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A localized basis that allows fast and accurate second order Moller-Plesset calculations

Description: We present a method for computing a basis of localized orthonormal orbitals (both occupied and virtual), in whose representation the Fock matrix is extremely diagonal-dominant. The existence of these orbitals is shown empirically to be sufficient for achieving highly accurate MP@ energies, calculated according to Kapuy's method. This method (which we abbreviate KMP2), which involves a different partitioning of the n-electron Hamiltonian, scales at most quadratically with potential for linearity in the number of electrons. As such, we believe the KMP2 algorithm presented here could be the basis of a viable approach to local correlation calculations.
Date: October 27, 2004
Creator: Subotnik, Joseph E. & Head-Gordon, Martin
Partner: UNT Libraries Government Documents Department

Terahertz radiation from laser accelerated electron bunches

Description: Coherent terahertz and millimeter wave radiation from laser accelerated electron bunches has been measured. The bunches were produced by tightly focusing (spot diameter {approx} 6 {micro}m) a high peak power (up to 10 TW), ultra-short ({ge}50 fs) laser pulse from a high repetition rate (10 Hz) laser system (0.8 {micro}m), onto a high density (>10{sup 19} cm{sup -3}) pulsed gas jet of length {approx} 1.5 mm. As the electrons exit the plasma, coherent transition radiation is generated at the plasma-vacuum boundary for wavelengths long compared to the bunch length. Radiation in the 0.3-19 THz range and at 94 GHz has been measured and found to depend quadratically on the bunch charge. The measured radiated energy for two different collection angles is in good agreement with theory. Modeling indicates that optimization of this table-top source could provide more than 100 {micro}J/pulse. Together with intrinsic synchronization to the laser pulse, this will enable numerous applications requiring intense terahertz radiation. This radiation can also be used as a powerful tool for measuring the properties of laser accelerated bunches at the exit of the plasma accelerator. Preliminary spectral measurements indicates that bunches as short as 30-50 fs have been produced in these laser driven accelerators.
Date: January 5, 2004
Creator: Leemans, W.P.; van Tilborg, J.; Faure, J.; Geddes, C.G.R.; Toth, Cs.; Schroe der, C.B. et al.
Partner: UNT Libraries Government Documents Department

Radiation from laser accelerated electron bunches: Coherent terahertz and femtosecond X-rays

Description: Electron beam based radiation sources provide electromagnetic radiation for countless applications. The properties of the radiation are primarily determined by the properties of the electron beam. Compact laser driven accelerators are being developed that can provide ultra-short electron bunches (femtosecond duration) with relativistic energies reaching towards a GeV. The electron bunches are produced when an intense laser interacts with a dense plasma and excites a large amplitude plasma density modulation (wakefield) that can trap background electrons and accelerate them to high energies. The short pulse nature of the accelerated bunches and high particle energy offer the possibility of generating radiation from one compact source that ranges from coherent terahertz to gamma rays. The intrinsic synchronization to a laser pulse and unique character of the radiation offers a wide range of possibilities for scientific applications. Two particular radiation source regimes are discussed: Coherent terahertz emission and x-ray emission based on betatron oscillations and Thomson scattering.
Date: October 1, 2004
Creator: Leemans, W.P.; Esarey, E.; van Tilborg, J.; Michel, P.A.; Schroeder, C.B.; Toth, Cs. et al.
Partner: UNT Libraries Government Documents Department

Laser triggered injection of electrons in a laser wakefield accelerator with the colliding pulse method

Description: An injection scheme for a laser wakefield accelerator that employs a counter propagating laser (colliding with the drive laser pulse, used to generate a plasma wake) is discussed. The threshold laser intensity for electron injection into the wakefield was analyzed using a heuristic model based on phase-space island overlap. Analysis shows that the injection can be performed using modest counter propagating laser intensity a{sub 1} < 0.5 for a drive laser intensity of a{sub 0} = 1.0. Preliminary experiments were preformed using a drive beam and colliding beam. Charge enhancement by the colliding pulse was observed. Increasing the signal-to-noise ratio by means of a preformed plasma channel is discussed.
Date: October 22, 2004
Creator: Nakamura, K.; Fubiani, G.; Geddes, C.G.R.; Michel, P.; van Tilborg, J.; Toth, C. et al.
Partner: UNT Libraries Government Documents Department

Coherent transition radiation from a laser wakefield accelerator as an electron bunch diagnostic

Description: The observation and modeling of coherent transition radiation from femtosecond laser accelerated electron bunches is discussed. The coherent transition radiation, scaling quadratically with bunch charge, is generated as the electrons transit the plasma-vacuum boundary. Due to the limited transverse radius of the plasma boundary, diffraction effects will strongly modify the angular distribution and the total energy radiated is reduced compared to an infinite transverse boundary. The multi-nC electron bunches, concentrated in a length of a few plasma periods (several tens of microns), experience partial charge neutralization while propagating inside the plasma towards the boundary. This reduces the space-charge blowout of the beam, allowing for coherent radiation at relatively high frequencies (several THz). The charge distribution of the electron bunch at the plasma-vacuum boundary can be derived from Fourier analysis of the coherent part of the transition radiation spectrum. A Michelson interferometer was used to measure the coherent spectrum, and electron bunches with duration on the order of 50 fs (rms) were observed.
Date: October 22, 2004
Creator: van Tilborg, J.; Geddes, C.G.R.; Toth, C.; Esarey, E.; Schroeder, C.B.; Martin, M.C. et al.
Partner: UNT Libraries Government Documents Department

Bicrystal YBCO dc squids with low noise

Description: We have fabricated 12 dc SQUIDs by laser depositing YBa{sub 2}Cu{sub 3}O{sub 7-x} on a SrTiO{sub 3} bicrystal substrate with a misorientation angle of 24*. At 77K all twelve devices had acceptable values of critical current, resistance and voltage modulation produced by an external magnetic field. The white noise energy of one device with an estimated inductance of 41 pH was 1.8 {times} 10{sup {minus}30} JHz{sup {minus}1}. The noise power scaled as 1/f at frequencies below about 1kHz, however, by using a bias current reversal scheme we were able to reduce this noise by two orders of magnitude at 1 Hz, to a value of about 1.5 {times} 10{sup {minus}29} JHz{sup {minus}1}. We made a magnetometer by coupling the SQUID to a flux transformer with a 5-turn input coil. The measured magnetic field gain was 60, and the white noise 36fT Hz{sup {minus}{1/2}}. However, the transformer produced relatively large levels of 1/f flux noise, not reduced by the bias reversal scheme, that limited the noise at 1 Hz to 1.7 pT Hz{sup {minus}{1/2}}. A single-layer magnetometer with a single-turn pick up loop is briefly described.
Date: August 1, 1992
Creator: Miklich, A. H.; Koelle, D.; Dantsker, E.; Nemeth, D. T.; Kingston, J. J.; Kromann, R. F. et al.
Partner: UNT Libraries Government Documents Department

Laser guiding at relativistic intensities and wakefield particle acceleration in plasma channels

Description: Electron beams with hundreds of picoCoulombs of charge in percent energy spread at above 80 MeV, and with few milliradian divergence, have been produced for the first time in a high gradient laser wakefield accelerator by guiding the drive laser pulse. Channels formed by hydrodynamic shock were used to guide acceleration relevant laser intensities of at least 1E18W/cm2 at the guide output over more than 10 Rayleigh lengths at LBNL's l'OASIS facility (10TW, 2E19W/cm2). The pondermotive force of the laser pulse drove an intense plasma wave, producing acceleration gradients on the order of 100 GV/m. Electrons were trapped from the background plasma and accelerated. By extending the acceleration length using the guiding channel, the energy of the electron beam was greatly increased, and bunches of small energy spread and low emittance were formed. Experiments varying gas jet length as well assimilations indicate that the high quality beams were formed when beam loading turned off injection after an initial load, producing an isolated bunch, and when that bunch was subsequently accelerated to the dephasing length at which point it rotated in phase space to produce low energy spread.
Date: August 1, 2004
Creator: Geddes, C.G.R.; Toth, Cs.; van Tilborg, J.; Esarey, E.; Schroeder, C.B.; Bruhwiler, D. et al.
Partner: UNT Libraries Government Documents Department

Bicrystal YBCO dc squids with low noise

Description: We have fabricated 12 dc SQUIDs by laser depositing YBa[sub 2]Cu[sub 3]O[sub 7-x] on a SrTiO[sub 3] bicrystal substrate with a misorientation angle of 24*. At 77K all twelve devices had acceptable values of critical current, resistance and voltage modulation produced by an external magnetic field. The white noise energy of one device with an estimated inductance of 41 pH was 1.8 [times] 10[sup [minus]30] JHz[sup [minus]1]. The noise power scaled as 1/f at frequencies below about 1kHz, however, by using a bias current reversal scheme we were able to reduce this noise by two orders of magnitude at 1 Hz, to a value of about 1.5 [times] 10[sup [minus]29] JHz[sup [minus]1]. We made a magnetometer by coupling the SQUID to a flux transformer with a 5-turn input coil. The measured magnetic field gain was 60, and the white noise 36fT Hz[sup [minus][1/2]]. However, the transformer produced relatively large levels of 1/f flux noise, not reduced by the bias reversal scheme, that limited the noise at 1 Hz to 1.7 pT Hz[sup [minus][1/2]]. A single-layer magnetometer with a single-turn pick up loop is briefly described.
Date: August 1, 1992
Creator: Miklich, A.H.; Koelle, D.; Dantsker, E.; Nemeth, D.T.; Kingston, J.J.; Kromann, R.F. et al.
Partner: UNT Libraries Government Documents Department