Revisiting the Longitudinal 90° Limit in High Intensity Linear Accelerators.

Parametric envelope and sum envelope resonances are analyzed to revisit the validity of an assumed stop band and design limit of high intensity linear accelerators at a longitudinal phase advance of 90° per focusing lattice period. While the 90° limit is unquestioned in the transverse plane, we show here that it can be dropped as longitudinal limit for lattices with two or more rf gaps per focusing period. A new limit arises, however, from a novel transverse-longitudinal parametric sum envelope instability. The resulting sum instability rule allows the phase advance to exceed 90° longitudinally provided that transversely it remains correspondingly under 90°. We suggest that the additional design freedom opens the possibility for larger accelerating gradients and stronger longitudinal focusing with potential length and cost saving in the design of advanced superconducting linear accelerator concepts-as long as technological cavity limits are not reached.

Space-Charge Structural Instabilities and Resonances in High-Intensity Beams.

The existence of a structural resonance stop band caused by space charge in high-current beams, where the resonance frequency is associated with 90° phase advance per focusing period, is well known and alternatively referred to in the literature as envelope instability or as fourth-order resonance. We show, however, that this stop band is actually a coincidence of a structural fourth-order resonance and the much stronger envelope instability as competing mechanisms--depending on the time scale and initial matching. A similar complexity of behavior--dependent on the distribution function--is also found between a third-order instability and a sixth-order resonance in a 60° stop band. We claim that these findings are a generic property of high-intensity beams in periodic focusing; they also allow a reinterpretation of the 90° linear accelerator stop band previously observed experimentally at the UNILAC accelerator.

Cellulose-based scaffold materials for cartilage tissue engineering.

Non-woven cellulose II fabrics were used as scaffolds for in vitro cartilage tissue engineering. The scaffolds were activated in a saturated Ca(OH)(2) solution and subsequently coated with a calcium phosphate layer precipitated from a supersaturated physiological solution. Chondrocyte cell response and cartilage development were investigated. The cell adherence was significantly improved compared to untreated cellulose fabrics, and the proliferation and vitality of the adhered chondrocytes were excellent, indicating the biocompatibility of these materials. A homogeneous distribution of the seeded cells was possible and the development of cartilageous tissue could be proved. In contact with a physiological chondrocyte solution, calcium is expected to be leached out from the precipitated layer, which might lead to a microenvironment that triggers the development of cartilage in a way similar to cartilage repair in the vicinity of subchondral bone.

Heterochromatin proteins and the control of heterochromatic gene silencing in Arabidopsis.

The SU(VAR)3-9 protein family was first identified in animals as heterochromatin-associated proteins and found to control establishment of heterochromatic chromatin domains by histone H3 lysine 9 methylation. In Arabidopsis ten SU(VAR)3-9 homologous SUVH genes are found where SUVH1, SUVH2 and SUVH4 represent different subgroups of genes. Also the SUVH1, SUVH2 and SUVH4 proteins represent heterochromatin-associated proteins and display differential effects on control of heterochromatic histone methylation marks. In Arabidopsis the heterochromatin specific histone methylation marks are mono- and dimethyl H3K9, mono- and dimethyl H3K27 and monomethyl H4K20. In contrast to animal systems trimethyl H3K9, trimethyl H3K27 and di- and trimethyl H4K20 do not index chromocenter heterochromatin in Arabidopsis. SUVH2 shows a central role in control of heterochromatin formation and heterochromatic gene silencing in Arabidopsis. Loss-of-function of SUVH2 results in significant reduction of all heterochromatin-specific histone methylation marks and causes DNA hypomethylation at chromocenter heterochromatin. SUVH2 overexpression leads to ectopic heterochromatisation accompanied with significant growth defects. SUVH2 shows strong dosage-dependent effects on transcriptional gene silencing. In Arabidopsis different experimental systems connected with transcriptional gene silencing have been used for genetic dissection of molecular mechanisms controlling epigenetic processes. Molecular analysis of the genes identified by the isolated modifier mutants suggests that transcriptional gene silencing in plants is caused by heterochromatisation. A new efficient experimental system for the analysis of transcriptional gene silencing has been established with the help of LUCIFERASE transgene repeats. The different lines established show either complete or partial silencing of the luciferase transgene repeats. These lines have been successfully used either for mutant isolation or for functional analysis of SUVH proteins in control of heterochromatic gene silencing.

Longitudinal impedance and shielding effectiveness of a resistive beam pipe for arbitrary energy and frequency.

The longitudinal coupling impedance of a cylindrical beam pipe for arbitrary relativistic gamma(0) and mode frequency is obtained analytically for finite wall conductivity and finite wall thickness. Closed form expressions for the electromagnetic fields excited by a beam perturbation are derived analytically. General expressions for the resistive-wall impedance in the presence of a metallic shield and for the rf shielding effectiveness of the beam pipe have been obtained and then compared with approximate expressions. The results are applied to the GSI synchrotron SIS, where the thickness of the vacuum chamber in the dipole magnets is much smaller than the skin depth at injection energy.

Pivotal role of AtSUVH2 in heterochromatic histone methylation and gene silencing in Arabidopsis.

SU(VAR)3-9 like histone methyltransferases control heterochromatic domains in eukaryotes. In Arabidopsis, 10 SUVH genes encode SU(VAR)3-9 homologues where SUVH1, SUVH2 and SUVH4 (KRYPTONITE) represent distinct subgroups of SUVH genes. Loss of SUVH1 and SUVH4 causes weak reduction of heterochromatic histone H3K9 dimethylation, whereas in SUVH2 null plants mono- and dimethyl H3K9, mono- and dimethyl H3K27, and monomethyl H4K20, the histone methylation marks of Arabidopsis heterochromatin are significantly reduced. Like animal SU(VAR)3-9 proteins SUVH2 displays strong dosage-dependent effects. Loss of function suppresses, whereas overexpression enhances, gene silencing, causes ectopic heterochromatization and significant growth defects. Furthermore, modification of transgene silencing by SUVH2 is partially transmitted to the offspring plants. This epigenetic stability correlates with heritable changes in DNA methylation. Mutational dissection of SUVH2 indicates an implication of its N-terminus and YDG domain in directing DNA methylation to target sequences, a prerequisite for consecutive histone methylation. Gene silencing by SUVH2 depends on MET1 and DDM1, but not CMT3. In Arabidopsis, SUVH2 with its histone H3K9 and H4K20 methylation activity has a central role in heterochromatic gene silencing.

Space-charge driven emittance growth in a 3D mismatched anisotropic beam.

We investigate the phenomenon of space-charge driven emittance growth in a three-dimensional mismatched anisotropic charged particle beam with relevance to high-intensity linear accelerators. The final emittance growth can be understood as a superposition of the contributions from the mismatch-induced halo formation and from the anisotropy-induced energy exchange. The averaged emittance growth per degree of freedom is bounded from above by the so-called "free energy limit" extended by the contributions from energy exchange. The partition of the growth into longitudinal or transverse is, however, a strong function of the tune ratio including the possibility that an initially equipartitioned beam is even driven substantially away from equipartition. The growth of the beam halo extent is dominated by the effect of mismatch, whereas anisotropy itself generates practically no halo.

BRU1, a novel link between responses to DNA damage and epigenetic gene silencing in Arabidopsis.

DNA repair associated with DNA replication is important for the conservation of genomic sequence information, whereas reconstitution of chromatin after replication sustains epigenetic information. We have isolated and characterized mutations in the BRU1 gene of Arabidopsis that suggest a novel link between these underlying maintenance mechanisms. Bru1 plants are highly sensitive to genotoxic stress and show stochastic release of transcriptional gene silencing. They also show increased intrachromosomal homologous recombination and constitutively activated expression of poly (ADP-ribose) polymerase-2 (AtPARP-2), the induction of which is associated with elevated DNA damage. Bru1 mutations affect the stability of heterochromatin organization but do not interfere with genome-wide DNA methylation. BRU1 encodes a novel nuclear protein with two predicted protein-protein interaction domains. The developmental abnormalities characteristic of bru1 mutant plants resemble those triggered by mutations in genes encoding subunits of chromatin assembly factor (CAF-1), the condensin complex, or MRE11. Comparison of bru1 with these mutants indicates cooperative roles in the replication and stabilization of chromatin structure, providing a novel link between chromatin replication, epigenetic inheritance, S-phase DNA damage checkpoints, and the regulation of meristem development.