844 research outputs found
ECR heating on the WEGA stellarator
The plasma in the WEGA stellarator is generated and heated by Electron Cyclotron Resonance Heating (ECRH). The microwave is emitted from the low field side mid-plane with power of up to 6+20 kW and with a frequency of 2.45GHz (λ=12.45 cm). The low cut-off density of ncutoff=7.5×10¹⁶m⁻³ makes ECRH on the WEGA stellarator inefficient in both O-mode and X-mode regime. This was confirmed in the first experimental campaign by perpendicular launch of the microwave with a TE11 antenna. In these experiments only edge heating was observed. Density and temperature profiles were hollow [1]. For the over dense plasma heating, mode conversion into the electrostatic electron Bernstein waves (EBW) is required. Two schemes have been tested: the direct X-B (X-mode to Bernstein mode) conversion, where an X-wave must be launched perpendicular to the magnetic field into an over dense plasma with a steep density gradient. In these experiments the strong reflection of the microwave power at the cut-off layer prohibited efficient plasma heating. Another possibility is the O-X-B conversion scheme [2]. The methods of its achievement with different antennas are the subject of this paper.Плазма у стеллараторі WEGA утворюється та нагрівається за допомогою НВЧ-нагріву на електронно- циклотронній частоті (ЕЦР). Ввід НВЧ-енергії здійснюється із зовнішнього боку тору у його екваторіальній площині, на частоті 2.45 ГГц (λ=12.45 см) та з максимальною потужністю до 26 кВт. Низька критична щільність плазми для цієї частоти (ncutoff=7.5×10¹⁶м⁻³ ) робить традиційний ЕЦР-нагрів неефективним, як у режимі «Звичайної» (З-хвиля), так і у режимі «Незвичайної» хвилі (Н-хвиля), що було підтверджено у ході першої експериментальної кампанії, коли НІЧ-енергія вводилась перпендикулярно силовим лініям магнітного поля за допомогою циліндричного ТЕ11 хвилеводу. У цих експериментах спостерігався нагрів периферичної плазми, профілі щільності та температури мали порожній характер [1]. Для нагріву плазми з щільністю вище щільності відсічки необхідна трансформація в електростатичну Бернштейн хвилю (ЕБХ). Два сценарії нагріву було випробувано на установці: перший з трансформацією Н-хвилі у ЕБХ, в цьому випадку Н-хвиля повинна бути введена перпендикулярно магнітному полю в плазму з щільністю, яка перевищує критичну. У цих експериментах сильне відбиття НВЧ-енергії у шарі відсічки перешкоджало ефективному нагріву плазми. У другому випадку використовується сценарій з подвійною конверсією спочатку З-хвилі у Н-хвилю з наступною трансформацією Н-хвилі в ЕБХ [2]. Методи реалізації такого сценарію і є темою цієї роботи.Плазма в стеллараторе WEGA создается и нагревается при помощи СВЧ-нагрева на электронно- циклотронной частоте (ЭЦР). Ввод СВЧ-энергии производится с внешней стороны тора в его экваториальной плоскости на частоте 2.45 ГГц (λ=12.45 см) и с максимальной мощностью до 26 кВт. Низкая критическая плотность плазмы для данной частоты (ncutoff=7.5×10¹⁶м⁻³ ) делает традиционный ЭЦР-нагрев неэффективным, как в режиме «обыкновенной» (О-волна), так и в режиме «необыкновенной» волны (Н-волна), что было подтверждено в ходе первой экспериментальной кампании, когда СВЧ-энергия вводилась перпендикулярно силовым линиям магнитного поля при помощи цилиндрического ТЕ11 волновода. В этих экспериментах наблюдался нагрев периферийной плазмы, профили плотности и температуры имели полый характер [1]. Для нагрева плазмы с плотностью, выше плотности отсечки, необходима трансформация в электростатическую Бернштейн волну (ЭБВ). Два сценария нагрева были протестированы на установке: первый с непосредственной трансформацией Н-волны в ЭБВ, в этом случае Н-волна должна быть запущена перпендикулярно магнитному полю в плазму с плотностью, превышающей критическую. В этих экспериментах сильное отражение СВЧ- энергии в слое отсечки препятствовало эффективному нагреву плазмы. Во втором случае используется сценарий с двойной конверсией сначала О-волны в Н-волну с последующим превращением Н-волны в ЭБВ[2]. Методы реализации такого сценария и есть тема настоящей работы
A consistent analysis of (e,e'p) and (d,3He) experiments
The apparent discrepancy between spectroscopic factors obtained in (e,e'p)
and (d,3He) experiments is investigated. This is performed first for
48Ca(e,e'p) and 48Ca(d,3He) experiments and then for other nuclei. It is shown
that the discrepancy disappears if the (d,3He) experiments are re-analyzed with
a non-local finite range DWBA analysis with a bound-state wave function that is
obtained from (e,e'p) experiments.Comment: 23 pages, 7 figure
Semigroup Closures of Finite Rank Symmetric Inverse Semigroups
We introduce the notion of semigroup with a tight ideal series and
investigate their closures in semitopological semigroups, particularly inverse
semigroups with continuous inversion. As a corollary we show that the symmetric
inverse semigroup of finite transformations of the rank
is algebraically closed in the class of (semi)topological inverse
semigroups with continuous inversion. We also derive related results about the
nonexistence of (partial) compactifications of classes of semigroups that we
consider.Comment: With the participation of the new coauthor - Jimmie Lawson - the
manuscript has been substantially revised and expanded. Accordingly, we have
also changed the manuscript titl
Production of mesons in proton-proton collisions
The cross section for the production of mesons in proton-proton
collisions has been measured in a previously unexplored region of incident
energies. Cross sections were extracted at 92 MeV and 173 MeV excess energy,
respectively. The angular distribution of the at =173 MeV is
strongly anisotropic, demonstrating the importance of partial waves beyond pure
s-wave production at this energy.Comment: 12 pages, 4 figures submitted to Physics Letters B v2: figure 1
added, discussion detailing the data analysis, figure 3 (fig. 2 in v1)
modified in line styles and systematic errors displayed on dat
Study of the p p -> p p pi+ pi- Reaction in the Low-Energy Tail of the Roper Resonance
Exclusive measurements of the p p -> p p pi+ pi- reaction have been carried
out at Tp = 775 MeV at CELSIUS using the PROMICE/WASA setup. Together with data
obtained at lower energy they point to a dominance of the Roper excitation in
this process. From the observed interference of its decay routes N* -> N sigma
and N* -> Delta pi -> N sigma their energy-dependent relative branching ratio
is determined
Relativistic versus Nonrelativistic Optical Potentials in A(e,e'p)B Reactions
We investigate the role of relativistic and nonrelativistic optical
potentials used in the analysis of () data. We find that the
relativistic calculations produce smaller () cross sections even in the
case in which both relativistic and nonrelativistic optical potentials fit
equally well the elastic proton--nucleus scattering data. Compared to the
nonrelativistic impulse approximation, this effect is due to a depletion in the
nuclear interior of the relativistic nucleon current, which should be taken
into account in the nonrelativistic treatment by a proper redefinition of the
effective current operator.Comment: Added one new figure, the formalism section has been enlarged and the
list of references updated. Added one appendix. This version will appear in
Phys. Rev. C. Revtex 3.0, 6 figures (not included). Full postscript version
of the file and figures available at
http://www.nikhefk.nikhef.nl/projects/Theory/preprints
Comparison of Isoscalar Vector Meson Production Cross Sections in Proton-Proton Collisions
The reaction was investigated with the TOF
spectrometer, which is an external experiment at the accelerator COSY
(Forschungszentrum J\"ulich, Germany). Total as well as differential cross
sections were determined at an excess energy of (). Using the total cross section of for the
reaction determined here and existing data for the reaction
, the ratio
turns out to be
significantly larger than expected by the Okubo-Zweig-Iizuka (OZI) rule. The
uncertainty of this ratio is considerably smaller than in previous
determinations. The differential distributions show that the
production is still dominated by S-wave production at this excess energy,
however higher partial waves clearly contribute. A comparison of the measured
angular distributions for production to published distributions for
production at shows that the data are consistent with an
identical production mechanism for both vector mesons
Donor states in modulation-doped Si/SiGe heterostructures
We present a unified approach for calculating the properties of shallow
donors inside or outside heterostructure quantum wells. The method allows us to
obtain not only the binding energies of all localized states of any symmetry,
but also the energy width of the resonant states which may appear when a
localized state becomes degenerate with the continuous quantum well subbands.
The approach is non-variational, and we are therefore also able to evaluate the
wave functions. This is used to calculate the optical absorption spectrum,
which is strongly non-isotropic due to the selection rules. The results
obtained from calculations for Si/SiGe quantum wells allow us to
present the general behavior of the impurity states, as the donor position is
varied from the center of the well to deep inside the barrier. The influence on
the donor ground state from both the central-cell effect and the strain arising
from the lattice mismatch is carefully considered.Comment: 17 pages, 10 figure
Fragmentation Function and Hadronic Production of the Heavy Supersymmetric Hadrons
The light top-squark \sto may be the lightest squark and its lifetime may
be `long enough' in a kind of SUSY models which have not been ruled out yet
experimentally, so colorless `supersymmetric hadrons (superhadrons)' (\sto
\bar{q}) ( is a quark except -quark) may be formed as long as the light
top-squark \sto can be produced. Fragmentation function of \sto to heavy
`supersymmetric hadrons (superhadrons)' (\sto \bar{Q}) ( or
) and the hadronic production of the superhadrons are investigated
quantitatively. The fragmentation function is calculated precisely. Due to the
difference in spin of the SUSY component, the asymptotic behavior of the
fragmentation function is different from those of the existent ones. The
fragmentation function is also applied to compute the production of heavy
superhadrons at hadronic colliders Tevatron and LHC under the so-called
fragmentation approach. The resultant cross-section for the heavy superhadrons
is too small to observe at Tevatron, but great enough at LHC, even when all the
relevant parameters in the SUSY models are taken within the favored region for
the heavy superhadrons. The production of `light superhadrons' (\sto \bar{q})
() is also roughly estimated. It is pointed out that the production
cross-sections of the light superhadrons (\sto \bar{q}) may be much greater
than those of the heavy superhadrons, so that even at Tevatron the light
superhadrons may be produced in great quantities.Comment: 20 pages, 9 figure
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