Abnormal transition of the electron energy distribution with excitation of the second harmonic in low-pressure radio-frequency capacitively coupled plasmas

The self-excited second harmonic in radio-frequency capacitively coupled plasma was significantly enhanced by adjusting the external variable capacitor.At a lower pressure of 3 Pa,the excitation of the second harmonic caused an abnormal transition of the electron energy probability function,resulting in abrupt changes in the electron density and temperature.Such changes in the electron energy probability function as well as the electron density and temperature were not observed at the higher pressure of 16 Pa under similar harmonic changes.The phenomena are related to the influence of the second harmonic on stochastic heating,which is determined by both amplitude and the relative phase of the harmonics.The results suggest that the self-excited high-order harmonics must be considered in practical applications of lowpressure radio-frequency capacitively coupled plasmas.

Influence upon specific absorption rate distribution at deep area by inserting an inner electrode during capacitive radio-frequency hyperthermia

In recent years, hyperthermia holds much attraction in the clinic practices, especially in the tumor treatment. In hyperthermia, the key to get much better therapeutic effects ts how to heat the tumor efficiently, that means how to keep the tumor in high temperature range (41—45℃), and avoid any damage of the surrounding normal tissues. Therefore, the first requirement for hyperthermic system is to produce an ideal heating field of which the main part can overcover the region for therapy.

Modeling of the nanoparticle coagulation in pulsed radio-frequency capacitively coupled C_2H_2 discharges

The role of pulse parameters on nanoparticle property is investigated self-consistently based on a couple of fluid model and aerosol dynamics model in a capacitively coupled parallel-plate acetylene（C2H2） discharge. In this model, the mass continuity equation, momentum balance equation, and energy balance equation for neutral gas are taken into account.Thus, the thermophoretic force arises when a gas temperature gradient exists. The typical results of this model are positive and negative ion densities, electron impact collisions rates, nanoparticle density, and charge distributions. The simulation is performed for duty ratio 0.4/0.7/1.0, as well as pulse modulation frequency from 40 kHz to 2.7 MHz for pure C2H2 discharges at a pressure of 500 mTorr. We find that the pulse parameters, especially the duty ratio, have a great affect on the dissociative attachment coefficient and the negative density. More importantly, by decreasing the duty ratio, nanoparticles start to diffuse to the wall. Under the action of gas flow, nanoparticle density peak is created in front of the pulse electrode,where the gas temperature is smaller.

Effect of Addition of Nitrogen to a Capacitively Radio-Frequency Hydrogen Discharge

A hybrid PIC/MC model is developed in this work for H2-xN2 capacitively coupled radio-frequency （CCRF） discharges in which we take into account 43 kinds of collisions reaction processes between charged particles （e-, H3＋, H＋, H＋, N＋, N＋） and ground-state molecules （H2, H＋ N2）. In addition, the mean energies and densities of electrons and ions （ 3, H＋, H＋）, and electric field distributions in the H2-N2 CCRF discharge are simulated by this model. Furthermore, the effects of addition of a variable percentage of nitrogen （0-30%） into the H2 discharge on the plasma processes and discharge characteristics are studied. It is shown that by increasing the percentage of nitrogen added to the system, the RF sheath thickness will narrow, the sheath electric field will be enhanced, and the mean energy of hydrogen ions impacting the electrodes will be increased. Because the electron impact ionization and dissociative ionization rates increase when N2 is added to the system, the electron mean density will increase while the electron mean energy and hydrogen ion density near the electrodes will decrease. This work aims to provide a theoretical basis for experimental studies and technological developments with regard to H2-N2 CCRF plasmas.

Influence of Discharge Parameters on Tuned Substrate Self-Bias in an Radio-Frequency Inductively Coupled Plasma

The tuned substrate self-bias in an rf inductively coupled plasma source is controlled by means of varying the impedance of an external LC network inserted between the substrate and the ground. The influencing parameters such as the substrate axial position, different coupling coils and inserted resistance are experimentally studied. To get a better understanding of the experimental results, the axial distributions of the plasma density, electron temperature and plasma potential are measured with an rf compensated Langmuir probe; the coil rf peak-to-peak voltage is measured with a high voltage probe. As in the case of changing discharge power, it is found that continuity, instability and bi-stability of the tuned substrate bias can be obtained by means of changing the substrate axial position in the plasma source or the inserted resistance. Additionally, continuity can not transit directly into bi-stability, but evolves via instability. The inductance of the coupling coil has a substantial effect on the magnitude and the property of the tuned substrate bias.

两极板电容式射频热疗中比吸收率分布的模拟计算和体模实验研究

温热疗法治癌时，单位质量生物组织所吸收的加热功率，即比吸收率分布，是检验其热场优劣和能否获得较好疗效的主要判据。本文用有限单元方法对两极板电容式射频热疗中比吸收率二维分布作了在均匀肌肉组织和不同尺寸电极条件下的模拟计算分析，其结果与均匀等效肌肉体模实验基本相符。

电容射频热疗中引入内电极对深部热场分布影响的仿真研究

深部热场控制是电容射频热疗用于深部治癌时尚未能有效解决的难点。本文研究了引入内部电极（接地或接极板）对电容射频（１３．５６ＭＨｚ）热疗深部热场的比吸收率（ＳＡＲ）和温度分布影响。模拟计算表明：引入内电极将显著改变深部热场分布，使８０％以上的ＳＡＲ集中于该电极周围，并使内电极附近成为温度最高的热点；调整内电极位置和加电方式可有效地控制深区ＳＡＲ和温度分布形态。

双极板射频电容热疗的动物实验研究

目的：研究双极板射频电容热疗时升温与双极板距离、加热时间的关系及腹部不同部位的升温情况。材料和方法：使用双极板直径为１３ｃｍ的ＲＦ－２０００射频透热治疗仪，双极板距离分为三组（２８、３０、３２ｃｍ）。１８只犬取右侧卧位，上腹部置于双极板之间，平均分三组，经历不同极板距离３ｈ的热疗，使用热电偶测量１、２、３ｈ时肝、脾、肾及腹腔的温度。结果：在极板距离２８ｃｍ时，各个测量部位所达温度相似（ｐ＞０．０５）。１、２、３ｈ分别从基础温度３８．２～３９．０℃上升到４０．５～４１．３℃、４２．２～４３．０℃及４２．７～４３．８℃。极板距离为３０、３２ｃｍ时，１、２、３ｈ各部位温度与距离２８ｃｍ时相似（ｐ＞０．０５）。三种设定极板距离情况下，各腹部测量部位的温度均表现出随着时间的延长而升高。结论：经过２ｈ（及更长时间）的热疗，受热部位均可以达到较高的温度（ ≤ ４２℃）。极板距离在２８到３２ｃｍ之间变化时，腹部各测量部位所达温度相似。

三电容射频热疗在均匀组织体模温度分布研究

用有限单元数值计算方法对三极板调相式电容射频（１３．５６ＭＨｚ）热疗装置在均匀肌肉组织体模和不同功率分配条件下的温度分布进行模拟计算分析．结果表明，可通过改变三个极板电压的相位、幅值和功率分配方式来调整热场分布形态，有较好的治疗热区调整能力．

电容射频在生物介质中热场分布图象的研究

热疗中热场（比吸收率和温度）分布图象是检验其装置优劣和能否获得较好疗效的主要依据。本文首次采用准稳电场模型和有限单元数值计算方法，对电容射频（１３．５６ＭＨＺ）热疗装置在均匀肌肉组织中的比吸收率和温度分布进行模拟计算，得到了在不同极板尺寸及功率分配条件下的热场分布图象。计算结果表明，两极板电容射频热疗主要靠电极大小来调整热场分布，其有效治疗热区多靠近小尺寸电极一侧；三极板调相式电容射频热疗则主要通过改变三个极板电压的相位、幅值和功率分配方式来调整热场分布形态，有更好的治疗热区调整能力。该研究对电容射频热疗临床选用电极尺寸和功率分配及了解相应的热区形态有指导意义。

电容射频热疗中深部热场分布的仿真研究(英文)

深部热场控制是电容射频热疗用于深部治癌时尚未能有效解决的难点．本文研究了引入内部电极（接地或接极板）对电容射频（１３．５６ＭＨｚ）热疗深部热场的比吸收率（ＳＡＲ）和温度分布影响．模拟计算表明：引入内电极将显著改变深部热场分布，使８０％以上的ＳＡＲ集中于该电极周围，并使内电极附近成为温度最高的热点；调整内电极位置和加电方式可有效地控制深区ＳＡＲ和温度分布形态。

Mode transition induced by gas pressure in dusty acetylene microdischarges: two-dimensional simulation

Radio-frequency microdischarge in acetylene is investigated by use of a fluid model and an aerosol dynamics model in a cylindrical discharge chamber.In this article,the results at a pressure of 100–500 Torr,a voltage of 80–150 V,and an electrode gap of 400–1000μm are carefully analyzed and discussed.It is shown that two electron heating modesαandγappear in the microdischarge,and the pressure-dependent transition fromαtoγwas accompanied by the abrupt decrease of electron density and electron temperature.The mode transition phenomenon is further confirmed by the variation of the electron temperature axial profiles,the profiles vary continuously from a center high at the pressure of 100 Torr to an edge high at the pressure of500 Torr.Furthermore,in theαmode(100 Torr)the plasma density increases linearly with the increase of electrode gap,but decreases sharply with the increase of electrode gap in theγmode(>100 Torr).The gas pressure and applied voltage effects on the nanoparticle density and degree of nonuniformity are also investigated.It has been shown that the gas pressure greatly influences the axial profiles of nanoparticle density and the values of the degree of nonuniformity,while the values of the plasma parameters(electron density and nanoparticle density)strongly depend on the applied voltage.