实时激光等离子体干涉诊断方法

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"Real-time-interferomet_2018_Nuclear-Instruments-and-Methods-in-Physics-Resea" 这篇研究文章发表在2018年的《核仪器与物理研究A》(Nuclear Instruments and Methods in Physics Research A)期刊上，探讨了实时干涉法在铷等离子体诊断中的应用。该研究属于CERNAWAKE项目的一部分，主要关注激光等离子体的实验研究，特别是利用迈克尔逊型干涉仪进行的实时探测。关键词包括：CERNAWAKE项目、激光等离子体、干涉法和等离子体诊断。这些关键词揭示了研究的核心领域和方法。CERNAWAKE项目可能是一个涉及高速流体动力学或高能粒子物理的研究计划，而激光等离子体是这个领域的一个重要研究对象，因为激光可以瞬间创造出高温高密度的等离子体环境，这对于理解基本物理过程和潜在的应用（如粒子加速或能源转化）至关重要。文章介绍了一种新的实时干涉测量技术，用于检测由强聚焦激光脉冲（在飞秒时间尺度内）在不同初始铷蒸气密度条件下产生的等离子体。迈克尔逊干涉仪是一种经典的光学设备，能够精确地测量光程差，因此在这项研究中被用来分析等离子体的特性。通过分析得到的时间依赖性干涉信号，研究人员采用余弦拟合和指数衰减的相对相位来量化等离子体的密度-长度乘积以及其再结合时间常数。等离子体的密度-长度乘积是评估等离子体状态的关键参数，它反映了等离子体的总电子数。再结合时间常数则提供了关于等离子体中电荷载体消失速度的信息，这是理解等离子体动态行为的重要指标。这项工作对等离子体物理的研究具有重要意义，它不仅提供了新的诊断工具，还可能推动对激光诱导等离子体特性的更深入理解。同时，这种方法的实时性意味着它能够捕捉到等离子体演化的快速变化，这对于未来在实验物理学、等离子体物理以及相关工程应用中设计更精确的控制策略具有潜在价值。

Nuclear Inst. and Methods in Physics Research, A 884 (2018) 25–30

Contents lists available at ScienceDirect

Nuclear Inst. and Methods in Physics Research, A

journal homepage: www.elsevier.com/locate/nima

Real-time interferometric diagnostics of rubidium plasma

G.P. Djotyan, J.S. Bakos, M.Á. Kedves *, B. Ráczkevi, D. Dzsotjan, K. Varga-Umbrich, Zs. Sörlei,

J. Szigeti, P. Ignácz, P. Lévai, A. Czitrovszky, A. Nagy, P. Dombi, P. Rácz

Wigner Research Centre for Physics of the Hungarian Academy of Sciences, 29-33 Konkoly-Thege Street, H-1121 Budapest, Hungary

a r t i c l e i n f o

Keywords:

CERN AWAKE project

Laser plasma

Interferometry

Plasma diagnostics

a b s t r a c t

A method of interferometric real-time diagnostics is developed and applied to rubidium plasma created by

strong laser pulses in the femtosecond duration range at different initial rubidium vapor densities using a

Michelson-type interferometer. A cosine fit with an exponentially decaying relative phase is applied to the

obtained time-dependent interferometry signals to measure the density–length product of the created plasma

and its recombination time constant. The presented technique may be applicable for real-time measurements of

rubidium plasma dynamics in the AWAKE experiment at CERN, as well as for real-time diagnostics of plasmas

created in different gaseous media and on surfaces of solid targets.

1. Introduction

Plasma generation in gases and on surfaces of solid targets by

intense ultrashort laser pulses has found an extremely broad spectrum

of applications in different fields of science and technology. These

include high order harmonics generation widely used for ultrashort laser

pulse generation, (see review paper [1] and references therein), plasma

wake-field based particle acceleration in novel compact accelerators for

charged particles, [2–6] and many others.

In this communication, we report results of our study of plasma

generation in rubidium (Rb) vapors by intense laser pulses of femtosec-

ond duration using the interferometry method. This study is directly

related to the Advanced Wake-Field Experiment (AWAKE) project at

CERN, [7,8], which will be the first ever in the world proton-driven

plasma wake-field experiment. The aim of this project is the construction

of a relatively compact (and cheaper) accelerator of electrons (positrons)

to TeV energies in a single acceleration stage utilizing the proton bunch

available at the Super Proton Synchrotron (SPS) at CERN. The length

of the SPS proton bunch is in the range of 10–30 cm. The estimations

and simulations have shown that the maximum wake-field acceleration

in plasma is achieved when the length of the proton bunch is equal to

the length of the plasma wave, which is in the range of 100 μm [7,9].

Since short (∼ 100 μm) proton bunches are not available at SPS up to

now, transverse modulation of the current proton bunch was proposed

by means of the self-modulation-instability in plasmas. Thus, it would

be possible to obtain micro-bunches with length equal to the plasma

wavelength, [10].

Corresponding author.

E-mail address: kedves.miklos@wigner.mta.hu (M.

Kedves).

As earlier estimations have shown, strict conditions on the plasma

density uniformity must be satisfied for an efficient self-modulation

instability effect [11], but according to recent numerical simulations,

some longitudinal plasma density gradient could be useful [12]. In any

case, the plasma density and its spatial distribution is a critical issue

for the successful transverse modulation of the proton bunch and the

efficient electron acceleration in the plasma wake field driven by the

produced proton micro-bunches. To assure the desired plasma density

it is necessary to ionize the single outer electron in all of the atoms

in the Rb vapor along the pass of the proton bunch. In this case, the

plasma density will be the same as the neutral Rb vapor initial density,

which can be controlled by the temperature of the Rb vapor. The strict

requirement on the plasma density uniformity can be fulfilled by confin-

ing Rb vapor in a steel tube with extremely homogeneous distribution

of the vapor temperature provided by external oil heating [11]. The

relatively low ionization potential of Rb, (4.18 eV) [13,14], makes it

relatively easy to ionize the single outer electron of Rb atoms. Thus,

one could fully ionize Rb vapor in the extended volume of the tube

by sufficiently intense (energetic) ultra-short laser pulses. It is worth

noting that the laser plasma source is an essential part of the setup of

the AWAKE experiment.

Diagnostics of the plasma generated by the ionizing laser pulses

in such a system is crucial for having an insight into the ionization

processes of the Rb atoms, and for detecting the density of the created

plasma as well as its time–space evolution. In Ref. [15], a method for

accurate measurement of the density of neutral Rb atoms was developed

https://doi.org/10.1016/j.nima.2017.12.004

Received 27 July 2017; Received in revised form 30 November 2017; Accepted 4 December 2017

Available online 7 December 2017

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