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The deteriorating effect of plasma density fluctuations on microwave beam quality

The deteriorating effect of plasma density fluctuations on microwave beam quality

Heating and diagnostics suffer, however, both from plasma density fluctuations existing at the plasma bound- ary. These fluctuations can reach levels of up to 100 % [6] and potentially spoil heating e ffi ciencies and result in am- biguous diagnostics results. This is in particular a prob- lem for the stabilization of so-called neo-classical tearing modes (NTMs), a magneto-hydrodynamic instability aris- ing from small perturbations in the plasma current pro- file [7, 8]. If not taken care of, they can result in disrup- tions which are to be avoided at all costs in large-scale tokamaks like ITER. One way to stabilize the NTMs con- sists in localized current drive in order to restore the orig- inal current profile. ECCD has been successfully used to provide this current [9]. Using numerical tools to evaluate the consequences of edge density turbulence on the quality of a microwave beam injected for NTM stabilization is the topic of this paper.
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Modulation of Observed Thomson Scattering Spectra in a Plasma Density Irregularity

Modulation of Observed Thomson Scattering Spectra in a Plasma Density Irregularity

Abstract—Thomson scattering of an electromagnetic wave in a plasma density irregularity is considered. A new effect is found that the scattered waves generation and superposition near the electron density extremum may result in a substantial modulation of the scattered signal frequency spectrum. Due to this effect, the observable spectrum shape will be substantially different from that for the electron density fluctuations. This fact should be taken into account when interpreting Thomson scattering experiments.

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Effect of Inhomogeneous Plasma Density on the Reflectivity in One Dimensional Plasma Photonic Crystal

Effect of Inhomogeneous Plasma Density on the Reflectivity in One Dimensional Plasma Photonic Crystal

Moreover, the optical properties and dispersion characteristics of PPC can also be controlled by considering inhomogeneous plasma in the unit cell. It is more practical also because homogeneous plasma having uniform density is rarely realized in the laboratory plasma [21]. Therefore in the present study we have taken inhomogeneous plasma in the unit cell of binary 1D-PPC which is yet not discussed by any researcher. One of the reasons for inhomogeneities in the plasma can be spatial dependence of plasma density. Here the transfer matrix method is employed to study the propagation of electromagnetic waves in 1D- PPCs having inhomogeneous plasma in the unit cell which is suitable for the analysis of 1D-PPCs [22, 23]. The inhomogeneous plasma with linear density profile [24] and exponentially density profiles [25] have chosen because close form solutions can be obtained in terms of special mathematical functions. The paper is organized as follow: in Section 2 formulas for the dispersion relation of the proposed structure is given. The other necessary formulas used in this paper are also presented. Section 3 is devoted to result and discussion. A conclusion is drawn in Section 4.
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Generation of attosecond electron bunches in a laser-plasma accelerator using a plasma density upramp

Generation of attosecond electron bunches in a laser-plasma accelerator using a plasma density upramp

Figure 1: Schematic of the plasma density profile (solid line) and the density-dependent plasma wave phase velocity (dashed line) as a function of position in the laser propagation direction z. The phase velocity is derived from Eq. (1) for a distance |ξ| of one nonlin- ear plasma wavelength λ np = λ p (1 + a 2 0 /2) 1/4 behind the laser

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The Dispersion Characteristics of a One Dimensional Plasma Photonic Crystal Having Inhomogeneous Plasma Density Profiles

The Dispersion Characteristics of a One Dimensional Plasma Photonic Crystal Having Inhomogeneous Plasma Density Profiles

The dispersion characteristics of binary 1D-PPCs having inhomogeneous plasma in the unit cell are studied. Using the transfer matrix method the required dispersion relations are obtained. Here the linear and exponential plasma density profiles are considered and compared with the homogeneous plasma having uniform density profile. It is observed that the inhomogeneity in plasma layer highly affect dispersion curves. By comparing the dispersion curves obtained in all considered cases, it is found that the widths of band gaps and phase velocities are always larger for exponential density profile than the linear uniform density profiles in the considered frequency range.
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The deteriorating effect of plasma density fluctuations on microwave beam quality

The deteriorating effect of plasma density fluctuations on microwave beam quality

Heating and diagnostics suffer, however, both from plasma density fluctuations existing at the plasma bound- ary. These fluctuations can reach levels of up to 100 % [6] and potentially spoil heating efficiencies and result in am- biguous diagnostics results. This is in particular a prob- lem for the stabilization of so-called neo-classical tearing modes (NTMs), a magneto-hydrodynamic instability aris- ing from small perturbations in the plasma current pro- file [7, 8]. If not taken care of, they can result in disrup- tions which are to be avoided at all costs in large-scale tokamaks like ITER. One way to stabilize the NTMs con- sists in localized current drive in order to restore the orig- inal current profile. ECCD has been successfully used to provide this current [9]. Using numerical tools to evaluate the consequences of edge density turbulence on the quality
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Plasma density shaping for attosecond electron bunch generation

Plasma density shaping for attosecond electron bunch generation

High energy attosecond electron bunches from the laser-plasma wakefield accelerator (LWFA) are potentially useful sources of ultra-short duration X-rays pulses, which can be used for ultrafast imaging of electron motion in biological and physical systems. Electron injection in the LWFA depends on the plasma density and gradient, and the laser intensity. Recent research has shown that injection of attosecond electron bunches is possible using a short plasma density ramp. For controlled injection it is necessary to keep both the laser intensity and background plasma density constant, but set to just below the threshold for injection. This ensures that injection is only triggered by an imposed density perturbation; the peak density should also not exceed the threshold for injection. A density gradient that only persists over a short range can lead to the injection of femtosecond duration bunches, which are then Lorentz contracted to attoseconds on injection. We consider an example of a sin 2 shaped modulation where the gradient varies until the downward slope exceeds the threshold for injection and then reduces subsequently to prevent any further injection. The persistence above the threshold determines the injected bunch length, which can be varied. We consider several designs of plasma media including density perturbations formed by shaped Laval nozzles and present an experimental and theoretical study of the modulated media suitable for producing attosecond-duration electron bunches.
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Simulation study of a passive plasma beam dump using varying plasma density

Simulation study of a passive plasma beam dump using varying plasma density

where all quantities are in SI units. As the density increases the plasma wavelength decreases, effectively shifting the bunch within the wakefield. Half of one plasma wavelength behind the drive bunch is the region of highest on-axis elec- tron density. If the plasma density is increased, decelerated particles will pass through a strong defocusing region and be removed from the axis. This will prevent their re- acceleration. For a stepped plasma, the change in plasma wavelength is instantaneous and the decelerated particles do not need to pass through the accelerating region. For a gradual plasma density increase the decelerated particles will gain energy in the accelerating region prior to being defocused. Figure 1 shows a diagram of the stepped and gradient plasma density schemes. The rate of change with position of the plasma wavelength can be calculated for a given plasma profile, by taking the derivative of k p
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Perturbing microwave beams by plasma density fluctuations

Perturbing microwave beams by plasma density fluctuations

Since a cold plasma description is used here, the mi- crowave interacts only with the plasma density turbulence which is seen by the microwave as electron density fluc- tuations. In the time frame of the microwave, the fluctua- tions are frozen. This is due to the typical frequency scale of the fluctuations which lies in the kHz range (to be com- pared with the GHz range of the microwaves). In addition, the group velocity of the microwave is orders of magni- tudes above the phase velocity of the density structures which can be approximated by the electron diamagnetic drift velocity and reaches values of 10 4 m / s [1]. There-
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Hybrid capillary discharge waveguide for laser wakefield acceleration

Hybrid capillary discharge waveguide for laser wakefield acceleration

This controllable plasma density range is very promising for laser wakefield acceleration. Besides, laser transmittance curves for a two-segment capillary are also measured for different pressures of the gas- injection, and the results are similar to the ones obtained in the single-segment capillary. Therefore, the hybrid capillary combines the advantages of the pure ablative capillary and the gas-filled capillary. Not only the hybrid capillary has a stable-discharge process and a long lifetime, but it can also be extended to a longer length (such as 10-cm-scale long) by adding multiple segments. All these characteristics make the hybrid capillary an attractive proposal for the GeV-scale, or even for multi-GeV-scale laser wakefield acceleration.
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Experiment and Numerical Simulation of Peculiarities in the Development of Helium DC Discharge in Reflex Geometry

Experiment and Numerical Simulation of Peculiarities in the Development of Helium DC Discharge in Reflex Geometry

With initial conditions for the plasma density used above in the case of H = 0, simulations carried out at a voltage of 0.6 kV in magnetic field H = 100 G show an in- crease in the ion current and concentration within ∼2 ms and pronounced tendency of discharge current stabiliza- tion on a level of ∼2 mA (Fig. 9 (a)). For the same dis- charge voltage in magnetic field H = 200 G, the current reaches 5.1 mA for a shorter period and continues linear growth with the time. In a field of H = 300 G, the current increases slower than in the case of H = 200 G but still faster than at 100 G. In a still stronger magnetic field of H = 400 G, the derivative dI/dt decreases to minimum and the current reaches a stationary value of 1 mA for 2 ms. An analogous behavior is observed for U = 1 kV (Fig. 10 (a)).
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Theoretical and Experimental Studies of 35 GHz
 and 96 GHz
 Electromagnetic Wave Propagation in Plasma

Theoretical and Experimental Studies of 35 GHz and 96 GHz Electromagnetic Wave Propagation in Plasma

orders of magnitude with the plasma density increases by one order of magnitude. Moreover, there are several valleys on the reflectance curves, and the values of the valleys decrease while the corresponding EM wave frequency increases with increasing plasma density. The phenomenon is owing to the cavity resonance effects; however, the effects of the plasma density on the resonance was not described by Yuan et al. [15]. The cavity resonance has a more obvious effect on the EM wave reflectance for higher EM wave frequency, while the effect becomes less obvious for higher plasma density. In essence, the cavity resonance effect is attributed to the multiple EM wave reflections at the interfaces z =0 and z = −d.
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Dispersion Characteristics and Optimization of Reflectivity of Binary One Dimensional Plasma Photonic Crystal Having Linearly Graded Material

Dispersion Characteristics and Optimization of Reflectivity of Binary One Dimensional Plasma Photonic Crystal Having Linearly Graded Material

while those having higher frequency than plasma frequency get transmitted. But these behaviors get modified when we have spatially periodic plasma structures. These periodic structures are called plasma photonic crystals (PPCs), in which one unit cell consists of homogeneous unmagnetized (magnetized) plasma and homogeneous dielectric (vacuum). Now, electromagnetic wave whose frequency is outside the photonic band gaps even smaller than the plasma frequency can also propagate through plasma. The electromagnetic waves with frequency inside the photonic band gaps, even higher than plasma frequency, is attenuated. The concept of PPCs was proposed by Hojo and Mase [6] as a plasma version of photonic crystal in 2004. They studied the dispersion characteristics of one dimensional (1D) PPCs. Subsequently, lots of theoretical, simulated, and experimental studies have been done in one and two dimensional PPCs [7–11]. Guo [12] studied obliquely incident electromagnetic wave propagation in 1D-PPCs and concluded that plasma density, collision frequency, plasma width, dielectric constant of background materials and incident angle have marked influence on dispersion. Prasad et al. [13, 14] have investigated the modal propagation characteristics of ternary 1D-PPCs and shown that the plasma frequency, plasma width and dielectric constant of dielectric media have influence on band gap, group index, group velocity and phase velocity.
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An investigation of self-focussing of a laser pulse in a laser-produced plasma

An investigation of self-focussing of a laser pulse in a laser-produced plasma

For short pulses, approximately 20-50ps, the self­ focussing threshold is very well defined because of the steepness of the left-hand side of the Curves 3a, 3b. For these short pulses the relativistic mechanism dominates the self-focussing behaviour and, because of the mecha­ nisms fast time response, filamentation occurs at the peak of the pulse once the threshold is exceeded. For longer pulses the competition between relativistic and ponderomotive force mechanisms causes a flattening of the left-hand side of the curves. The threshold for efficient self-focussing can be seen to be a strong function of the parameters of pulse duration and electron temperature. A decrease in the plasma electron temperature causes an increase in the efficient self­ focussing threshold reflecting the dominant role of absorption in determining the in-filament flux density. Figure 4 illustrates this point for two different pulse durations of 20 and 100 ps. In both cases the threshold increases rapidly for Te < 500 eV. This behaviour differs from that predicted for steady-state ponderomotive force self-focussing in the absence of absorption [6,15]. In that situation an increase in the plasma temperature, and hence a proportional rise in hydrodynamic pres­ sure, increases the critical power for self-focussing [6]. In our calculations with short pulses, which represents a situation far removed from the steady-state, and where relativistic effects are included, a variation in plasma temperature in the absence of absorption has been found to have only a marginal effect on the self­ focussing thresholds. In view of this we have examined the variation in the on-axis plasma density when a nanosecond pulse propagates through a non-absorbing plasma at different temperatures. As expected from the steady-state model the maximum perturbation of the on-axis plasma density is reduced substantially at high plasma temperatures. However, it should be noted that this maximum perturbation occurs long after self­ focussing has occurred [17] and hence it is the initial growth of the density perturbation, not its maximum value, which affects the self-focussing behaviour. Our results suggest that the initial growth is not strongly
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Simulation of density measurements in plasma wakefields using photon acceleration

Simulation of density measurements in plasma wakefields using photon acceleration

example. In the AWAKE experiment, the plasma density is n 0 ¼ 7 × 10 14 cm −3 , laser wavelength is 800 nm, propa- gation length is 10 m and the relative density perturbation is about Δ n=n 0 ∼ 0 . 1–0 . 5 . Based on Eq. (1), the frequency change is about ∼0 . 3% of the initial frequency. Thus one needs diagnostics with frequency precision up to ∼0.003% around 800 nm to make a good measurement with 1% precision of the frequency shift. With the technology of FROG that can measure the frequency change up to ∼0 . 0003% , this technique can be applied to a plasma with density as low as ∼1 × 10 14 cm −3 , using the same propa- gation distance and density perturbation with the AWAKE experiment. The number can be lower for longer plasma column or with more precision and accurate diagnostic tools. Moreover, a short pulse can have a broad frequency spectrum. If the pulse is too short and the frequency change is too small, it would be also too hard to observe the frequency change. In order to make the measurement easier, the laser ’ s frequency change should be greater than bandwidth of the pulse or Δω > ω bw . By considering the Gabor limit [31], τ pulse ðω bw = 2πÞ ≥ 1 = 2 , and Eq. (1), the
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Direct observation of plasma waves and dynamics induced by laser-accelerated electron beams

Direct observation of plasma waves and dynamics induced by laser-accelerated electron beams

To obtain few-cycle probe pulses suitable for the shadowgraphy of plasma waves, a small part of the laser pulse (about 1 mJ), is coupled out before the focusing optics and sent into an argon-filled hollow-core fiber. Self- phase modulation (SPM) inside the fiber leads to spectral broadening and allows temporal compression of the beam to below 10 fs, while its timing is adjusted with a delay stage (see the Appendix for more details). It is sent through the target perpendicularly to the main pulse. The plane of interaction is imaged by a long-working-distance micro- scope objective ( 5 × or 10 × magnification, depending on the configuration) to form shadowgrams with a spatial resolution of approximately 2 μ m. Because of the short pulse duration even rapidly moving structures like plasma waves can be resolved. The measured diffraction signal directly reflects periodic modulations of the plasma density distribution, i.e., the laser- or beam-driven plasma wave. In the quasilinear regime of wakefield acceleration, the periodicity of the plasma-wave train is equal to the plasma wavelength λ p , which is 10 to 30 μm for densities n 0 of 10 19 and 10 18 cm −3 , respectively [cf. Eq. (2)]. Meanwhile,
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Helicon wave propagation and plasma equilibrium in high-density hydrogen plasma in converging magnetic fields

Helicon wave propagation and plasma equilibrium in high-density hydrogen plasma in converging magnetic fields

In Figure 8a, the ion density increases approximately linearly away from the antenna, reaches a maximum and plateaus near the maximum magnetic field strength and rapidly decays in the vicinity of the end plate. In addition, the electron temperature is largest under the antenna and monotonically decreases towards the end plate where it has very low temperatures. In part (b), we provide a close up of the electron temperature near the end plate. The fast decay in plasma density near the end plate is associated with very low electron temperature (< 0.5 eV). In addition, the structures in the electron temperature under the antenna may indicate localized RF heating; however, further work is required to settle this point. Figure 9 suggests the presence of an axial electric field for z > 0 cm that accelerates ions and decelerates electrons in the direction of the target region. Figure 10 indicates that the electron pressure peaks downstream of the source region while the bulk of the thermal proton generation occurs in the vicinity of the antenna in the region -20 < z < 20 cm. In Chapter 7, we analyse these measurements using a two-fluid approach and provide an explanation for the observed equilibrium plasma density. Radial ion density and electron temperature profiles measured at z = 11 cm are shown in Figure 11. The triangular radial density profile and the edge peaked electron temperature are typical of MAGPIE discharges for conditions as per Table 2. The corresponding radial floating and plasma potential measurements are shown in Figure 12 and indicate an ion confining radial electric field.
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A Review Study on Amplification of X Ray Free Electron Laser Pulse in Plasma

A Review Study on Amplification of X Ray Free Electron Laser Pulse in Plasma

regime needed for efficient BRA is still looking far away for practical realization of XFEL pulse amplification. Since the efficient operation of the process needs the joint facility of NIF and LCLS. Hence due to unavail- ability of practical tools, we may test the efficient BRA X-regime via the simulation codes. However we need to modify our existing Particle-In-Cell (PIC) simulation codes to test BRA in x-regime where we expect to have very high plasma density and very intense pump FWHM field. Since most of PIC codes are considering the colli- sionless plasma but in high density plasma, we can not ignore the role of electron-ion collisions and we need to implement the collisional process (including absorption of laser energy and Landau damping [18]) in simulation code.
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Density and temperature characterization of long scale length, near critical density controlled plasma produced from ultra low density plastic foam

Density and temperature characterization of long scale length, near critical density controlled plasma produced from ultra low density plastic foam

Point projection X-ray radiography. X-ray radiography has been successfully used in the past to deter- mine plasma density. Here, we used a similar technique to measure the density profile of the foam inside the cyl- inder. As mentioned above, the x-ray source was created by shooting a glass stalk having a diameter of 50 μ m by a short pulse laser to create the 1.74 keV K-alpha line emission. The spatial uniformity of the backlighting x-rays was measured to be ± 0.3% across 500 μ m at the object plane (from it, we derive the uncertainty in the transmis- sion measurements shown in Fig. 4), and the spatial resolution is better than 20 μ m. The backlighter short pulse beam was triggered 600 ps after the beginning of the long pulse beam heating the foam. The radiograph was recorded on an imaging plate (FujiFilm TR) with 13 μ m thick black Kapton in front of it acting as a light-tight filter. As shown in Fig. 1(a), we do not use an imaging system, but rather simply have a projection of the target on the imaging plate. In this projection scheme, the target is magnified by a factor 10 on the detector.
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Simulation of small size divertor tokamak plasma edge at low density of plasma

Simulation of small size divertor tokamak plasma edge at low density of plasma

there are several commonly studied aspects. These in- clude E  B flow that stabilizes ion or electron tempera- ture gradient mode or electron modes [10-12]. Improved confinement due to the formation of an internal transport barrier (ITB) has been studied in order to develop an ad- vanced tokamak operation scenario [13,14]. It important to clarify what condition is required for formation of ITBs. Since many parameters appear to affect the conditions for ITB formation (e.g. applying sufficient heating power, toroidal momentum input...). Since the reduced of plasma density for ITB formation is one of the critical issues for the application of ITBs to many tokamaks, it is important to investigate the relation between ITB formation and reduced plasma density. ITB’s on small size divertor to- kamak are produced by applying neutral beam injection NBI [15]. The detailed study into the reduction of plasma density condition for ITB formation has thus far not been undertaken at small size divertor tokamak. In order to understanding of “ITB” formation at low density plasma, discharge with neutral beam injection “NBI” has been modeling by B2SOLPS0.5.2D fluid transport code [16,17] and analyses are presented in this paper for edge plasma of small size divertor tokamak. The results of modeling demonstrated that, “ITBs” at edge plasma of small size divertor tokamak strong form at low plasma density, due to a steeper normalized density gradient.
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