 3:30PM - 4:00PM |
CI2.00004: Generalized Phase-Space Tomography for Intense Beams Invited Speaker: Many applications of accelerators, such as free electron lasers, pulsed neutron sources, and heavy ion drivers for warm dense matter experiments require good quality beams with high intensity, i.e., cold, high-current beams. At the low-energy end of such machines, collective interactions from space charge dominate the beam dynamics and the beam can be viewed as a nonneutral plasma capable of carrying waves. Consequently, the initial beam distribution significantly affects its downstream behavior and beam characterization at the source is an important requirement to understand its evolution. This work reports on a novel diagnostic for time-dependent beam phase space characterization by using tomographic techniques. Tomography here is the reconstruction of phase space from a number of projections onto configuration space.~Application of tomography to beams with space charge is non-trivial since it involves assumptions about the beam distribution one is trying to measure.This talk will address this issue, as well as the implementation of this diagnostic to both solenoidal and quadrupole focusing lattices. Also discussed will be a series of proof-of-principle experiments conducted at the University of Maryland to test the diagnostic. The tomography is benchmarked both against self-consistent simulation using a particle-in-cell code and against a pinhole-scan direct experimental sampling of phase-space. [Preview Abstract] |
Monday, November 2, 2009 4:00PM - 4:30PM |
CI2.00005: Advances in Modeling of Beam-Wave Interaction in Multi-Megawatt Gyrotrons Invited Speaker: High-power gyrotrons, capable to produce several megawatts of CW radiation in millimeter wave range, are used in many magnetic fusion facilities, and planned to be used in ITER. The gyrotrons employ an interaction between a gyrating electron beam and very high order modes of open cylindrical or co-axial cavities to keep Ohmic losses on cavity walls on acceptable level. Since the gyrotron cavity supports a large number of eigenmodes with different azimuthal and radial indexes many of which are capable of interaction with electron beam at different frequencies. The code MAGY [1,2] has been developed to address the mode competition issue in gyro-devices. MAGY model is based on multi time-scale approach and uses electromagnetic fields expansion into series of eigenfunctions of local transverse cross-section. This approach leads to computationally efficient solution of the Maxwell's Equations. MAGY has been used for design and modeling of gyro-devices in CPI, MIT, UMD, NRL for last decade and demonstrated excellent agreement with experimental data. Modeling of Multi-Megawatt gyrotrons operating at high frequencies (170 GHz and above) presents a new challenge due to the unprecedented level of spectral mode density and higher level of beam current. A co-axial cavity gyrotron has been introduced to reduce this spectral density. To address these computational physics challenges a new MAGY model for mode interaction in gyrotrons with co-axial cavities has been implemented. MAGY has been used to model the FZK (Germany) 170 GHz co-axial gyrotron [3,4]. The results of this modeling will be presented. Further advances in the theoretical models for comparison with the existing experimental data will be discussed. \\[4pt] [1] S.C. Cai, et al, \textit{Int. J. Elect.}, 72, p. 759, 1992.\\[0pt] [2] M. Botton, et al, \textit{IEEE Trans on P S, }26, p. 882, 1998.\\[0pt] [3] B. Piosczyk, et al, \textit{IEEE Trans. on P S}., 32, 413, 2004.\\[0pt] [4] A.N. Vlasov, et al, \textit{IEEE Trans. on P S}., 36, p. 606, 2008. [Preview Abstract] |
Monday, November 2, 2009 4:30PM - 5:00PM |
CI2.00006: Wakefields in Photonic Crystal Accelerator Cavities Invited Speaker: The RF properties of photonic crystals (PhCs) can be exploited to avoid the parasitic higher order modes (HOMs) that degrade beam quality in accelerator cavities and reduce efficiency and power in RF generators. For example, an accelerator cavity can be designed using a PhC structure that traps only modes within a narrow frequency range, so that the cavity has only a single mode. Although the lack of HOMs is perhaps the most drastic difference between PhC cavities and traditional metal cavities, PhC cavities should allow a much wider range of materials and shapes, which could potentially lead to cavities that operate at higher electric fields and at higher frequencies (with lower losses). However, this greater flexibility introduces many challenges for building actual structures. A hybrid cavity that uses a dielectric 2D PhC along with metal plates to trap fields in the third dimension may offer the advantages of a PhC cavity while being relatively easy to construct. Although the 2D photonic structure may allow only a single mode, the 3D structure can in principle trap HOMs, such as guided modes in the dielectric rods that form the PhC; however, computer simulations show that long-range wake fields can be significantly reduced in such hybrid structures. For a 3D cavity based on a triangular lattice of dielectric rods, the rod positions can be optimized (breaking the lattice symmetry) to reduce radiation leakage using a fixed number of rods; moreover, the optimized structure can further reduce the wake fields. [Preview Abstract] |
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