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"In spite of abundant experimental evidences supporting the viability of the laser induced shock wave plasma model for the explanation of the important features ofthe plasma and the associated spectroscopic characteristics, a controversy on the atomic excitation mechanism in the plasma has remained to be completely resolved. In this study the contributions of the shock wave model and two other most popular models, the electron-ion recombination model and thc electron collision model were thoroughly investigated. For that purpose, a special technique has been developed for the direct detection of the charge current in conjunction with plasma emission measurement dining the laser plasma generation and expansion. The current detection was performed by placing a partially transmitting metal mesh electrode at a distance in front of the sample surface with the sample target sewing as the counter electrode. The electric Held between the mesh and sample surface was set up and varied by applying a variable DC voltage (0-400 Volt) between them. The laser plasma was generated by a YAG laser (64 ml, 8 ns) tightly focused on a Cu target through the mesh electrode in low-pressure surrounding gas. It was found that the charge current time profiles obtained at various gas pressures invariably exhibit a lack of consistent correlation with the emission time profile of the plasma throughout most of the emission period. The result of this study has thus practically eliminated any significant roles ofthe electron-ion recombination and electron collision models in the excitation process. We are therefore led to conclude that the shock wave model proposed earlier is most plausible for the consistent explanation of the secondary plasma emission, while the other two models may have some contribution only at the very initial stage ofthe secondary plasma generation."

"ABSTRACTIn spite of abundant experimental evidences supporting the viability of the laserinduced shock wave plasma model for the explanation of the important features oftheplasma and the associated spectroscopic characteristics, a controversy on the atomicexcitation mechanism in the plasma has remained to be completely resolved. In thisstudy the contributions of the shock wave model and two other most popular models,the electron-ion recombination model and thc electron collision model werethoroughly investigated. For that purpose, a special technique has been developed forthe direct detection of the charge current in conjunction with plasma emissionmeasurement dining the laser plasma generation and expansion. The current detectionwas performed by placing a partially transmitting metal mesh electrode at a distancein front of the sample surface with the sample target sewing as the counter electrode.The electric Held between the mesh and sample surface was set up and varied byapplying a variable DC voltage (0-400 Volt) between them. The laser plasma wasgenerated by a YAG laser (64 ml, 8 ns) tightly focused on a Cu target through themesh electrode in low-pressure surrounding gas. It was found that the charge currenttime profiles obtained at various gas pressures invariably exhibit a lack of consistentcorrelation with the emission time profile of the plasma throughout most of theemission period. The result of this study has thus practically eliminated anysignificant roles ofthe electron-ion recombination and electron collision models in theexcitation process. We are therefore led to conclude that the shock wave modelproposed earlier is most plausible for the consistent explanation of the secondaryplasma emission, while the other two models may have some contribution only at thevery initial stage ofthe secondary plasma generation.Key words: charge current, shock wave, electron-ion recombination and electroncollision.Praiseci is to the Lord for He is my reason in everything I do.This manuscript is never be done without the guidance by Pro£ Tjia May On, towhom I am extremely grateful. He also provided the support without which this thesiswould not possible. He is more than just a teacher for me for his words have deeplytouched me. Moreover, he also introduced me that knowledge is something we shouldshare among others and to improve the education in my country.I am also indebted to Prof. Kiichiro Kagawa at the Fukui University for providingthe atmosphere and the physical resources to make thesis writing in these times of fastpaced research. I am also thankful for the opportunity which is given to me to joinresearch together with him in his laboratory in Japan.Extra special thanks go to Dr. Hendrik Kurniawan for providing me withencouragement and support for this project. He is the first one who encouraged me totake Doctor Cotuse Program which seemed impossible at the beginning. Hiscompanion during research at Applied Spectroscopy Laboratory at University ofIndonesia is a leading experience in research for me.I am particularly grateful to the excellent team of referees who provided criticalcomments on this thesis. Their feedback was a great benefit to me.I gratefully acknowledge all my colleagues: Rinda Hedwig, Mangasi A.Marpaung, Hery Suyanto, MM. Suliyanti, Wahyu S. Budi, and Emon in AppliedSpectroscopy Laboratory at University of Indonesia, for their assistance and supportduring my study.My never-ending thanks to my beloved family, especially to my parents whoexhibited thoughtful patience over extended periods of time when I seemed to beinvisible. Thanks also to Loviana who helped me in all situations which I no longercan resist by myselflFinally, I apologize to all those who helped that I did not acknowledge specifically.I know there were many and greatly appreciate your assistance.August, 2002Author"

"Instabilities of fluid flows and the associated transitions between different possible flow states provide a fascinating set of problems that have attracted researchers for over a hundred years. This book addresses state-of-the-art developments in numerical techniques for computational modelling of fluid instabilities and related bifurcation structures, as well as providing comprehensive reviews of recently solved challenging problems in the field. "

"Aluminium (Al) adalah logam yang memiliki ketahanan korosi serta sifat mekanik yang baik. Terdapat beberapa paduan aluminium, salah satunya paduan aluminium seri 7075-T651 (AA7075-T651), paduan ini memiliki ketahanan korosi yang lebih baik dikarenakan memiliki paduan utama seng (Zn) didalamnya. Untuk mengoptimalkan sifat unik yang dimiliki AA7075-T651 dilakukan rekayasa material dengan metode Plasma Electrolytic Oxidation (PEO). Dalam proses pengoptimalannya, variasi rapat arus dilakukan untuk melihat arus yang paling optimal digunakan dalam PEO untuk AA7075-T651. PEO dilakukan menggunakan elektrolit 30 g/l + 30 g/l KOH + 30 dengan waktu selama 15 menit. Rapat arus digunakan dengan variasi 200 A/m2, 300 A/m2, dan 400 A/m2. Pengamatan permukaan plasma yang terbentuk dilakukan karakterisasi menggunakan Scanning Electron Microscopy-Energy Dispersive x-ray Spectroscopy (SEM-EDS) dan Optical Microscope (OM). Selanjutnya, lapisan hasil PEO dikarakterisasi menggunakan X-Ray Diffractometer (XRD) untuk menganalisis komposisi fasa kristal. Uji elektrokimia dilakukan untuk mengetahui ketahanan korosi paduan setelah perlakuan PEO. Uji kekerasan dilakukan menggunakan alat Vickers microhardness machine dan untuk mengetahui ketahanan aus dilakukan uji aus menggunakan alat Ogoshi. Unsur P, Si, dan O adalah lapisan pelindung terhadap korosi. Hasil XRD menunjukan adanya lapisan Al2O3 dan AlPO4. Hasil PDP dan EIS menunjukan PEO300 yang memiliki rapat arus korosi sebesar 4,18 × 10-8 A.cm−2 . Untuk ketahanan optimum dalam ketahanan aus dimiliki oleh PEO300 yang memiliki ketahanan aus terbesar, yaitu sebesar 2,9 mm3/mm, sedangkan nilai kekerasan tertinggi dimiliki oleh PEO200 sebesar 156 HV. Ketebalan yang paling tebal dimiliki oleh PEO400 yang memiliki ketebalan 18,11 ± 1,13 um.

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Aluminum (Al) is a metal known for its corrosion resistance and good mechanical properties. There are several aluminum alloys, one of which is the 7075-T651 series aluminum alloy (AA7075-T651). This alloy has better corrosion resistance due to its main alloying element, zinc (Zn). To optimize the unique properties of AA7075-T651, material engineering is conducted using the Plasma Electrolytic Oxidation (PEO) method. In the optimization process, variations in current density are applied to determine the most optimal current for PEO of AA7075-T651. PEO is carried out using an electrolyte of 30 g/l + 30 g/l KOH + 30 for 15 minutes. The current densities used are varied at 200 A/m2, 300 A/m2, dan 400 A/m2. Surface observations of the formed plasma are characterized using Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (SEM-EDS) and Optical Microscope (OM). Subsequently, the samples are characterized using an X-Ray Diffractometer (XRD) to analyze the crystal phase composition. The PEO layer undergoes electrochemical testing to determine the corrosion resistance of the engineered layer. Hardness testing is conducted using a Vickers microhardness machine, and wear resistance is assessed using an Ogoshi wear testing machine. The elements P, Si, and O form a protective layer against corrosion. The XRD results show the presence of Al2O3 and AlPO4 layers. The PDP and EIS results indicate that PEO300 has a corrosion current density of 4,18 × 10-8 A.cm−2. The optimal wear resistance is exhibited by PEO300, with the highest wear resistance of 2,9 mm^3/mm, while the highest hardness value is found in PEO200 at 156 HV. The thickest layer is observed in PEO400, with a thickness of 18,11 ± 1,13 um."