Ultrafast olivine-ringwoodite transformation during shock compression

Meteorites from interplanetary space often include high-pressure polymorphs of their constituent minerals, which provide records of past hypervelocity collisions. These collisions were expected to occur between kilometre-sized asteroids, generating transient high-pressure states lasting for several seconds to facilitate mineral transformations across the relevant phase boundaries. However, their mechanisms in such a short timescale were never experimentally evaluated and remained speculative. Here, we show a nanosecond transformation mechanism yielding ringwoodite, which is the most typical high-pressure mineral in meteorites. An olivine crystal was shock-compressed by a focused high-power laser pulse, and the transformation was time-resolved by femtosecond diffractometry using an X-ray free electron laser. Our results show the formation of ringwoodite through a faster, diffusionless process, suggesting that ringwoodite can form from collisions between much smaller bodies, such as metre to submetre-sized asteroids, at common relative velocities. Even nominally unshocked meteorites could therefore contain signatures of high-pressure states from past collisions.

On second-order accuracy schemes for modeling of plasma oscillations

Для моделирования колебаний холодной плазмы как в нерелятивистском случае, так и с учетом релятивизма предложены модификации классических разностных схем второго порядка точности: метода МакКормака и двухэтапного метода Лакса-Вендроффа. Ранее для подобных расчетов в эйлеровых переменных была известна только схема первого порядка точности. Для задачи о свободных плазменных колебаниях, инициированных коротким мощным лазерным импульсом, с целью тестирования представленных схем проведены численные эксперименты по сохранению энергии и других величин. Сделан вывод о достоверности численного анализа колебаний как на основе схемы МакКормака, так и на основе схемы Лакса-Вендроффа, однако для расчетов долгоживущих процессов первая схема более предпочтительна. Теоретическое исследование аппроксимации и устойчивости вместе с экспериментальным наблюдением за количественными характеристиками погрешности для наиболее чувствительных величин существенно повышает достоверность вычислений. Ключевые слова: численное моделирование, плазменные колебания, эффект опрокидывания, схемы МакКормака и Лакса-Вендроффа, порядок точности разностной схемы, законы сохранения. For modeling cold plasma oscillations in the non-relativistic and relativistic cases, some modifications of classical difference schemes of the second order of accuracy are proposed: the McCormack method and the two-stage Lax-Wendroff method. Previously, only the first-order accuracy scheme was known for calculations in Euler variables. For the problem of free plasma oscillations initiated by a short high-power laser pulse, the results of numerical experiments on energy conservation and other quantities were performed in order to test the proposed schemes. It is concluded that the numerical analysis of oscillations is reliable both for the McCormack scheme and for the Lax-Wendroff scheme however, for the calculation of long-lived processes, the first scheme is more preferable. The theoretical analysis of approximation and stability together with experimental observations of quantitative characteristics of errors for the most sensitive quantities significantly increases the reliability of calculations.