Photodissociation of RbI in an Intense Laser Field: Detection of the Product Metal Atom Rb Using RIMS

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Photodissociation of RbI in an Intense Laser Field: Detection of the Product Metal Atom Rb Using RIMS

It is reported on the recent research using resonant ionization spectroscopy (RIS) and resonant ionization mass spectroscopy (RIMS) to detect the photodissociation product of Rubidium iodide molecules in a molecular beam in an intense laser field. Time of flight mass spectra is recorded. In particular, the appearances of multiphoton ionization are discussed.     

Photodissociation of sodium iodide and resonant ionization of sodium atom produced

Resonant ionization spectroscopy (RIS) and resonant ionization mass spectroscopy (RIMS) are employed to detect the photodissociation product of sodium iodide molecules in a molecular beam in an intense laser field in the absence of the buffer gases. Time of flight mass spectra is recorded. In particular, the appearances of multiphoton ionization are discussed.     

充氦气对多波切伦柯夫振荡器工作特性的影响

采用PIC模拟方法,研究了具有谐振腔的多波切伦柯夫振荡器在充入不同密度的氦气下工作的物理过程,分析了等离子体产生物理机制及其对微波输出影响。结果表明,等离子体的产生是由于电子束对氦原子的碰撞电离及其雪崩效应引起的。由于电离产生的正离子有利于束的传输和群聚,当在一定范围内增加氦气密度时,可减小微波起振时间,提高束波能量转换效率,但并不改变微波频率;进一步增大气体密度,微波起振时间增大、效率下降,甚至出现脉冲缩短现象。     

铀原子多色三光子共振电离谱研究

利用Ｎｄ：ＹＡＧ二倍频激光同步泵浦的脉冲染料激光系统、原子束装置、飞行时间质谱和Ｂｏｘｃａｒ技术测量了＾２３８Ｕ的多色三光子共振电离谱。研究了电场对铀原子第一激发态的影响。解决了如何确定三角三光子共振电离谱图中所含的单色,双色和三色共振电离峰的问题。     

High-frequency emissions during the propagation of an electron beamin high-density plasma

A relativistic annular electron beam passing through a high-density plasma excites Langmuir waves via Cerenkov interaction. The Langmuir waves are backscattered off ions via nonlinear ion Landau damping. At moderately high amplitudes these waves are parametrically up-converted by the beam into high-frequency electromagnetic radiation, as observed in some recent experiments. A nonlocal theory of this process is developed in a cylindrical geometry. It is seen that the growth rate of the Langmuir wave scales as one-third the power of beam density. The growth rate of parametric instability scales as one-fourth the power of beam density and the square root of beam thickness     

Decay instability in a plasma channel

Near the critical layer, a large-amplitude laser beam creates a transverse plasma channel due to the ponderomotive force. The axis of the channel is normal to the ambient density gradient. Inside the channel, the laser beam decays into counter-propagating ion acoustic and Langmuir waves. A nonlocal theory of the process is developed for a parabolic density profile. The growth rate of the instability is significantly reduced due to the nonlocal effects     

静电探针测量强流电子束电离气体产生的等离子体密度

 用电离理论和核物理学中讨论电子束通过介质后的能量损耗方法分别估算了强流电子束电离中性气体产生的等离子体的密度。在实验中将静电探针应用于测量强流电子束电离氮气产生的等离子体的密度,得出等离子体密度随气压变化的曲线。实验结果表明在1～15帕气压范围内,等离子体密度在量级,与理论结果相符,证明静电探针用于诊断强流相对论电子束电离中性气体产生的等离子体的密度是可行的。     

Mechanisms of resonant laser ionization

An experimental investigation and numerical simulation of resonant laser breakdown are performed. As a result, quantitative agreement between the experimental data on the parameters of a dense resonant plasma (the electron density and the electron temperature) and the results of calculations in the range of detunings of the laser radiation from resonance Δλ>2–2.5 nm, in which the spatial instability of the intense resonant laser beam and the absorption of radiation are minimal, is obtained for the first time. It is shown that the previously proposed mechanism of resonant breakdown associated with laser-induced associative ionization introduces only a small correction to the final extent of ionization of the resonant plasma and scarcely alters its temperature. The influence of quantum stimulated inverse bremsstrahlung processes, which are usually described as collisions of the second kind in the resonance case, on the energy gain by electrons is analyzed for the first time in reference to specific experimental findings. The numerical calculations show that at detunings of the order of the Rabi frequency, the mechanism by which electrons gain energy through the resonant system does not reduce to collisions of the second kind and can significantly increase the density of the resonant plasma. However, in this range of detunings the laser beam is still strongly perturbed by instability processes, precluding a proper comparison of the theory with experiment. At large Δλ the classical and quantum cases differ from one another only slightly, and the values of N _e calculated for both mechanisms lie within the measurement error. Zh. éksp. Teor. Fiz. 111, 1274–1296 (April 1997)     

Anfachung und Dämpfung magneto-akustischer Wellen in MHD-Kanälen und ihr Einfluß auf die mittlere Stromdichte und die mittlere Hall-Feldstärke

Magneto-acoustic waves generated by fluctuations in the Hall parameter, the electric conductivity and the stream velocity are theoretically investigated in a weakly ionized plasma streaming across a strong external magnetic field and bearing a current flowing perpendicular to both magnetic field and stream velocity. The investigations hold for seeded rare gas plasmas at any degree of seed ionization but are resticted to waves propagating in parallel or antiparallel direction to the current density vector and in parallel or antiparallel direction to the stream velocity vector and to wave lengths which are small in comparsion to the interaction length which occurs as a characteristic wave length. The influence of these waves on the mean current density and the mean Hall field intensity is calculated in case of small amplitudes and low degree of seed ionization up to second order terms. Omitting Ohmic heating the dispersion equation can be solved exactly. A phase shift exists between the fluctuations in gas density and gas velocity. The phase velocity and the amplification rate depend on the wave length. Typical results are represented in a diagram. For both types of waves the phase velocity slightly rises with increasing wave length, while the amplification rate decreases. Waves propagating in opposite direction to the current density vector are amplified, if the electron velocity exceeds a critical value. They reduce the mean current density and the mean Hall field intensity. Waves propagating in opposite direction to the stream velocity vector are also amplified except for very high degrees of seed ionization. The threshold current density is greater than that for the waves of the first type approximately by the Hall parameter as factor. At extremely high degree of seed ionization the phase velocity is directed opposite to the direction occuring at weakly ionized seed. Waves of the second type decrease the mean current density, but increase the mean Hall field intensity.     

Observations of resonant modes formation in microwave generated magnetized plasmas

Ion sources have a significant number of applications in accelerator facilities and in industrial applications. In particular, the electron cyclotron resonance ion sources (ECRIS) are nowadays the most effective devices that can feed particle accelerators in a continuous and reliable way, providing high current beams of low and medium charge state ions and lower, but still remarkable, beam current for highly charged ions. In recent years several experiments have shown that the current, the charge states and even the beam shape change by slightly varying the microwave frequency (the so-called frequency tuning effect – FTE). The theoretical explanation of these results is based on the difference in the electromagnetic field pattern over the resonance surface, i.e. that region where the electrons resonantly interact with the incoming wave. In order to be consistent with the experiments, this model requires that standing waves are formed also in presence of a dense plasma. The proof was sought by means of a series of measurements performed with a network analyzer and with a plasma reactor operating at 2.45 GHz, according to the principles of the microwave discharge ion sources (MDIS). The measurements have been carried out with the aim to achieve the electromagnetic characterization of the plasma chamber in terms of possible excited resonant modes with and without plasma, and they reported that resonant modes are excited inside the cavity even in presence of a dense plasma. It was observed that the plasma dynamics strongly depends on the structure of the standing waves that are generated. The measurement of the eigen-frequencies' shifts were carried out for several values of pressure and RF power, thus linking the shift with the plasma density measured by a Langmuir probe. The changes in plasma shape, density and electron temperature have been also monitored for different operating conditions. A strong variation of plasma properties has been observed as a consequence of the introduction of the Langmuir probe inside the resonant cavity, thus demonstrating that the standing wave can be strongly perturbed even by means of relatively small metallic electrodes. The measurements reported hereinafter are relevant also for ECRIS, because they confirm the validity of the theoretical model that describes the frequency tuning.     

Development of a gas cell-based laser ion source for RIKEN PALIS

We developed a prototype laser ionization gas cell with a beam extraction system. This device is for use of PArasitic Laser Ion-Source (PALIS), which will be implemented into RIKEN’s fragment separator, BigRIPS as a part of SLOWRI. Off-line resonant laser ionization for stable Co, Cu, Fe, Ni, Ti, Nb, Sn, In and Pd inside the gas cell, ion extraction and transport to the high-vacuum region via SPIG and QMS have been confirmed (Sonoda et al, Nucl Instrum Meth B 295:1, 2013).     

吸收谱线Doppler展宽对原子多步光电离的影响

原子法激光同位素分离的核心之一是如何高效地将原子激发电离.本文从原子法激光同位素分离的实际情况出发,研究了原子吸收谱线的Doppler展宽对原子电离率的影响.研究中使用的理论工具是原子激发电离的密度矩阵方程,并利用数值计算方法对方程进行求解.研究结果表明:当吸收谱线有Doppler展宽时,在激光参数不变的条件下原子电离率会降低;且当激光功率固定时,存在使原子电离率为最大的线宽值.这与已发表文献中无Doppler展宽时的计算结果有很大不同.为了追求最佳的原子电离效果,在原子法激光同位素分离系统中激光应该尽可能地工作在最佳线宽条件下.如果激光线宽有不可控的随机波动,在技术上让激光线宽略大于最佳线宽更为有利.无论如何控制激光线宽,尽可能地降低原子吸收谱线的Doppler展宽都有利于原子电离率的提高.     

Yttrium ionization scheme development for Ti:Sa laser based RILIS

Resonant ionization laser ion sources (RILIS) are popular ion sources if intense, radioactive ion beams (RIBs) with minimal isobaric contamination are required. The intensity of the ion beam depends strongly on the applied resonant laser ionization scheme. Based on the all solid state laser system TRIUMF’s RILIS (TRILIS) is using, the off-line development towards an efficient ionization scheme for yttrium is presented. Several continuous wavelength scans have been performed to compare different nonresonant ionization schemes and to identify suitable Rydberg or autoionizing states for resonant ionization schemes.     

Ionization of atoms by the spatial gradient of the pondermotive potential in a focused laser beam

Ionization of atoms by the spatial gradient of the pondermotive potential in a focused laser beam is investigated. Rydberg ions, formed during the interaction of noble gas atoms with an intense laser pulse, are used to probe the gradient field. Rydberg ion species with higher ionization potentials are produced at locations where the gradient field is largest. The measured Rydberg ion yields differ dramatically from estimates that ignore gradient-field ionization, but are in good agreement with predictions that include the effect.     

Nonlinearly-coupled Langmuir and Ion-Acoustic Waves

In a plasma acted on by an intense laser field, one encounters the problem of nonlinearly-coupled Langmuir and ion-acoustic waves. We use a generalized perturbation theory to discuss the nature of coupled oscillatory waves in this system, and the possibility of formation of an ionsheath.     

Evolution of subnanosecond pulsed electric breakdown of gas gaps for uniform gas preionization

Initiation and evolution of breakdown of gas gaps by surge voltage pulses with a rise time of ≤1 ns are investigated experimentally and theoretically. The propagation of ionization waves for a uniform initial electron distribution in the gap is analyzed. The results of calculation are in qualitative agreement with the experiment. It is shown that the evolution of ionization waves leads to electric field redistribution in the discharge gap, and a region of an enhanced field with the strength sufficient for initiating emission processes and generation of a short fast electron beam in the cathode region is formed at the cathode for a very short time (up to 100 ps).     

激光脉冲在气体中产生的离化波前的演化及其对光脉冲传播的影响

利用带离化子程序的一维粒子模拟程序，对光脉冲与离化波前的相互作用进行了模拟研究，讨论了在原子多度电离的情况下光脉冲与离化波前的相互作用关系.研究表明，由于原子各级离化势的不连续性使得离化波前在空间上出现平台区；离化过程导致激光频率发生蓝移，这个蓝移又对脉冲形状产生调制，使脉冲前沿呈阶跃型增长；阶跃型的光脉冲包络使离化波前的平台区变短，直至整个离化波前分为几个不同梯度的区域.还讨论了不同元素和不同密度的气体中产生的离化波前的特点.同时分析了光脉冲频率随传播距离的关系，指出由于光脉冲宽度的增加，将导致出射的光 关键词： 激光等离子体 光电离 离化波前 激光频率上转换     

Electromagnetic interaction between local surface plasmon polaritons and an atmospheric surface wave plasma jet

We analyze the electromagnetic interaction between local surface plasmon polaritons （SPPs） and an atmospheric surface wave plasma jet （ASWPJ） in combination with our designed discharge device. Before discharge, the excitation of the SPPs and the spatial distribution of the enhanced electric field are analyzed. During discharge, the critical breakdown electric field of the gases at atmospheric gas pressure and the surface wave of the SPPs converted into electron plasma waves at resonant points are studied. After discharge, the ionization development process of the ASWPJ is simulated using a two- dimensional fluid model. Our results suggest that the local enhanced electric field of SPPs is merely the precondition of gas breakdown, and the key mechanism in maintaining the discharge development of a low-power ASWPJ is the wave-mode conversion of the local enhanced electric field at the resonant point.