Plasma conditions for improved energy coupling into the gain region of the Ni-like Pd transient collisional x-ray laser

Abstract

We have directly probed the conditions in which the Ni-like Pd transient collisional x-ray laser is generated and propagates by measuring the near-field image and by utilizing picosecond resolution soft x-ray laser interferometry of the preformed Pd plasma gain medium. The electron density and gain region of the plasma have been determined experimentally and are found to be in good agreement with simulations. We observe a strong dependence of the laser pump-gain medium coupling on the laser pump parameters. The most efficient coupling occurs with the formation of lower density gradients in the preformed plasma and when the duration of the main heating pulse is comparable to the gain lifetime ͑ϳ10 ps for mid-Z Ni-like schemes͒. This increases the output intensity by more than an order of magnitude relative to the commonly utilized case where the same pumping energy is delivered within a shorter heating pulse duration ͑Ͻ3 ps͒. In contrast, the higher intensity heating pulses are observed to be absorbed at higher electron densities and in regions where steep density gradients limit the effective length of the gain medium. A detailed understanding of the plasma that constitutes the gain medium is crucial for the development of efficient x-ray lasers. Use of the prepulse technique has allowed x-ray lasers to achieve saturated output using many different elements for the lasing media ͓1͔. However, even the best laser-pumped x-ray lasers typically have an efficiency of 10 −6 . In the transient collisional excitation ͑TCE͒ scheme a low intensity long pulse preforms a plasma, which is allowed to expand and cool before being heated by a high intensity short pulse ͓2͔. This short pulse, in some cases with subpicosecond duration, rapidly heats the plasma to generate a high gain coefficient, saturated x-ray laser output ͓3͔, and x-ray laser pulses as short as 2 ps ͓4͔. In experiments reported on high power laser drivers the pulse duration of the short pulse generated by chirped pulse amplification ͑CPA͒ is in the range of 0.3-3 ps ͓3-7͔. It has been assumed to some extent that by maximizing the intensity of the main heating pulse the temperature, collisional pumping, and local gain coefficient will also be maximized. Under these conditions the lowest saturated wavelength currently demonstrated is 7.3 nm for Nilike Sm ͓5͔.To improve the efficiency we need to better understand the laser-plasma coupling and plasma characteristics of the x-ray laser media. In this paper we combine the techniques of near-field imaging with recently developed picosecond x-ray laser interferometry ͓8͔ to characterize the lasing medium for a Ni-like Pd x-ray laser. It is observed that a combination of controlling and reducing the plasma density gradients while matching the duration of the main pumping pulse to the gain lifetime at a specific density optimizes the coupling efficiency. This increases the x-ray laser output by an order of magnitude over the case where the same pumping energy is delivered into a higher intensity, shorter pulse. In contr...

Relativistic many-body calculations of multipole (E1, M1, E2, M2, E3, and M3) transition wavelengths and rates between 3l−14l′ excited and ground states in nickel-like ions

“…In Table III, we present results for the zeroth-, first-, and second-order Coulomb contributions, E (0) , E (1) , and E (2) , and the first-and second-order Breit-Coulomb corrections, B (1) hf and B (2) . It should be noted that corrections for the frequency-dependent Breit interaction [44] are included in the first order only.…”

“…In Table IV, we list the following contributions to the energies of 13 excited states in W 46+ : the sum of the zeroth and first-order energies E (0+1) = E (0) +E (1) +B (1) hf , the second-order Coulomb energy E (2) , the second-order Breit-Coulomb correction B (2) , the QED correction E LAMB , and the sum of the above contributions E tot . The QED correction is approximated as the sum of the one-electron self energy and the first-order vacuum-polarization energy.…”

“…The first-and second-order Coulomb corrections (Z (1) , Z (2) ), and second-order Breit-Coulomb corrections (B (2) ) for E1, E2, and E3 uncoupled matrix elements for transitions between excited and ground states calculated in length (L)and velocity (V ) forms in Ni-like ions.…”

“…To determine the first-order energies of the states under consideration, we diagonalize the symmetric first-order effective Hamiltonian, including both the Coulomb and Breit interactions. The first-order expansion coefficient C N [av(J)] (often a mixing coefficient)is the N -th eigenvector of the first-order effective Hamiltonian and E (1) [N ] is the corresponding eigenvalue. The resulting eigenvectors are used to determine the second-order Coulomb correction E (2) [N ], the second-order Breit-Coulomb correction B (2) [N ] and the QED correction E Lamb [N ].…”

“…The Ni-isoelectronic sequence has been studied extensively in connection with x-ray lasers [1,2,3,4,5,6,7,8,9,10,11]. Recently, an investigation into the use of atomic databases in simulation of Ni-like gadolinium x-ray laser was presented by King et al in Ref.…”