A new ion acceleration method, namely, phase-stable acceleration, using circularly-polarized laser pulses is proposed. When the initial target density n(0) and thickness D satisfy a(L) approximately (n(0)/n(c))D/lambda(L) and D>l(s) with a(L), lambda(L), l(s), and n(c) the normalized laser amplitude, the laser wavelength in vacuum, the plasma skin depth, and the critical density of the incident laser pulse, respectively, a quasiequilibrium for the electrons is established by the light pressure and the space charge electrostatic field at the interacting front of the laser pulse. The ions within the skin depth of the laser pulse are synchronously accelerated and bunched by the electrostatic field, and thereby a high-intensity monoenergetic proton beam can be generated. The proton dynamics is investigated analytically and the results are verified by one- and two-dimensional particle-in-cell simulations.

A novel fast electron beam emitting along the surface of a target irradiated by intense laser pulses is observed. The beam is found to appear only when the plasma density scale length is small. Numerical simulations reveal that the electron beam is formed due to the confinement of the surface quasistatic electromagnetic fields. The results are of interest for potential applications of fast electron beams and deep understanding of the cone-target physics in the fast ignition related experiments.

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Self-injection in a laser-plasma wakefield accelerator is usually achieved by increasing the laser intensity until the threshold for injection is exceeded. Alternatively, the velocity of the bubble accelerating structure can be controlled using plasma density ramps, reducing the electron velocity required for injection. We present a model describing self-injection in the short-bunch regime for arbitrary changes in the plasma density. We derive the threshold condition for injection due to a plasma density gradient, which is confirmed using particle-in-cell simulations that demonstrate injection of subfemtosecond bunches. It is shown that the bunch charge, bunch length, and separation of bunches in a bunch train can be controlled by tailoring the plasma density profile.

It is found that stable proton acceleration from a thin foil irradiated by a linearly polarized ultraintense laser can be realized for appropriate foil thickness and laser intensity. A dual-peaked electrostatic field, originating from the oscillating and nonoscillating components of the laser ponderomotive force, is formed around the foil surfaces. This field combines radiation-pressure acceleration and target normal sheath acceleration to produce a single quasimonoenergetic ion bunch. A criterion for this mechanism to be operative is obtained and verified by two-dimensional particle-in-cell simulation. At a laser intensity of ∼5.5×10(22)  W/cm(2), quasimonoenergetic GeV proton bunches are obtained with ∼100  MeV energy spread, less than 4° spatial divergence, and ∼50% energy conversion efficiency from the laser.

The radiation pressure of next generation ultra-high intensity (>10 23 W cm −2 ) lasers could efficiently accelerate ions to GeV energies. However, nonlinear quantum-electrodynamic effects play an important role in the interaction of these laser pulses with matter. Here we show that these effects may lead to the production of an extremely dense (∼10 24 cm −3 ) pair-plasma which absorbs the laser pulse consequently reducing the accelerated ion energy and laser to ion conversion efficiency by up to 30%-50% and 50%-65%, respectively. Thus we identify the regimes of laser-matter interaction, where either ions are efficiently accelerated to high energy or dense pair-plasmas are produced as a guide for future experiments.

By the use of two-dimensional particle-in-cell simulations, we clarify the terahertz (THz) radiation mechanism from a plasma filament formed by an intense femtosecond laser pulse. The nonuniform plasma density of the filament leads to a net radiating current for THz radiation. This current is mainly located within the pulse and the first cycle of the wakefield. As the laser pulse propagates, a single-cycle and radially polarized THz pulse is constructively built up forward. The single-cycle shape is mainly due to radiation damping effect.

Summary The c-Ha-ras gene was analysed by Southern blot hybridisation in 67 specimens of lymph node metastases and in 25 specimens of primary tumours obtained from 85 untreated patients with head and neck squamous cell carcinoma. The loss of one c-Ha-ras allele was observed in 10/46 (22%) tumours from heterozygous patients for this locus. Different genes, located as the c-Ha-ras gene on the short arm of chromosome 11, were also found to be deleted suggesting that the deletion of other genes could play a role in aggressiveness of head and neck carcinomas. Using polymerase chain reaction, mutation at codon 12 was detected in only 2/54 (3.8%) tumours but no mutation involving codon 61 was found. Neither gene amplification nor gene rearrangement could be observed. Total RNA was prepared from 79 of these tumour specimens and analysed by Northern and slot blot hybridisation. A 1.2 kb c-Ha-ras transcript band was detected in all the RNA preparations. Relatively high c-Ha-ras transcript levels were found in 18% of lymph node metastases and in 21% of primary tumours, indicating no significant differences between these cancers. Moreover, the c-Ha-ras mRNA levels were not significantly greater in the primary tumours than in the normal mucosae in 10/12 cases for which both tissues were analysed. These data indicate that c-Ha-ras gene does not seem to be strongly involved in head and neck carcinomas at that advanced stage of the disease, as this was previously reported for earlier clinical stages.It is now well established that genetic alterations are implicated in the biological deregulation of cancer cells and that the cellular oncogenes are involved in the cancer process (Bishop, 1987;Merkel & McGuire, 1988;Nordenskjold & Cavenee, 1988). Among those cellular oncogenes, the ras genes were thought to play an important role and many studies were initiated to detect alterations and aberrant expression of these genes. Somatic mutations, resulting in the substitution of a single base at particular positions in the gene locus were found to be responsible for oncogenic activity by transfection assay, in about 15% of cancers (Barbacid, 1988). Recently, using more sensitive methods, high rates of mutation affecting the c-Ki-ras gene were detected in DNA from pancreas and colon carcinomas (Bos et al., 1987;Forrester et al., 1987;Almoguera et al., 1988;Bos, 1989). In colon tissues the c-Ki-ras mutations were even found in the precancerous lesions which were assumed to progress in invasive carcinomas. Mutations of the ras gene were also found in thyroid adenomas and carcinomas (Lemoine et al., 1988;Suarez et al., 1988). C-Ha-ras mutation at codon 12 was shown to be associated with cervical cancers of poor prognosis . A c-Ha-ras restriction fragment length polymorphism (RFLP) (Capon et al., 1983) (Barbacid, 1988). On the contrary, studies on the expression of ras genes indicated that high levels of ras-specific messenger RNA and ras p21 protein were associated with tumour progression in human cancers of different origins (Viola et al...