Excitation of ion-wave wakefield by the resonant absorption of a short pulsed microwave with plasma Md. Kamal-Al-Hassan, Mikhail Starodubtsev, Hiroaki Ito, Noboru Yugami, and Yasushi Nishida Department of Energy and Environmental Science, Graduate School of Engineering, Utsunomiya University, 7-1-2 Yoto, Utsunomiya, Tochigi 321-8585, Japan Received 17 February 2003; published 29 September 2003 Unmagnetized, inhomogeneous laboratory plasma irradiated by a high power (  =E 0 2 /4 n e kT e 5.0 10 -2 ) short pulsed microwave with pulse length of the order of ion-plasma period (  pi 2  /  pi ) is studied. Large density perturbation traveling through the underdense plasma with a velocity much greater than the ion sound speed produced by the resonant absorption of the microwave pulse has been observed. In the beginning the density perturbation has large amplitude (  n / n 0 40%) and propagates with a velocity of the order of 10 6 cm/s. But later its amplitude as well as the velocity decrease rapidly, and finally the velocity arrives with twice the ion sound speed. The oscillating incident electromagnetic waves enhance highly localized electric field by the resonant absorption process and develop time-averaged force field which pushes plasma electrons from the resonant layer. As the electrons are accelerated to be ejected, they pull plasma ions as a bunch with them by means of self-consistent Coulomb force. This suprathermal ion bunch can excite an ion-wave wake- field. DOI: 10.1103/PhysRevE.68.036404 PACS numbers: 52.35.Mw, 52.35.Fp, 52.40.Db, 52.90.+z I. INTRODUCTION An interesting topic in modern plasma physics is plasma based accelerators and plasma wakefield phenomena. Since the first proposal by Tajima and Dawson 1, many success- ful phenomena have been demonstrated in this area. Some new accelerator schemes 2–4 have been proposed and suc- cessfully demonstrated in proof-of-principles experiments. Plasma waves in the plasma wakefield accelerator PWFA are driven by one or more electron bunches. In the plasma wakefield accelerator concepts 5–8, plasma waves wake- field are excited by relativistic electron bunches, in which the electron beam should be terminated in a time shorter than the plasma wave period (  pe -1 ). When the beam density ap- proaches one-half the plasma density, the excited electron plasma wave approaches the nonrelativistic wave-breaking limit to give the electric field E → pe mc / e . Jones and Kein- igs 9 described the possibility to excite ion waves by using an ion bunch with proper falloff time. Using an ion with mass M may increase the limiting electric field for fixed  pe by the ratio of the ion mass to the electron mass M / m . It has been shown that the optimal falloff time (  fall ) of the driving beam depends on the plasma frequency as  pe = fall -1 . How- ever, in the case of ion movement, the ambipolar field limits the maximum field strength to be smaller than the case of electron bunch wakefield. Nishida et al. 10 have success- fully excited the wake field in the ion-wave regime with a variety of shapes of ion bunches injected into a plasma in the double plasma machine. The experimental results show that a ramp-shaped ion bunch with a sharp falloff time can excite a large amplitude wakefield with dn / n 0 17%. The ampli- tude of the wakefield decreases with an increase of falloff time constant of the incident ion bunch, along with the os- cillation of amplitude. Experiment by Aossey et al. 11 has demonstrated plasma wakefield excitation in both a positive ion-electron plasma and a positive ion-negative ion-electron plasma system. The main difficulty when exciting an ion wakefield is to produce sharp ion bunch shape with a proper falloff time. One of the possible ways to meet this requirement is to expel quickly the plasma electrons out from a small plasma region leaving behind a bare ion column. Resulting ambipolar elec- tric field can accelerate plasma ions producing a short ener- getic ion bunch. In the present experiment we have produced an ion bunch with sharp falloff time by the above method using the resonant absorption process to expel plasma elec- trons from the resonant plasma layer. Indeed, a nonuniform unmagnetized plasma has been irradiated by a short micro- wave pulse with its duration shorter than the ion oscillation period (  pi =2  /  pi ). The key to excite strong wakefield is that the microwave pulse width should be of the order of ion oscillation period (  pi =2  /  pi ). The physical aspects are straightforward. The ponderomotive force created at the resonant area pushes out the electrons from that area, leaving behind bare ions. These ions are accelerated as an energetic ion bunch by the ambipolar force. Typical ion energy in a bunch has been measured to be about 58 eV, which is more than 100 times greater than the thermal ion. This suprather- mal high energy ion bunch acts as the driving bunch to excite ion wakefield in the present experiment. The typical ion- plasma period  pi 95 ns is estimated by using the resonant density parameter ( n c =1 10 11 cm -3 ) at the critical layer in the argon plasma-microwave interactions. The wakefield excited by the incident microwave of pulse width 85–90 ns is described in the experimental results. The maximum am- plitude of the wakefield is observed about  n / n 0 =40%. This paper is organized as follows. The experimental setup is described in Sec. II, results are presented in Sec. III, and Sec. IV discusses the results in the view of different theoretical aspects. Finally, the results of the paper are con- cluded in Sec. V. II. EXPERIMENTAL ARRANGEMENTS A schematic drawing of the experimental arrangements is shown in Fig. 1a. A cylindrical, unmagnetized, nonuniform, PHYSICAL REVIEW E 68, 036404 2003 1063-651X/2003/683/0364048/$20.00 ©2003 The American Physical Society 68 036404-1