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Active plasma lensing is a compact technology for strong focusing of charged particle beams, which has gained considerable interest for use in novel accelerator schemes. While providing kT/m focusing gradients, active plasma lenses can have aberrations caused by a radially nonuniform plasma temperature profile, leading to degradation of the beam quality. We present the first direct measurement of this aberration, consistent with theory, and show that it can be fully suppressed by changing from a light gas species (helium) to a heavier gas species (argon). Based on this result, we demonstrate emittance preservation for an electron beam focused by an argon-filled active plasma lens.

The fast ignitor scheme for inertial confinement fusion requires forward driving of the critical density surface by light pressure (hole boring) to allow energy deposition close to the dense fuel. The recession velocity of the critical density surface has been observed to be v/c=0.015 at an irradiance of 1.0×1019 W cm−2 at a wavelength of 1.05 micron, in quantitative agreement with modeling.

We report on the use of an active Langmuir probe with three-harmonic compensation to diagnose rf discharge plasmas driven at 13.56 MHz. The plasma generates many harmonics on the fundamental, the first few being strongest. This gives a multi-harmonic rf voltage across the probe sheath that is removed by applying a rf signal to the probe that is matched both in amplitude and in phase for each harmonic. The probe I-V characteristic can then be analysed using dc theory. We show here that only when the rf harmonic amplitude approaches about 2T e is it necessary to compensate for that harmonic. For the commercial processing rig used this only occurs for the second harmonic at low pressures, <5 × 10 −3 mbar. Here the addition of second-harmonic compensation shifts the probe I-V curve, making it markedly more positive than for fundamental-only compensation. Despite this the values obtained for the electron density n e and temperature T e changed by less than 10%. For most plasmas in which the harmonics have amplitudes below T e the use of fundamental-only compensation is adequate for all but the most precise measurements.

We report on Langmuir probe measurements of low pressure (0.1-10 Pa) O2 plasmas in an RF discharge at 13.56 MHz. The probe is actively compensated using a three harmonic box and the collisionless theory of Amemiya et al (1999) for electronegative plasmas is used to fit to the resulting I-V curves. However, it has been shown for example by Shih and Levi (1971) and Sudit and Woods (1994) that in electropositive plasmas such as He and N at these low pressures the ion saturation current can be reduced to as much as one-half the expected value due to a small number of collisions in the probe's presheath. This effect has been treated theoretically by Shih and Levi (1971) for both spherical and cylindrical probes. In this paper we extend this theory to cover the case of electronegative plasmas. Here collisions have a greater effect as the resulting ohmic drop has to be compared to the smaller collisionless presheath voltage drop in units of (kBTe/e) of -1/2γ, for a negative ion temperature ratio γ = Te/Tn. Using the correction factors from this theory we are able to obtain the negative ion fraction α in our weakly collisional plasmas. The results using a commercial processing rig indicate a maximum in the negative ion density at around 2.5 Pa consistent with measurements based on laser photodetachment by Stoffels et al (1995).

We report on Langmuir probe measurements of low-pressure (0.1-20 Pa) electropositive plasmas in an RF discharge at 13.56 MHz. From the probe I-V characteristic it is found that the electron density inferred from the ion current in the ion saturation region using radial motion (Allen, Boyd and Reynolds, ABR) theory can be up to one-half that obtained directly from the electron current at the plasma potential. The reduction in the ion current is attributed to orbital motion (OM) of the ions and also to a small number of ion-neutral collisions in the presheath. We show that if a sufficiently large probe is chosen so as to minimize the OM effects then the collisional theory developed by Shih and Levi (1971) can be used to give an appropriate correction factor over a narrow pressure range. The corrected electron density is found to agree with the knee current value to typically 10% for Ar, N2 and Kr plasmas.