Fast Ion Beam Research Articles

Lower hybrid turbulence excited by a fast transverse ion beam in a magnetized plasma

Experimental and theoretical investigations of lower hybrid (LH) turbulence in the W7-AS stellarator are presented. The turbulence is excited by an ion beam, which is generated by a weak neutral hydrogen beam injected transversely to the confining magnetic field. The instability is detected by collective Thomson scattering of powerful gyrotron radiation. From the measured density dependence of the frequency it was identified as an LH type of instability. The spectrum is characterized by a narrow bandwidth in spite of the inherently poor radial resolution of the backscattering geometry. The theoretical model of an LH instability driven by a transverse fast ion component under the double resonance condition (coincidence of the LH frequency with a high cyclotron harmonic of the fast ions) is developed. The instability growth rate is derived. The stabilizing effect of high bulk ion temperatures was observed experimentally, in accordance with theoretical modelling. An instability saturation mechanism similar to the well known stochastic ion heating is proposed.

Experimental results on the fast beam-ion instability

The so-called fast beam-ion instability, if it really exists, is going to be a serious problem for future accelerators such as $B$ factories. A series of experiments has been conducted at the Pohang Light Source to test the existence of this new instability. An adequate amount of He gas was injected into the ring to enhance the ion effects. The results of the experiment strongly support the existence of the instability. The measured ion frequencies agree well with the linear theory. They even appear in normal operation condition with enough bunch current and bunch train length. Above all, measurements of the single pass beam position monitor clearly show the coherent oscillations with the amplitude increasing along the length of the bunch train, as predicted by the theory of this new instability.

Near-Threshold Photodissociation of ColdCH+in a Storage Ring

Photodissociation of ${\mathrm{CH}}^{+}$ molecules ( ${\mathrm{CH}}^{+}+h\ensuremath{\nu}\ensuremath{\rightarrow}{\mathrm{C}}^{+}+\mathrm{H}$) is studied by collinear laser spectroscopy on a rotationally relaxed fast molecular ion beam in a storage ring. We have detected the resonances predicted between the thresholds related to the two fine-structure levels of the ${\mathrm{C}}^{+}$ fragment for low initial rotations, and found that no heating processes hinder the complete rovibrational thermalization of the fast stored beam in a room temperature environment.

Transportation of intensive ion beams (invited)

This proceeding considers excitation of ion and electron oscillations in a plasma formed by the beam of fast positive ions. Linear and nonlinear stages of the oscillations developing, and the influence of these oscillations on the beam transportation properties are described. Also briefly described are some techniques for the stabilization of instabilities of electron and ion oscillations in compensated ion beams.

Dissociative Photodetachment Dynamics of Isomeric Forms of N3O2-

The dissociative photodetachment of N3O2- at 532, 355, and 266 nm has been studied using coincident photoelectron and photofragment translational spectroscopy in a fast-ion beam. The photoelectron spectra confirm previous experimental evidence for a weakly perturbed NO-(N2O) complex and show for the first time a broad continuum at large electron-binding energies. Translational energy release spectra exhibit both low- and high-energy-release channels, indicating that two regions of the NO−N2O neutral potential energy surface with very different intermolecular repulsion are accessed. Photoelectron−photofragment coincidence data confirm that the high translational energy release events are correlated with the continuum in the photoelectron spectrum. These dissociative photodetachment dynamics are consistent with the existence of two isomeric species, an ion−dipole complex with the charge localized on the NO moiety and a molecular anion with an estimated stability of 2.0 ± 0.2 eV relative to NO + N2O + e-.

Transportation of intensive ion beams (invited)

This proceeding considers excitation of ion and electron oscillations in a plasma formed by the beam of fast positive ions. Linear and nonlinear stages of the oscillations developing, and the influence of these oscillations on the beam transportation properties are described. Also briefly described are some techniques for the stabilization of instabilities of electron and ion oscillations in compensated ion beams.

Development of the radio frequency driven H- ion source for the National Spallation Neutron Source

The ion source for the 1 MW National Spallation Neutron Source (NSNS) is required to provide 35 mA of H- beam current (1 ms pulses at 60 Hz) at 65 keV with a normalized root-mean-square emittance of <0.2 pi mm mrad. The same ion source should be able to produce 70 mA of H- at 6% duty factor when the NSNS is upgraded to 2 MW of power. For this application, a radio-frequency driven, magnetically filtered multicusp source is being developed at Lawrence Berkeley National Laboratory. The design of this R and D ion source, which is equipped with a cesium dispenser-collar, a fast ion beam prechopper (rise times <100 ns) and a strong permanent-magnet insert for electron deflection, will be presented.

A new type of electrostatic ion trap for storage of fast ion beams

A new technique for trapping of fast (keV) ion beams is presented. The trap, which is electrostatic, works on a principle similar to that of optical resonators. The main advantages of the trap are the possibility to trap fast beams without need of deceleration, the well-defined beam direction, the easy access to the trapped beam by various probes, and the simple requirement in terms of external beam injection. Results of preliminary experiments related to the radiative cooling of molecular ions are also reported.

Effects of fast beam ions on ion-acoustic solitons and shocks excited in a double plasma device

The limitation in amplitude of ion-acoustic solitons or laminar shocks excitable in a double plasma device is clarified to result from the effect of pulsed fast beam ions, generated by the pulse externally applied for wave excitation. Beam ions faster than the soliton, which suppress the growth of the proto-soliton amplitude at an early stage of wave excitation, are always generated in the case of the double plasma (DP)-excitation, when a pulse high enough is applied so as to excite a large amplitude soliton. Controlling the generation of such fast beam ions by adjusting the rise time of the applied pulse, ion-acoustic solitons or laminar shocks of high Mach numbers up to M≃1.4 are observed to be formed even in a plasma with a finite ion temperature such as Ti≃Te/20.

First Observations of a “Fast Beam-Ion Instability”

We report the results of observations of a new regime of ion instabilities at the Advanced Light Source (ALS). With artificially increased pressure and gaps in the bunch train large enough to avoid multiturn ion trapping, we observed a factor of 2{endash}3 increase in the vertical beam size along with coherent beam oscillations which increased along the bunch train. The observations are qualitatively consistent with the {open_quotes}fast beam-ion instability{close_quotes} [T.O. Raubenheimer and F. Zimmermann, Phys.Rev.E {bold 52}, 5487 (1995); G.V. Stupakov {ital et al.,} Phys.Rev.E {bold 52}, 5499 (1995)], which can arise even when the ions are not trapped over multiple beam passages. This effect may be important for many future accelerators. {copyright} {ital 1997} {ital The American Physical Society}

The design and performance of a high resolution translational energy loss spectrometer

An instrumental development is described that achieved high energy resolving power when studying collisions between fast ion beams, at keV energies, interacting with thermal molecular and atomic gaseous targets. This technique is referred to as translational energy spectroscopy (TES) and can reveal spectroscopic information on the states of the participating species, the dynamics of the collision, lifetimes of species formed, state-to-state collision cross-sections and populations of states. The instruments described are based on an unique symmetrical arrangement of cylindrical electrostatic analysers. These are placed in a double focusing ion-optical geometry leading to very small first and second order image aberrations, sufficient to obtain a primary energy resolving power of 105. Two such instruments were developed and their relative performances are compared experimentally and through computer modelling; the second, and larger, instrument has an improved sensitivity by an order of magnitude and a three times increase in resolving power. This has enabled collision spectroscopy to achieve performance similar to electron energy-loss spectroscopy and photoelectron spectroscopy, with a resolution on the unscattered ion beam measured to be 0.01 eV, at 3 keV collision energy, on energy loss processes of ≥ 0.03 eV. These developments allow TES to observe, (i) the vibrational/electronic spectroscopy of keV ion/molecule collisions; (ii) spin-selective spectra; (iii) the highly excited states close to and beyond the ionization limit of the target species; and (iv) involatile targets using a high temperature cell. The validity of optical quantum selection rules, relying on the electric-dipole interaction governing photon-induced transitions, has been tested for TES, which relies on the more general ion/neutral interaction potential responsible for collisional excitation. Spin conservation was found to be strongly adhered to, whilst orbital angular momentum and total angular momentum were not. The specific selection rules for homonuclear diatomics g<–>g and u<–>u were found to be allowed in TES, in contradiction to the case of optical spectroscopy, whilst there was some preliminary evidence that the optical Σ+–Σ− symmetry-forbidden transition was also forbidden for the TES reactions studied. For H2+ projectiles, excitation to triplet states is strongly favoured over excitation to singlet states. © 1997 John Wiley & Sons, Ltd.

Stability of global drift-kinetic Alfvén eigenmodes in DIII-D

Global Alfvén eigenmodes are studied using two different models for the plasma and the results compared with the instability threshold measured experimentally. Fluid-resistive models predict that continuum damped toroidicity induced Alfvén modes (TAE) are formed in the frequency range of the experimental instabilities, but using realistic values of the resistivity makes a stability analysis impossible for computational reasons. The kinetic plasma model solves this problem by taking into account the ion Larmor radius and the resonant Landau interactions between the particles and the wavefield. As a consequence drift-kinetic Alfvén eigenmodes (DKAE) are created by toroidal coupling between the global TAE wavefield, the mode converted kinetic Alfvén, ion-acoustic and drift waves. The power transfers between the wave and the particles show that the drift character of the wavefield in the core destabilizes DKAE modes through resonant interactions with the fast beam ions. The predicted beam pressure instability threshold is in agreement with the one measured experimentally.

Electrostatic bottle for long-time storage of fast ion beams

A technique for storage of fast-ion beams (keV) using only electrostatic fields is presented. The fast-ion trap is designed like an optical resonator, whose electrode configuration allows for a very large field-free region, easy access into the trap by various probes, a simple ion loading technique, and a broad acceptance range for the initial kinetic energies of the ions. Such a fast-ion storage device opens up many experimental possibilities, a few of which are presented.

Excited state dynamics in clusters of oxygen

Considerable insights into the dynamics of both ionic (photodissociation) and neutral (dissociative photodetachment) decomposition pathways of O4- and O6- have been gained using photoelectron and photofragment translational spectroscopy in a fast-ion beam. The O4- data at 532 nm reveal a novel process involving sequential photodetachment of an electron with a near-zero binding energy from photodissociating O4-. Studies of O6- at 532 nm reveal that addition of a third O2 to the O4- core leads to a dramatic change in the photodissociation dynamics, producing highly vibrationally excited O2- photofragments not observed in the case of O4-. At 355 nm, both O4- and O6- yield vibrationally excited O2- photofragments, as observed by autodetachment of the nascent O2- (v5)→O2+e-. At 266 nm, photofragment time-of-flight (TOF) measurements on O6- and O4- show that the dynamics of dissociative photodetachment in O6- are only slightly perturbed relative to O4-. The anisotropic product angular distribution previously observed in O4- is observed to persist in the three-body neutral decomposition O6-+hν→O2+O2+O2+e-. The origins of these diverse phenomena in O4- and O6- are discussed.

Translational spectroscopy studies of the photodissociation dynamics of O−4

An investigation of the photodissociation dynamics of the dimer anion O−4 at 523.6, 349.0, and 261.8 nm is reported. Product translational energy and angular distributions have been obtained using photofragment translational spectroscopy in a fast ion beam. At all wavelengths photodissociation (O−4+hν→O2+O−2) is observed to proceed via a rapid parallel electronic transition, with the photofragment angular distribution strongly peaked along the laser electric vector. The lowest energy photodissociation channel produces O2(a1Δg) and ground state O−2(X2Πg), indicating that O−4 is a doublet anion. The partitioning of energy in the dissociation reveals a complicated wavelength dependence.

Fast-ion beam laser spectroscopy of 14N2+ and 15N2+: high-resolution study of the (1, 2) band of the B2Σu+X2Σg+ system

Abstract We used laser-induced fluorescence combined with fast-ion beam laser spectroscopy to study at high resolution the rotational structure of the (1, 2) band of the first negative system (B2Σu+X2Σg+ of 14N2+ and 15N2+. Combining our laser-induced fluorescence data for the BX band and the new Fourier-transform emission data for the Meinel A-X system, we removed the correlation between the X and B state and we determined more accurate values of the molecular parameters for the excited and perturbed states of 14N2+ and 15N2+. The vibrational level v′ = 1 of the B state was found to be unperturbed within the observed rotational levels (4 A 2 Π u state.

Efficient, Indirect Transverse Laser Cooling of a Fast Stored Ion Beam.

Three-dimensional laser cooling of a fast stored ion beam has been demonstrated at the Heidelberg Test Storage Ring. With a purely longitudinal cooling force applied to a $7.3\mathrm{MeV}{}^{3}{\mathrm{Be}}^{+}$ beam, we have observed an efficient transverse cooling effect. We interpret this observation as being due to a thermal intrabeam relaxation between the different degrees of freedom that is caused by Coulomb collisions of the stored particles.

Hyperfine structure measurements in Kr II

The fast ion beam laser technique has been utilized to study the hyperfine structure in the transitions 4d4D7/2-5p4D72/0 at 6420 A and 4d4D7/2-5p4D52/0 at 6303 A in singly-charged krypton. The magnetic dipole and electric quadrupole hyperfine constants have been extracted for the upper levels of the transitions.

Fast Ion Beam Laser Spectroscopy of13CO+2Laser Induced Fluorescence of theÃ2Πu–X2ΠgElectronic Transition

Laser induced fluorescence of a fast ion beam has been applied to13CO+2for its (0, 0, 0)Ã2Πu–(0, 0, 0)X2ΠgUV electronic transition. The high resolution (0.005 cm−1) of the FIBLAS method, due to a highly reduced Doppler broadening, allows us to evaluate the isotopic shift, to appreciate the contribution of the central atom when determining molecular parameters, and, for the first time, to determine the influence of the central atom on hyperfine structure. Absolute calibration of the13CO+2spectra is obtained by means of a12CO+2–13CO+2spectrum, allowing calibration via FT spectra of12CO+2.

Photodissociation spectroscopy and dynamics of the N2O−2 anion

The spectroscopy and dissociation dynamics of the N2O−2(C2v) anion have been investigated using the technique of fast ion beam translational spectroscopy. A newly developed pulsed supersonic discharge source is described for the production of internally cold N2O−2. A structured absorption band beginning near 580 nm is observed, and is assigned to the B̃(2A2)←X̃(2B2) transition with the aid of ab initio calculations. Two dissociation channels from the upper state are observed: (1) O−+N2O and (2) NO−+NO. Translational energy and angular distributions are measured for both channels at several excitation energies. The translational energy distribution for channel (1) at 570 nm shows resolved structure corresponding to N2O vibrational excitation. The translational energy distributions for channel (1) are reasonably well described by prior distributions, indicating this channel results from dissociation from the N2O−2 ground electronic state. In contrast, channel (2) appears to result from dissociation on a repulsive excited electronic state. From the translational energy distributions for channel (1), we obtain the bond dissociation energy and heat of formation (at 0 K) for N2O−2: D0(O–N2O)=1.40±0.03 eV and ΔH0f,0(N2O−2)=0.58±0.04 eV.