H. Hamnén

Studies of fusion burn control

Findings from a general study of issues associated with operation control of burning fusion plasmas are reported, and applications to the International Thermonuclear Experimental Reactor (ITER) are given. A number of control variables are discussed. A zero-dimensional system has been developed, and stability against coupled temperature and density variations is studied. Space-dependent energy balance and transition to thermonuclear burn are analyzed as are maximum obtainable Q values under subignited operation conditions. Control designs with different input-output strategies are analyzed and numerically simulated, and a numerical experiment on system identification is performed. Requirements on diagnostics are discussed, and areas for further study are identified. 67 refs., 21 figs., 3 tabs.

A model for steady‐state breakdown plasmas in microwave transmit‐receive tubes

A model is proposed for the consistent description of the steady‐state interaction between a high‐power microwave and a breakdown plasma in transmit‐receive (TR) tubes often used as switches in radar microwave systems. The electron‐density equilibrium in the TR tube is established as a balance between the ionization produced by the wave and various electron‐loss processes. In the currently proposed model, it is shown that diffusive losses of electrons in the direction of wave propagation is the most important loss process. An approximate analytical solution is derived from the model and compared with the numerically obtained full solution. Finally, the predictions obtained from the model concerning the scaling of absorbed and reflected microwave power are shown to be in good agreement with experimental results.

Breakdown phenomena in microwave transmit-receive switches

A detailed experimental and theoretical investigation is made of the physical phenomena involved in the generation of a breakdown plasma in a microwave transmit‐receive switch and in the subsequent interaction between the plasma and incident microwave. Particular emphasis is given to characteristics like breakdown and sustainment levels, turn‐on time, and reflected, transmitted, and absorbed powers.

Distortion of ion velocity distributions in the presence of ICRH: a semi-analytical analysis

On the assumption that the distribution function of ions heated by ion cyclotron resonance absorption is essentially isotropic, analytical and semi-analytical approximations are derived for the distribution function. The result is used to evaluate the weighted velocity space averages of the distribution, which determine the fusion reactivity and the collisional power transfer to plasma background particles, and to study their scaling with RF parameters such as absorbed power and perpendicular wave number. The importance of higher order finite Larmor radius effects on the formation of RF induced high energy tails is particularly emphasized. Comparisons based on full 2-D numerical calculations show good agreement with the semi-analytical results.

α‐particle ripple losses during slow down in a tokamak reactor

An investigation is made of the effect of toroidal field ripple mirrors on the confinement of α particles. It is found that the diffusion of α particles into the loss regions of the ripple mirrors during slow down gives rise to significant particle and energy losses.

Effects of velocity-space loss regions on the alpha-particle distribution function and collisional alpha-particle losses in tokamak reactors

The steady-state distribution function for alpha-particles in the presence of loss regions in velocity space is derived in an analytical form on the basis of the Fokker-Planck equation. The particle losses and the associated energy loss of alpha-particles due to scattering into the loss region are estimated for parameters characteristic of a tokamak reactor.

Transition to thermonuclear burn in fusion plasmas

An analytical investigation is made of the time evolution of the 1-D temperature profile in a fusion reactor plasma where the nonlinear energy balance equation is dominated by alpha-particle heating and thermal conduction losses. Special emphasis is given to the problem of establishing sufficient conditions for the transition to thermonuclear burn for given initial temperature profiles. In particular, it is demonstrated that for strongly nonlinear alpha-particle heating, temperature profiles initially peaked on-axis are more easily ignited than profiles similar in form to the equilibrium profile of the energy balance equation. Simple analytical criteria for ignition are established and are shown to compare favourably with results of numerical calculations.

Excitation of thermonuclear cone instabilities due to alpha-particle ripple losses in tokamaks

The possibility of a new set of thermonuclear loss‐cone instabilities is demonstrated. These instabilities are caused by an anisotropy in the alpha‐particle distribution function which arises because of alpha‐particle losses due to the toroidal field ripple in tokamaks. The conditions for the excitation of a number of instabilities are studied.

Alpha-particle losses during slowing-down in an open-field-line fusion plasma

Alpha-particle losses due to pitch-angle scattering into the loss cone of an open-field-line configuration are investigated. Specifically, the particle loss and the corresponding energy loss are given as explicit functions of energy during slowing-down. It is found that the alpha-particle energy loss due to pitch-angle scattering for a pure DT-plasma is of the order of 5–10%, in contrast to the 35–40% obtained in a recent paper

Enhancement of high-energy ion tails in the presence of combined neutral-beam and ICRF heating of a two-component plasma

An investigation is made of the effect of tuning ICRF heating to neutral-beam-injected ions in a two-component plasma. The corresponding RF-induced velocity diffusion is found to cause a significant enhancement of the high-energy tail of the beam ions. This is also shown to give rise to a subsequent increase in the fusion power multiplication factor, indicating an efficient use of the available heating power.