**7. Acknowledgements**

The authors of this chapter would like to acknowledge Prof. Chengwei Sun and Dr. Fuli Tan, Ms. Jia He, Mr. Jianjun Mo and Mr. Gang Wu for the good work and assistance in our simulation and expeimental work. We would also like to express our thanks to the referee for providing invaluable and useful suggestions. Of cousre, the work is supported National Natural Science Foundation of China under Contract NO. 10927201 and NO.11002130, and the Science Foundation of CAEP under Contract NO. 2010A0201006 and NO. 2011A0101001.
