JOURNAL OF INTENSE PULSED LASERS AND APPLICATIONS IN ADVANCED PHYSICS

Vol. 4, No. 1, 2014, p. 11 - 16

Ultrahigh laser acceleration of plasma blocks with ultrahigh ion density for fusion and hadron therapy R. BANATIa,b, H. HORAc*, P. LALOUSISd, S. MOUSTAIZISe a Australian Nuclear Science and Technology Organisation, Lucas Heights, Australia b Faculty of Health Sciences, National Imaging Facility at the Brain and Mind Research Institute, (BMRI), Sydney University c Department of Theoretical Physics, University of NSW, Sydney, Australia d Institute of Electronic Structure and Laser FORTH, Heraklion, Crete, Greece e Technical University of Crete, Cania, Greece

At ultrahigh acceleration of plasma blocks at irradiation with laser pulses of shorter than ps duration and powers in the range of petawatt (PW) approaching exawatt (EW) optical energy is converted into macroscopic directed motion with few thermal processes. This allows the exploration of unknown physics properties and the development of a wide range of new technologies , such as the generation of fusion energy using proton-Boron11 fuel and hadron (particle) therapy for the treatment of cancer. Here, we draw attention to existing experimental data on ultrahigh acceleration of plasma blocks that appear in agreement with plasma hydrodynamics. These observations call for more detailed investigations as the basis for future applications, such as in nuclear energy with substantially reduced generation of radioactivity as well as the development of small-size hadron (particle) therapy units. (Received December 4, 2014; January 16, 2014)

1. Introduction The continual advances in laser technology [1] to[10] based on the chirped pulse amplification [6] have opened new avenues of exploration in physics. It enables basic research into the Heisenberg-Schwinger pair production in vacuum [7], particles of increasing energies up to 1000 times higher than the presently highest values achieved by accelerators those at CERN [4], or the exploration of processes of attosecond duration [8][9]. The potential use of laser-driven acceleration of plasma for nuclear fusion energy with near negligible radioactivity [2][3] [11], as well as medical applications [1] [10] is based on the ability to generate laser pulses of picoseconds (ps) or shorter duration and powers above petawatt (PW) up to exawatt (EW) and higher powers [4][5]. One new and important aspect is that optical energy of laser pulses can be directly converted into macroscopic motion by avoiding thermal processes, thus avoiding the delays known from nanosecond laser pulse interactions and losses by instabilities and bremsstrahlung emission [12][13][14]. It reflects the existence of ordered atomistic states appearing as macroscopic entities without the disturbing chaotic states of thermodynamics. This nonlinearity of laser physics with the nonlinear force [15][16][17], was foreshadowed in electrostatics in the discovery of the ponderomotive force by William Thomson (Lord Kelvin) in 1845 [18]. This force acts on neutral dielectric materials without having an electric charge in contrast to the Coulomb force [19]. In addition to the extensively explored and classically described acceleration and scattering mechanisms, see [1], it is the

collective mechanism of the extremely short time interactions [20] as seen under Chirped-Pulse Amplification (CPA) [4][5] that underpins the theoretical and experimental development of the Extreme Light Infrastructure (ELI) [1]. 2. Ultrahigh laser acceleration Using plsma hydrodynamics, the force density f is determined by the gas-dynamical pressure p = npkT where np is particle density, k is Boltzmann’s constant and T the temperature and by the presence of electric and magnetic fields E and H. f = p + fNL

(1)

For fields of a laser of frequency  defining a complex optical constant n  (2) n2 = 1 – (p)2/(1 – i) with the plasma frequency p and the electron collision frequency  in the plasma, the nonlinear force is given by [15][16] fNL = [EE + HH - 0.5(E2 + H2)1 + +(1+(/t)/)(n2-1)EE]/(4) - (/t)E  H/(4c)

(3)

This force is dominating over the gasdynamic pressure if the quiver energy of the electrons in the laser field is larger than the energy of thermal motion [16]. For simplified one-dimensional geometry and perpendicular laser irradiation, the force (3) can be reduced to the time

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R. Banati, H. Hora, P. Lalousis, S. Moustaizis

averaged value

of [26] than in [23] is related to the hundred times lower intensity.

fNL = - (x)(E2+H2)/(8) = -(p/)2(x)(Ev2/n)/(16) (4)

where the first expression is the force density definition from the gradient of laser field energy density, and where Ev is the amplitude of the electric field of the laser radiation in vacuum. This special case reminds of Kelvin’s ponderomotive force of electrostatics. It should be noted that for cases with complete accuracy, all terms of the tensor are needed [21]. Computations for plane geometry interaction with inclusion of the nonlinear force interaction, and of thermal laser absorption by collisions and equipartition processes in the dynamically developing optical plasma properties for interaction of neodymium glass laser irradiation of 1018 W/cm2 intensity irradiating deuterium having an initial Double-Rayleigh density profile [22, Figures 10.18a&b], Fig. 1 shows the result after 1.5 ps interaction time for the velocity distribution and for the electromagnetic energy density. The laser was irradiating from the right hand side and a plasma block of about 20 vacuum wave length thickness was moving against the laser light and another one into the deeper target. The velocity of the deuterium plasma at this time at the closest part to the laser was more than 109 cm/s. This corresponds to an average acceleration of more than 5×1020 cm/s2. The results of the computation were initially published in 1978 [22] but it took a long time [23] before an experimental confirmation of these ultrahigh accelerations was measured. The reason was not only the question how to produce the ps laser pulses of more than terawatt (TW) power, but there was the difficulty of relativistic self-focusing [24]. Each laser prepulse produced a plasma plume where any very intense laser beam was relativistic squeezed to less than wave length diameter producing very high intensities and emission of highly charged ions to energies far beyond MeV into all directions. Sauerbrey had to cut off the prepulses by a factor above 108 (contrast ratio), to avoid focusing, and the then plane laser wave fronts were highly directed plasma blocks accelerated by 2×1020 cm/s2 against the laser as immediately measured by Doppler shift. This was in full agreement with the nonlinear force acceleration [25] as computed before, Fig. 1. Variations with respect to experimental accuracy were of minor nature for comparison in view of the significant fact that the ultrahigh accelerations were 100,000 times higher than ever measured before in a laboratory or by the thermal pressure acceleration with the largest NIF laser. The measurement of the ultrahigh acceleration needed a high quality lateral intensity profile of the laser beam as this was realized in the used KrF laser used by Sauerbrey [23]. Only under these high quality condition was it possible again with KrF lasers, to reproduce the ultrahigh acceleration, see Fig. 2, [26][27][28]. The exact reproduction of the measured values [23][26] was shown [25] leading to a dielectric swelling factor for the laser radiation of about 3 which value is comparable with other experiments. The 10 times lower acceleration in the case

Fig. 1. Hydrodynamic computations in 1978 using 1018 W/cm2 laser irradiation on deuterium close to the critical density resulted in a plasma block moving to the right against the laser light after 1.5 ps showing an acceleration of about 1020 cm/s2 [22].

Another result apart from the ultrahigh acceleration is the generation of ultrahigh ion densities in the accelerated space charge neutral plasma blocks. The blocks were identified as macroscopic directed moving plasma volumes of the thickness of the (dielectrically increased) skin layer of the irradiated target. This was for the first time discovered by the measurements by Badziak et al. [29] showing the fully directed motion of the plasma blocks as realized in more details later [30][31] where the direct transfer of the optical energy of the laser pulses into the directed motion of the ions (apart from some negligible thermal losses) was verified. The ion densities were in the range up to 1013Amps/cm2 where the space charge neutralizing electrons between the ions are of the same directed velocity as that of the ions. These ultrahigh ion current densities are more than million times higher than from accelerators.

Fig.2. Intensity dependence of the velocity of the plasma front from the Doppler shift of the reflected 700fs KrF laser pulses from Al target [26].

   

 

Ultrahigh laser acceleration of plasma blocks with ultrahigh ion density for fusion and hadron therapy

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3. Medical application of laser-driven ultrahigh density ion pulses To be useful in cancer therapy, an ion beam needs capable of penetrationg in a depth from 3.5 cm to 32 cm with sufficient current to deliver 2 Grays of radiation to a targeted tumor for the period of approximately 2 – 3 minutes. Current designs, such as the CERN Proton Ion Medical Machine Study (PIMMS), achieve energy range for C12 beams of 120 - 430 MeV/u and for protons, 60 250 MeV, whereby the beam is applied in a raster scanning approach. Laser-driven ion pulses with a range of intensities have potential medical applications for example in the treatment of surface tissue [1] as well as deep-seated inaccessible tissues, such as tumors, which are the main target of from hadron therapy [10]. The latter utilizes the favourable property of accelerated particles of usually about up to 200MeV to deposit their energy in a narrow peak, the Bragg-maximum. Especially the use of fully ionized carbon (C6+) allows for the spatially highly confined targeting of tumor tissue with maximal sparing of the surrounding healthy tissue [33]. At present, the ability to produce ion beams for medical use relies on mid-size to often large-scale accelerator technology, that requires substantial physics and engineering expertise and is often only found in substantial national and international expert teams. Examples of the medical application of accelerator technology include medium size cyclotrons [32] as well as larger size synchrotrons, that have emerged from the conceptual and technical development activities at CERN in Geneva [33]. The successful use of accelerated proton or ions, such as carbon, for the treatment of cancers otherwise not treatable by either surgery or chemotherapy has led to the design and establishment of new large accelerator (usually synchrotron) facilities dedicated to the treatment of a range of cancers, notably in the nervous system and in children. The CERN design has recently been realized at CNAO (Centro Nazionale di Adroterapia Oncologica, Pavia, Italy) and MedAustron (Wiener Neustadt, Austria), while HIT (Heidelberger IonenstrahlTherapiezentrum, Heidelberg, Germany) builds on a design by GSI (Helmholtz Centre for Heavy Ion Research, Darmstadt, Germany). While the conceptually and technically largely mature, synchrotron-based hadron therapy facilities are beginning to define the most useful therapeutic indications and treatment protocols, their high cost are likely to limits in the longer term the available capacity of hadron therapy. Therefore, to turn particle therapy into a readily available best treatment option for localized cancers, nextgeneration more affordable technology, such as the use of laser generated ion beams, is needed [13][14]. It has been stated: “Laser plasma acceleration can potentially replace large and expensive cyclotron or synchrotrons for radiotherapy and ions” [10] .

Fig. 3. Irradiation of a 0.5 ps 248 nm wave length long laser pulse of 1021 W/cm2 on solid DT. Ion velocity U1 and velocity of the alpha particle fluid depending on depth X in the target at time 1.4 ps. Left hand curve is the ion velocity Ui and the other curve is the velocity Ua of the alpha particles [35][36].

We propose, that the future scope of research into new applications of laser-plasma interactions mechanisms [20] should include the description and further investigation of ultrahigh acceleration of plasma blocks with ultrahigh ion densities terms of hydrodynamic [12]. Existing results by Steinke at al. [13] indicate that hydrodynamic treatment can be applied successfully. However further investigation and validation is necessary. While the both the measurements of Sauerbrey [23] and Földes et al [26] as well as more detailed evaluation [25] suggest, based on plasma hydrodynamics, an agreement with predicted ultrahigh acceleration (see p. 179 of [22]), it will be important to evaluate the conditions under which the highly non-equilibrium state of the plasma at the generation of the nonlinear force driven plasma blocks is happening in details. First numerical evaluations [35][36][47] indicate that the complicated process of shock generation with the expected comparable low electron temperatures during the very short time of picoseconds interaction in the laser-driven blocks is indeed detectable and different from the shock generation at nanosecond interaction [48]. The measurements of Steinke et al [13] using 1.2J/45fs laser pulses on a 5.6nm diamond-like target resulted in proton energies of 13 MeV and C6+ ions of 71 MeV. This factor of about 6 proves that the acceleration follows the hydrodynamic theory of nonlinear (ponderomotive) force acceleration by the laser field [12][16]. The authors [13] also conclude that for single particle computations by sheath acceleration “scaling laws based on analytical models failed to interpret” the measurements. This was confirmed by the Sandner [34]. A further agreement with plasma hydrodynamics is the result of a computation of relativistic interaction, Fig. 3 [35][36] at a laser intensity of 1021 W/cm2. The ion energy of the plasma block for deuterons resulted in 60 MeV. This is comparable with the results of Nam et al. [37] where the hydrodynamic calculated proton energy is in the range of 80 MeV in agreement with simultaneous measurements [37].

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Generally, these results indicate that experimental physics research into compact very high density space charge neutral ion pulses in the range of 200 MeV energy is still at an early stage and requires more detailed studies using, e.g. 3 PW laser pulses [38] and reaching 10 PW as achievable by the ELI-beamline [39]. Also, a substantial amount of research supporting the theoretical understanding of the ultrahigh acceleration of plasma blocks with ultrahigh ion current densities is needed. 4. Ultrahigh acceleration for radical new laser fusion of solid density p-11B (HB11)   A dramatic change by Chirped Pulse Amplification (CPA) of laser pulses may lead to fusion energy without nuclear radiation problems. One may remembering that before 2008, the usual reaction of hydrogen with boron-11 (HB11) was about 100,000 times more difficult than burning heavy and superheavy deuterium (deuterium and tritium DT), i.e. it is impossible, when using pulses of nanosecond (ns) duration. This factor of difficulty can be reduced to only 5 by CPA with ICAN laser pulses [4][5][6] of less than picoseconds (ps) duration. This is based on the discovery that these pulses produce the ultrahigh acceleration of plasma blocks 100,000 times higher than measured in laboratories which values were predicted in 1978 [22] and measured since 1996 [23]. The plasma blocks generated within ps duration can initiate a Bobin-Chu fusion flame [40][41] in uncompressed solid density fuel. Using HB11, energy is produced with less nuclear radiation than from burning coal, what is negligible. The problem is that the CPA generated ps-laser pulses need energy fluxes above 100PW. 3 PW have been measured [38] but for the reaction of solid density HB11 one has to reach 1000PW=Exawatt(EW). This is anyhow the aim needed for fundamental experiments [4][5]. Thanks to the perfect optical quality of ICAN fiber lasers [4] it is estimated that arranging fiber laser ends as a sphere of a meter radius can produce a spherical converging laser pulse (see Fig. 4) such that the conditions of a power station for burning HB11 may be reached [2]. The steps for application of these developments for fusion energy, after the theoretical-numerical prediction [22] and the Doppler measurement [23] of the ultrahigh acceleration, began with the detailed discovery of the acceleration of the dielectrically increased skin layer acceleration as measured in 1999 [29] and as realized [30] as a basically different phenomenon in contrast to preceding observations. The use of the picosecond laser pulses with powers above TW opened for the first time the possibility, to the initiation of the fusion flame in solid state density for which the Bobin-Chu mechanism needed the enormous energy flux density of E*=4×108 J/cm2

(5)

Fig. 4. Generation of a spherical shrinking laser pulse from radial directed fiber ends at a radius 1 to irradiate a spherical solid state fusion fuel HB(11) 3 with the concentric grid 2 of positive electric charge for slowing down the generated alpha particles an gain their energy as electric power [2].

for DT fusion. A pulse duration in the range of ps was needed and as seen from the numerical results (Fig. 10.18a&b of Ref. [22]). This initiation of the fusion flame [30][31] in uncompressed solid density DT was described in a Patent application [42] and highlighted in the Foreword and Introduction to the Edward Teller Lectures in 2003 [43][44]. An update of Chu’s detailed computations of the block ignition [40] arrived at a reduction of E* by a factor 20 [45]. A surprise was that the threshold E* was only about 5 times higher when using the fusion cross section for the HB11 reaction instead of DT. This reduced the difficulty for the HB11 reaction from the former known value of 100,000 to only a value of 5 [46][47]. Shock generation and the picoseconds details of generation of shock fronts were studied with generation of shock velocities up to 10,000 km/s and more [11][20]. In addition, it has to be mentioned that the fusion reactions were based on a collective stopping power by electrons only. This is acceptable for DT, but for HB11 there is a change due to the ion stopping power by an avalanche process of the generated alpha particles when colliding with boron nuclei [3][35][36]. This mechanism and the changing of the computations from plane geometry to spherical geometry in order to avoid lateral energy losses did strongly increase the fusion gains for HB11, however a little only for decreasing of E*. For fusion of HB11 in a spherically irradiated pellet 3 of Fig. 4, an enormous power of Exawatt is necessary to produce a spherically shrinking ps laser pulse generated by the output of fiber amplifiers on the sphere 1 of Fig. 4. This is possible for a radius of the sphere 1 of one meter taking into account that it was estimated that a ps laser pulse of kJ energy may be generated per 100 cm2 cross section emitted at the ends of the fibers.

Ultrahigh laser acceleration of plasma blocks with ultrahigh ion density for fusion and hadron therapy

Under these conditions [2] it was estimated that a HB11 pellet of 0.56 mm diameter in Fig. 4 will convert 1 MJ laser energy in 69 MJ energy of alpha particles. This should be sufficient for a GW power station operating with 14 reactions per second. Estimating the cost of polymer fibers for the ICAN [4] in the range of less than $100million for a radius 1 of one meter, this is realistic taking into account that the fusion energy goes mostly into electricity with low heat generation. 5. Conclusions Good agreement between predictions based on theoretical plasma hydrodynamics and reported experimental observation has been shown. It, therefore, seems justified to pursue further investigations into the use of nonlinear force generated plasma blocks by ultrahigh acceleration and with ultrahigh space charge neutral ion densities for near radiation-free fusion energy production or to the generation of 200 MeV hadrons for cancer therapy. To realize these potential applications further advances in the development laser hardware and a better understanding of the interaction mechanisms of >>PW laser pulses of less than ps duration with targets still have to be achieved. Acknowlegement Special thanks are to Professor Gerard Mourou for invitation to his IZEST conference 18/20 November in Tokyo and numerous discussions and for including this project into his worldwide activities. Discussions about the paper with Prof. Helma Buttner, ANSTO, are gratefully acknowledged.

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