324 research outputs found
Perturbed Defects and T-Systems in Conformal Field Theory
Defect lines in conformal field theory can be perturbed by chiral defect
fields. If the unperturbed defects satisfy su(2)-type fusion rules, the
operators associated to the perturbed defects are shown to obey functional
relations known from the study of integrable models as T-systems. The procedure
is illustrated for Virasoro minimal models and for Liouville theory.Comment: 24 pages, 13 figures; v2: typos corrected, in particular in (2.10)
and app. A.2, version to appear in J.Phys.
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Laser conditioning study of KDP on the optical sciences laser using large area beams
Considerable attention has been paid over the years to the problem of growing high purity KDP and KD*P to meet threshold requirements on succeeding generations of inertial confinement fusion lasers at LLNL. While damage thresholds for these materials have increased over time, the current National Ignition Facility (NIF) maximum fluence requirement (redline) for KD*P frequency triplers of 14.3 J/cm{sup 2} at 351 nm, 3 ns has not been reached without laser (pre)conditioning. It is reasonable to assume that, despite the rapid increase in damage thresholds for rapidly grown crystals, -a program of large scale conditioning of the 192 NIF triplers will be required. Small area ramp (R/1) tests on single sites indicate that KDP damage thresholds can be raised on average up to 1.5X the unconditioned values. Unpublished LLNL 3{omega} raster conditioning studies on KDP, however, have not conclusively shown that off-line conditioning is feasible for KD*P. Consequently, investigating the feasibility of on-line conditioning of NIF triplers at 3{omega} has become a high priority for the KDP damage group at LLNL. To investigate the feasibility of on-line conditioning we performed a series of experiments using the Optical Sciences Laser (OSL) on numerous samples of conventional and rapid growth KDP and KD*P. The experiment entailed exposing sites on each sample to a range of ramped shot (N/l) sequences starting at average fluences of -2 J/cm{sup 2} (in a 7 mm ``top hat`` beam @ 351 nm, 3 ns) up to peak fluences of approximately 13 J/cm{sup 2}. Test results indicated that the most effective conditioning procedure entailed a 7-8 shot ramp starting at 2 J/cm{sup 2} and ending at 12-13 J/cm{sup 2}. The pinpoint onset fluence for the 8/1 tests was 1.4 times that of the unconditioned site. Damage evolution appears to be exponential as a function of increasing fluence. When damage occurs after conditioning however, pinpoint density evolution exhibits a greater slope than less conditioned sites. The overall reduction in the total pinpoint number can be as high as 30OX. Despite laser conditioning , the pinpoint onset for the samples considered is below the NIF redline fluence of 14.3 J/cm{sup 2}. In addition, the exponential pinpoint evolution curves indicate that damage levels at NIF redline fluences will be on the order of 10{sup 4} pinpoints/mm{sup 2}. This suggests that there will be significant damage in NIP triplers, however, substantial damage has not been observed in the large Beamlet tripler (conventionally grown KD*P) under similar exposure conditions. By applying the OSL damage evolution curves to model NIF THG output spatial profiles it is possible to show damage in NIF triplers will be slight, consisting of isolated clusters with a few pinpoints at high fluence portions of the beam. This prediction has been verified by scatter mapping the 37 cm Beamlet tripler crystal. These results will be discussed in a future memo. These results indicate the feasibility of on-line conditioning for the NIF laser
Timelike Boundary Liouville Theory
The timelike boundary Liouville (TBL) conformal field theory consisting of a
negative norm boson with an exponential boundary interaction is considered. TBL
and its close cousin, a positive norm boson with a non-hermitian boundary
interaction, arise in the description of the accumulation point of
minimal models, as the worldsheet description of open string tachyon
condensation in string theory and in scaling limits of superconductors with
line defects. Bulk correlators are shown to be exactly soluble. In contrast,
due to OPE singularities near the boundary interaction, the computation of
boundary correlators is a challenging problem which we address but do not fully
solve. Analytic continuation from the known correlators of spatial boundary
Liouville to TBL encounters an infinite accumulation of poles and zeros. A
particular contour prescription is proposed which cancels the poles against the
zeros in the boundary correlator d(\o) of two operators of weight \o^2 and
yields a finite result. A general relation is proposed between two-point CFT
correlators and stringy Bogolubov coefficients, according to which the
magnitude of d(\o) determines the rate of open string pair creation during
tachyon condensation. The rate so obtained agrees at large \o with a
minisuperspace analysis of previous work. It is suggested that the mathematical
ambiguity arising in the prescription for analytic continuation of the
correlators corresponds to the physical ambiguity in the choice of open string
modes and vacua in a time dependent background.Comment: 28 pages, 1 figure, v2 reference and acknowledgement adde
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A System for Measuring Defect Induced Beam Modulation on Inertial Confinement Fusion-class Laser Optics
A multi-wavelength laser based system has been constructed to measure defect induced beam modulation (diffraction) from ICF class laser optics. The Nd:YLF-based modulation measurement system (MMS) uses simple beam collimation and imaging to capture diffraction patterns from optical defects onto an 8-bit digital camera at 1053, 527 and 351 nm. The imaging system has a field of view of 4.5 x 2.8 mm{sup 2} and is capable of imaging any plane from 0 to 30 cm downstream from the defect. The system is calibrated using a 477 micron chromium dot on glass for which the downstream diffraction patterns were calculated numerically. Under nominal conditions the system can measure maximum peak modulations of approximately 7:1. An image division algorithm is used to calculate the peak modulation from the diffracted and empty field images after the baseline residual light background is subtracted from both. The peak modulation can then be plotted versus downstream position. The system includes a stage capable of holding optics up to 50 pounds with x and y translation of 40 cm and has been used to measure beam modulation due to solgel coating defects, surface digs on KDP crystals, lenslets in bulk fused silica and laser damage sites mitigated with CO{sub 2} lasers
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Overview of recent KDP damage experiments and implications for NIF tripler performance
Considerable attention has been paid over the years to the problem of growing high purity KDP and KD*P to meet damage threshold requirements of ICF lasers at LLNL. The maximum fluence requirement for KD*P triplers on the National Ignition Facility (NIF) is 14.3 J/cm2 at 351 nm in a 3 ns pulse. Currently KD*P (conventional or rapid grown) cannot meet this requirement without laser (pre)conditioning. In this overview, recent experiments to understand laser conditioning and damage phenomena in KDP and KD*P will be discussed. These experiments have lead to a fundamental revision of damage test methods and test result interpretation. In particular, the concept of a damage threshold has given way to measuring performance by damage distributions using millimeter sixed beams. Automated R/l (conditioned) damage tests have shown that the best rapidly grown KDP crystals exhibit the same damage distributions at the best conventionally grown KD*P. Continuous filtration of the growth solution and post growth thermal sealing are shown to increase the damage performance as well. In addition, centimeter size beams from multijoule lasers have been used to study stepwise laser conditioning in KDP. These tests have shown that an increase in the damage threshold of ~1.5X is attainable with 8-12 shots of increasing fluence. The experiments show that the damage density (pinpoints/mm3) evolves exponentially and once formed, the micron sized bulk pinpoints remain stable against increases in local fluence. The information obtained from damage distributions and conditioning studies has been used with model NIF spatial profiles to determine the probability of damage and the local pinpoint density generated in a tripler. Calculations based on test data have shown that .for well conditioned, high quality rapid growth KDP or conventional growth KD*P the damage probability is less than 3%. Furthermore, the fluence profiles expected on NIF lead to only small numbers of generated pinpoints which are not expected to adversely affect NIF operations. To check the validity of the results, the 37 cm KD*P tripler from the Beamlet laser was mapped for damage. The inspection revealed pinpoint densities of the order of predicted by the damage evolution calculation
The limit of N=(2,2) superconformal minimal models
The limit of families of two-dimensional conformal field theories has
recently attracted attention in the context of AdS/CFT dualities. In our work
we analyse the limit of N=(2,2) superconformal minimal models when the central
charge approaches c=3. The limiting theory is a non-rational N=(2,2)
superconformal theory, in which there is a continuum of chiral primary fields.
We determine the spectrum of the theory, the three-point functions on the
sphere, and the disc one-point functions.Comment: 37 pages, 3 figures; v2: minor corrections in section 5.3, version to
be published in JHE
Boundary Liouville theory at c=1
The c=1 Liouville theory has received some attention recently as the
Euclidean version of an exact rolling tachyon background. In an earlier paper
it was shown that the bulk theory can be identified with the interacting c=1
limit of unitary minimal models. Here we extend the analysis of the c=1-limit
to the boundary problem. Most importantly, we show that the FZZT branes of
Liouville theory give rise to a new 1-parameter family of boundary theories at
c=1. These models share many features with the boundary Sine-Gordon theory, in
particular they possess an open string spectrum with band-gaps of finite width.
We propose explicit formulas for the boundary 2-point function and for the
bulk-boundary operator product expansion in the c=1 boundary Liouville model.
As a by-product of our analysis we also provide a nice geometric interpretation
for ZZ branes and their relation with FZZT branes in the c=1 theory.Comment: 37 pages, 1 figure. Minor error corrected, slight change in result
(1.6
W-Extended Fusion Algebra of Critical Percolation
Two-dimensional critical percolation is the member LM(2,3) of the infinite
series of Yang-Baxter integrable logarithmic minimal models LM(p,p'). We
consider the continuum scaling limit of this lattice model as a `rational'
logarithmic conformal field theory with extended W=W_{2,3} symmetry and use a
lattice approach on a strip to study the fundamental fusion rules in this
extended picture. We find that the representation content of the ensuing closed
fusion algebra contains 26 W-indecomposable representations with 8 rank-1
representations, 14 rank-2 representations and 4 rank-3 representations. We
identify these representations with suitable limits of Yang-Baxter integrable
boundary conditions on the lattice and obtain their associated W-extended
characters. The latter decompose as finite non-negative sums of W-irreducible
characters of which 13 are required. Implementation of fusion on the lattice
allows us to read off the fusion rules governing the fusion algebra of the 26
representations and to construct an explicit Cayley table. The closure of these
representations among themselves under fusion is remarkable confirmation of the
proposed extended symmetry.Comment: 30 page
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