0．53μm激光产生的超热电子的实验观测

THE EXPERIMENTAL OBSERVATION OF HOT ELECTRONS PRODUCED BY LASER OF 0.53μm WAVELENGTH

介绍了在“神光”Ｉ号装置上利用波长０．５３ｐμｍ、脉宽τ约７５０ｐｓ、能量６０～２３０Ｊ激光（靶面激光强度１×１０￣（１３）～５×１０￣（１５）Ｗ／ｃｍ￣２）照射Ａｕ盘靶和Ａｕ拄黑腔靶产生超热电子的实验观测结果与分析。实验测量１０ｋｅＶ以上硬Ｘ光谱和通量表明：采用倍频激光可以使超热电子能量明显比基频光小一个量级左右，超热电子温度Ｔ＿ｈ、热电子温度Ｔ＿ｅ均降低一半左右，受激Ｒａｍａｎ散射光能量Ｅ＿（ＳＲＳ）减少二个多量级。在我们的实验条件下，Ａｕ盘靶（等离子体定标尺度Ｌ≤１００μｍ）产生超热电子的主要机制可能是双等离子体衰变和共振吸收，此外还有受激Ｒａｍａｎ散射（ｎ≈ｎ＿ｃ／４），１００μｍ＜Ｌ≤２４０μｍ超热电子产生的主要机制是ＴＰＤ，此外还有ＳＲＳ（ｎ≈ｎ＿ｃ／４）；黑腔靶（Ｌ≥３００μｍ）超热电子产生的主要机制是ＳＲＳ（ｎ＜ｎ＿ｃ／４）。

AbstractShorter wavelengths laser should improve inertial-confinement fusion performanee byreducing the number of hot electrons and thus reducing target preheat.Many experiments haveconfirmed that hot electrons can be reduced to an aceeptable level by shortening laser wavelengthto 0.35μm￣[1,2,3].Since we use the laser beams from Nd:glass laser facility at the wavelength0. 53μm,it is neeessary to study the processes and property of hot electron production and seekthe methods to suppress them to a harmless level.The experiments were conducted at the“Shen Guang-I#”Nd:glass laser facility(λ=0.53μm,τ=850ps,focal spot Φ_0≈80-240μm,E_L=50～230J,I=5× 10￣(13)～5 × 10￣(15)W/cm￣2)Laser light is focused on target through lens of f/1.7. Targets used is experiments were gold-disk target with a thickness of 15μm and Auhohlraum target. An absolutely-calibrated X-ray spectrometer has been equipped with a ten-channel fil-ter-fluorescer system and a five-channel K-edge filter system. The bandpass of the spectrometerchannels can be adjusted to fit the experimental requirements.In experiments three channels wereused to measure the thermal X-ray(1.5～10keV),five channels to measure the hot component(10～88keV).The K-edge channels use K-edge filters along a line of sight to obtain highersensitivity.Borth types use scintillators and photomultiplier tubes as detectors. In this report,we focus on the hot eletrons behavior of Au disk targets and hohlraum tar-gets irradiated by 0.53μm laser light.T_H and T_e,the temperature of hot electrons and thermalelectrons are commonly deduced from the slope of hard X-ray spectrum. We used an iterative un-folding procedure that did not constrain the shape of the spectrum to evaluate the X-ray spectra.The measured fluxes and spectra above several-keV X-ray indicate hot electrons of Au disk tar-get and hohlraum target fractions of～1/1000 and～1/100, respectively,with T_H of 8.5 to15keV and 11 to 22keV, respectively,and with T_e of 1.1 to 1.5keV. T_H of Au disk is the greatsinularity to the results obtained by The experimental results also show that hot electron amount produced by laser of 0.53μmwavelength is about one order of magnitude less than that of 1.053μm;both of hot electron tem-perature T_h and thermal electron temperature T_e reduce about one half,SRS energy decreases abouttwo orders of magnitude. It has also been observed that total energy of hot electrons has a linearrelation with total yield of SRS light in the hohlraum targets,an total energies of hot electronsand SRS light are about one order of magnitude,two order of magnitude greater than Audisk-target,respectively, so we can conclude that SRS is the primary mechnism for producinghot electrons in hohlraum(plasma scale length L ≥ 300μm )targets.Under our experimental condi-tions,first probably two-plasma decay(TPD )and resonance absorption are theprimary mechanism for producing hot electrons in Au disk targets(L≤100μm)and next SRS(n≈n_c/4).First probable TPD is the primary mechanism for producing hot electrons in Au disktargets(100μm<L ≤240μm)and next SRS(n≈n_c/4).