PIC mode simulation of solenoid collimation channel

A new SSC (Separated Sector Cyclotron)-Linac is being designed to serve as an injector for the SSC at the HIRFL (Heavy Ion Research Facility Lanzhou). The beam intensity at the LEBT (Low Energy Beam Transport) for the heavy ions after the selection is typically low and the space charge effects are inconspicuous. The space charge effects become obvious when the beam current increases to a few hundred microamperes. The emittance growth deriving from the space charge effects may be particularly troublesome for the following linac and cyclotron. An optical system containing three solenoids has been designed for the LEBT to limit the beam emittance and to avoid the unnecessary beam loss in the cyclotron, as well as for the purpose of immunizing the LEBT emittance growth due to the space charge effects. The results of the PIG (Particle-In-Cell) mode simulation illustrate that this channel could limit the beam emittance growth and increase the beam brightness.

Drift distance survey in direct plasma injection scheme for high current beam production

In a laser ion source, plasma drift distance is one of the most important design parameters. Ion current density and beam pulse width are defined by plasma drift distance between a laser target and beam extraction position. In direct plasma injection scheme, which uses a laser ion source and a radio frequency quadrupole linac, we can apply relatively higher electric field at beam extraction due to the unique shape of a positively biased electrode. However, when we aim at very high current acceleration such as several tens of milliamperes, we observed mismatched beam extraction conditions. We tested three different ion current at ion extraction region by changing plasma drift distance to study better extraction condition. In this experiment, C6+ beam was accelerated. We confirmed that matching condition can be improved by controlling plasma drift distance.

The Compact Pulsed Hadron Source: A Design Perspective

During the past. decades, large-scale national neutron sources have been developed in Asia, Europe, and North America. Complementing such efforts, compact hadron beam complexes and neutron sources intended to serve primarily universities and industrial institutes have been proposed, and some have recently been established. Responding to the demand in China for pulsed neutron/proton-beam platforms that are dedicated to fundamental and applied research for users in multiple disciplines from materials characterization to hadron therapy and radiography to accelerator-driven sub-critical reactor systems (ADS) for nuclear waste transmutation, we have initiated the construction of a compact, yet expandable, accelerator complex-the Compact Pulsed Hadron Source (CPHS). It consists of an accelerator front-end (a high-intensity ion source, a 3-MeV radio-frequency quadrupole linac (RFQ), and a 13-MeV drift-tube linac (DTL)), a neutron target station (a beryllium target with solid methane and room-temperature water moderators/reflector), and experimental stations for neutron imaging/radiography, small-angle scattering, and proton irradiation. In the future, the CPHS may also serve as an injector to a ring for proton therapy and radiography or as the front end to an ADS test facility. In this paper, we describe the design of the CPHS technical systems and its intended operation.

ECR离子源发射度探测器的研制

成功研制一套适合于ECR离子源引出束流发射度测量的探测器ESS（Electric-SweeP Scanner）。该探测器主要由测量探头、机械驱动装置、计算机控制及数据获取系统组成。ESS探测器所采用的测量方法是双狭缝加电扫描法，具有速度快、精度高，能够比较直观的反映束流的发射度相图、相空间密度分布等特点，是目前ECR离子源引出束流发射度测量的有效手段。本论文对发射度探测器ESS的原理、结构及物理和技术设计作了较详细的描述，并给出了相应的设计图纸及数值模拟结果。最后，利用Ess探测器对LECR3（Lanzhou No.3）离子源引出的04＋离子束的发射度作了初步测量。当04＋引出束流为343μA日寸，初步测量所得水平方向的发射度为227mm·mrad（约90％束流），并给出了相应的束流发射度相图和束流相空间密度分布。

高电荷态ECR离子源的设计与实验研究

本文详细论述了三种各具特色的高电荷态ECR离子源的设计与研制，它们分别是：14．SGHz全永磁高电荷态ECR离子源LAPEC咫、传统常温ECR离子源GTS、混和超导ECR离子源HECRIS。另外，还从理论与实验两个方面对ECR离子源多电荷态离子束流的引出进行了详细研究。设计研制了一台高性能14．SGHz全永磁ECR离子源LAPECRZ，该离子源的设计尺寸为杯50mm*560mm，重量可达9ookg以上，为目前世界上体积与重量最大、设计参数最高的全永磁ECR离子源。全永磁离子源的尺寸增大一些，其加工与研制所存在的难度将会有质的变化。研制LAPEC形所存在的技术难度大大超过了以往所有全永磁ECR离子源。LAPECR2的磁场参数为：注入磁场1 ．4T，如果加注入端辅助铁扼，磁场可达2．6T；引出磁场为1 ．1T；六极永磁体在弧腔内壁产生的最大径向磁场的设计值为1．22T，实际加工得到的磁场大小为1．2T，与计算结果基本一致。该离子源的设计采用了多种特殊技术与方法以提高引出高电荷态离子束流强度，如：双层含铝衬底的等离子体弧腔、铝制等离子体电极、冷阴极电子枪（注入端负偏压电极）、旁轴微波直接馈入与等离子体位置相对于磁场可调等等。目前，我们已开始源体的组装。GTS离子源为目前世界上运行性能最好的常温ECR离子源之一。它的设计指导思想是ECR离子源的经验结论scaling Laws，高磁场、高磁镜比，高磁场梯度是该离子源磁场设计的主要特点。设计时充分考虑了多频加热机制，允许多频功率同时馈入弧腔：IOGHz＋14GHz＋18GHz＋ZSGHz。良好的磁场约束、灵活的磁场结构设计、高频率大功率微波馈入、大体积弧腔、足够的冷二次电子注入、优良的真空抽气系统、以及有效的束流引出系统等等都是该离子源高性能表观的原因。经过近一年半的优化与调试，该离子源运行性能优良，能产生很多领先国际伺类ECR源的离子束流，如：1.95emAO6+，O·semAArll”，o．15e拜AArl犷，31Oe瘩建叙e，哪se拼A豁岁”与0.4eμAXe39+为18GHz时调试的结果：1.OemAAr8+，1.8eμAAr17+24OeμAXe20+ ，20eμAxe30+与0.3oe从Axe38＋为在14GHz调试的最好结果。该离子源已多次为原子物理实验提供了Ar17+与Ar18+等高电荷态离子束流。HECRIS是一台高性能混和超导ECR离子源，其各项设计参数均高于GTS源，以保证该离子源能具有比GTS更好的性能。它的最小B场型设计继续沿袭现在ECR离子源设计惯用的Scaling Laws设计思想。该离子源采用较高频率的微波馈入，设计运行范围为18GHz-24GHz。该离子源着重于在运行调试中实现对轴向最小磁场Bmin的调节与匹配，这里采用两组超导线包进行辅助调节。径向磁约束由一个能在弧腔内壁产生1．6T径向磁场的六极永磁体提供，它强磁场设计与部分磁块存在的退磁问题的解决也是该离子源的一大特点与难点。文中还详细讨论一个高性能的Halbach结构六极永磁体的设计方法。分别从永磁材料选取、物理尺寸优化、结构选择等角度，结合理论分析与PolssON，PERMAG和TOscA3D程序的模拟计算结果，对六极永磁体的性能进行了全面的分析，提出两种确定六极永磁体中存在退磁磁块的办法，并且提出了一种用高极化矫顽力永磁材料替代退磁磁块中所采用的永磁材料的解决办法。对多电荷态ECR离子源的引出进行了理论与实验两方面的研究。利用PBGUNS程序，对ECR离子源的引出系统进行了一定的理论分析。通过模拟计算，详细地论述了引出系统的各主要物理参数对ECR离子源引出离子束流品质的影响。这些得到的结果，对设计一套高性能的ECR离子源引出系统具有一定的指导意义。结合理论分析，我们还设计完成了一系列关于ECR离子源引出系统的实验，来研究多电荷态离子束流的引出与短距离传输。采用电扫描发射度测量系统（ESS）探测离子束流在x与y方向的发射度，用两个荧光靶分别在束运线不同位置探测束流的束斑形状，并利用一个亮度探测器对束斑相对亮度进行探测。通过多组实验结果的处理，分别分析了离子源的几个主要参数如磁场、微波、掺气、偏压等对引出束流品质的影响并结合相关理论给予了解释，通过对混合多电荷态离子束流的三电极引出实验结果的讨论，建立了一个关于高电荷态ECR离子源束流引出的较为清晰的物理图象。

Electrical control of dynamic spin splitting induced by exchange interaction as revealed by time-resolved Kerr rotation in a degenerate spin-polarized electron gas

Manipulation of the spin degree of freedom has been demonstrated in a spin-polarized electron plasma in a heterostructure by using exchange-interaction-induced dynamic spin splitting rather than the Rashba and Dresselhaus types, as revealed by time-resolved Kerr rotation. The measured spin splitting increases from 0.256 meV to 0.559 meV as the bias varies from -0.3 V to -0.6 V. Both the sign switch of the Kerr signal and the phase reversal of Larmor precessions have been observed with biases, which all fit into the framework of exchange-interaction-induced spin splitting. The electrical control of it may provide a new effective scheme for manipulating spin-selected transport in spin FET-like devices. Copyright (C) EPLA, 2008.

Electron spin quantum beats and room temperature g factor in GaAsN

We report on the investigation of electron spin quantum beats at room temperature in GaAsN thin films by time-resolved Kerr rotation technique. The measurement of the quantum beats, which originate from the Larmor precession of electron spins in external transverse magnetic field, yields an accurate determination of the conduction electron g factor. We show that the g factor of GaAs1-xNx thin films is significantly changed by the introduction of a small nitrogen fraction.

Spin relaxation and dephasing mechanism in (Ga,Mn)As studied by time-resolved Kerr rotation

Spin dynamics in (Ga,Mn)As films grown on GaAs(001) was investigated by Time-resolved magneto-optical Kerr effect. The Kerr signal decay time of (Ga,Mn)As without external magnetic field applied was found to be several hundreds picoseconds, which suggested that photogenerated polarized holes and magnetic ions are coupled as a ferromagnetic system. Nonmonotonic temperature dependence of relaxation and dephasing (R&D) time and Larmor frequency manifests that Bir-Aronov-Pikus mechanism dominates the spin R&D time at low temperature, while D'yakonov-Perel mechanism dominates the spin R&D time at high temperature, and the crossover between the two regimes is Curie temperature.

Photoinduced spin alignment of the magnetic ions in (Ga,Mn)As

Time-resolved Kerr rotation measurement in the (Ga,Mn)As diluted magnetic semiconductor allows direct observation of the dynamical properties of the spin system of the magnetic ions and the spin-polarized holes. Experimental results show that the magnetic ions can be aligned by the polarized holes, and the time scales of spin alignment and relaxation take place in tens and hundreds of picoseconds, respectively. The Larmor frequency and effective g factor obtained in the Voigt geometry show an unusual temperature dependence in the vicinity of the Curie temperature due to the exchange coupling between the photoexcited holes and magnetic ions. Such a spin coherent precession can be amplified or destructed by two sequential excitation pulses with circularly copolarized or oppositely polarized helicity, respectively. (c) 2006 American Institute of Physics.

COMPARISONS OF PHASE TIMES WITH TUNNELING TIMES BASED ON ABSORPTION PROBABILITIES

The times spent by an electron in a scattering event or tunnelling through a potential barrier are investigated using a method based on the absorption probabilities. The reflection and transmission times derived from this method are equal to the local Larmor times if the transmission and reflection probability amplitudes are complex analytic functions of the complex potential. The numerical results show that they coincide with the phase times except as the incident electron energy approaches zero or when the transmission probability is too small. If the imaginary potential covers the whole space the tunnelling times are again equal to the phase times. The results show that the tunnelling times based on absorption probabilities are the best of the various candidates.

Beam dynamics design of an RFQ for a planned accelerator, which uses a direct plasma injection scheme

To meet the requirements of providing high-intensity heavy ion beams the direct plasma injection scheme (DPIS) was proposed by a RIKEN-CNS-TIT collaboration. In this scheme a radio frequency quadrupole (RFQ) was joined directly with the laser ion source (LIS) without a low-energy beam transport (LEBT) line. To find the best design of the RFQ that will have short length, high transmission efficiency and small emittance growth, beam dynamics designs with equipartitioning design strategy and with matched-only design strategy have been performed, and a comparison of their results has also been done. Impacts of the input beam parameters on transmission efficiency are presented, too. (C) 2008 Elsevier B.V. All rights reserved.

Commissioning test of LAPECR2 source on the 320kV HV platform

The high charge state all permanent Electron Cyclotron Resonance Ion Source (ECRIS) LAPECR2 (Lanzhou All Permanent magnet ECR ion source No.2) has been successfully put on the 320kV HV platform at IMP and also has been connected with the successive LEBT system. This source is the largest and heaviest all permanent magnet ECRIS in the world. The maximum mirror field is 1.28T (without iron plug) and the effective plasma chamber volume is as large as circle divide 67mm x 255mm. It was designed to be operated at 14.5GHz and aimed to produce medium charge state and high charge state gaseous and also metallic ion beams. The source has already successfully delivered some intense gaseous ion beams to successive experimental terminals. This paper will give a brief overview of the basic features of this permanent magnet ECRIS. Then commissioning results of this source on the platform, the design of the extraction system together with the successive LEBT system will be presented.