Compact Toroid Injection Experiment Research Articles

Impurity production and acceleration in CTIX

The Compact Toroid Injection Experiment (CTIX) produces a high density, high velocity hydrogen plasma that maintains its configuration in free space on a MHD resistive time scale. In order to study the production and acceleration of impurities in the injector, several sets of silicon collector probes were exposed to spheromak-like CT’s exiting the accelerator. Elemental analysis by Auger Electron Spectroscopy indicated the presence of O, Al, Fe, and Cu in films up to 200 Å thickness (1000 CT interactions). Using a smaller number of CT interactions (10–20), implantation of Fe and Cu was measured by Auger depth profiling. The amount of impurities was found to increase with accelerating voltage and number of CT interactions while use of a solenoidal field reduced the amount. Comparison of the implanted Fe and Cu with TRIM simulations indicated that the impurities were traveling more slowly than the hydrogen CT.

Poloidal field amplification in a coaxial compact toroid accelerator

The Compact Toroid Injection Experiment (CTIX) produces spheromak-like compact toroids (SCTs) without external power switching, initiating a discharge by pulsed gas injection into a formation region containing a seed magnetic field generated by a solenoidal coil. After formation, the plasma is driven by an inductively delayed capacitor bank into an acceleration region, where surface axial and toroidal magnetic fields are measured at several axial positions. Due to strong eddy-current effects, formation-region magnetic field cannot be simply computed; instead, it is measured using the response of axial and radial test coils in the formation region to short solenoid test current pulses. A temporal and spatial reconstruction method is developed allowing formation-region field to be computed from the test-coil data for any CTIX discharge of identical solenoid geometry. By varying the peak value and timing of solenoidal current, curves of peak accelerator-region field as a function of initial formation-region field are developed. Curves of peak accelerator-region axial magnetic field are thereby found to be highly nonlinear functions of formation-region field, showing a threshold value for the formation-region field of approximately 5 G, above which acceleration-region field saturates at values between 2 and 12 kG. The direction of acceleration-region axial field reverses sign when the direction of solenoid current is reversed. Saturated accelerator-region axial field is a function of axial position and accelerator voltage, and is typically comparable to toroidal field at the same location. The ratio of accelerator-region to formation-region axial field commonly exceeds 1000 near the onset of saturation. This large amplification is of practical advantage for delayed plasma breakdown on CTIX, allowing a modest seed field to produce high poloidal fields, which are necessary for intense SCT acceleration. The results may also provide a useful benchmark for numerical simulation of the conversion of abundant toroidal field into poloidal field in a plasma with comparatively low dissipation.

Profiling Line-Averaged Compact Toroid Electron Density on CTIX With Arrayed Laser Deflectometry

The compact toroid (CT) injection experiment (CTIX) is a coaxial plasma rail gun originally designed for tokamak refueling. CTIX can form and accelerate CT plasmas of controllable densities and velocities. For a given chord through a plasma, changes in line-integrated transverse electron density gradient can be resolved by measuring the angle of deflection of a laser beam traversing the plasma. A vertical array of four temperature-controlled laser deflectometers was installed in the viewing section of CTIX and used to profile the line-averaged density of an unconstrained CT drifting at high velocity.

Spectroscopy on magnetically confined plasmas using electron beam ion trap spectrometers

Multiple spectrometers originally designed for and used at the University of California Lawrence Livermore National Laboratory’s electron beam ion trap have found use at various magnetically confined plasma facilities. Three examples will be described. First is a soft X-ray/EUV grating spectrometer (6–150 Å), which is operating at the National Spherical Torus Experiment. Second is an EUV spectrometer with wavelength coverage up to 400 Å, which has just recently started operating at the Sustained Spheromak Physics Experiment. The last is a high-resolution transmission-grating spectrometer for visible light that has been used at the Compact Toroid Injection Experiment and is currently at the Alcator C-Mod tokamak.PACS Nos.: 39.30.+w, 52.55.–s, 32.30.Rj, 07.60.Rd, 52.70.La

Method of reconstructing a moving pulse

We present a method of analyzing a set of N time signalsfi(t) that consist of local measurements of the same physicalobservable taken at N sequential locations Zi along the length ofan experimental device. The result is an algorithm for reconstructingan approximation F(z,t) of the field f(z,t) in the inaccessibleregions between the points of measurement. We also explore theconditions needed for this approximation to hold, and test thealgorithm under a variety of conditions. We apply this method toanalyze the magnetic field measurements taken on the Compact ToroidInjection eXperiment (CTIX) plasma accelerator; providing a directmeans of visualizing experimental data, quantifying global properties,and benchmarking simulation.

Visible spectrometer at the Compact Toroid Injection Experiment and the Alcator C-Mod tokamak for Doppler width and shift measurements

A novel Doppler spectrometer is currently being used for ion or neutral velocity and temperature measurements on the Alcator C-Mod tokamak. The spectrometer has an f∕# of ∼3.1 and is appropriate for visible light (3500–6700Å). The linewidth from a line emitting calibration source has been measured to be as small as 0.4Å. The ultimate time resolution is line brightness light limited and on the order of milliseconds. A new photon efficient charge coupled device detector is being used at C-Mod. Time resolution is achieved by moving the camera during a plasma discharge in a perpendicular direction through the dispersion plane of the spectrometer, causing a vertical streaking across the camera face. Initial results from C-Mod as well as previous measurements from the Compact Toroid Injection Experiment are presented.

CT Injection Experiment in JFT-2M

A compact toroid (CT) injector is considered to comprise one of the advanced refueling methods for the fusion reactor. In JFT-2M, it was demonstrated for the first time that a CT injected into a neutral beam-heated plasma penetrated deeply into the plasma and caused a rapid increase in the electron density. We also observed interesting motions such as shift and reflection of the injected CT plasma as well as magnetic fluctuations induced just after CT injection. A power spectrum analysis suggested that this fluctuation was related to magnetic reconnection between the CT plasmoid and the toroidal field. We also modified the shape of the CT injector electrodes to improve CT injection efficiency. As a result, the CT parameters were superior to those of previous experiments, and the operational window became broader. Finally, it has been successfully demonstrated that a CT could be transported smoothly through curved drift tubes, which is one of the key technologies to avoid adverse effects of the toroidal field and to extend flexibility of the design and layout of the CT injector to access larger devices.

Plasma density gradient measurement using laser deflection

For a given chord through a plasma, changes in the line integrated index of refraction as a result of a transverse density gradient can be observed by measuring the angle of deflection of a laser beam. In contrast to laser interferometers, this method of density profile measurement places modest requirements on laser quality and alignment procedures, allowing measurements to be conducted with short coherence length commercial laser diodes and segmented photodiode detectors. A prototype implementation of this scheme has been constructed and tested on the compact toroid injection experiment (CTIX). At densities comparable to magnetic fusion plasmas, laser deflections in the nanoradian range were measured. By assuming a particular density profile, a sensitivity of ∼1012cm−3∕nrad was obtained. This produced estimates of CTIX peak density in reasonable agreement with conventional interferometry data. The final goal of this diagnostic is a simple, reliable, array deployable density profile diagnostic.

Compact toroid dynamics in the Compact Toroid Injection Experiment

Work on the acceleration of a compact toroid plasma configuration between coaxial electrodes is reported. In the experiment the maximum poloidal field component and the full width at half maximum of the poloidal field are shown to increase and decrease with the accelerator voltage, respectively. The velocity of the compact toroid is shown to increase with accelerator voltage and then saturate as the accelerator voltage is increased above approximately 11 kV. The saturation in the velocity and field components of the compact toroid is due to crowbarring of the accelerator insulator. The crowbarring of the insulator is consistent with the onset of the `blowby' effect, which is the most likely triggering source.

Observation of pinching in the compact toroid injection experiment: Implications for a plasma opening switch

A compact toroid inductively stores the energy released by a capacitor bank as it is being accelerated. This energy can be stored for a period of more than ten microseconds and then transferred to a load on a much shorter time scale. This article presents framing camera images of the radial compression of plasma trailing behind a compact toroid as the compact toroid leaves its inner electrode. This compression illustrates the basic principles of a compact toroid plasma opening switch which could be used to drive fast z pinches.

Dynamics of spheromak-like compact toroids in a drift tube

In order to supply plasma fuel confined inspheromak-like compact toroids (SCTs) to a fusion device,the SCTs must be successfully guided through a drift tube region,in which they might be influenced by the magnetic field leakingfrom the fusion device. To reveal the SCT dynamics in a drift tube,MHD numerical simulations, where the SCTs are acceleratedin a co-axial perfectly conducting cylinderwith an external magnetic field, are carried out. In addition,the effect of an extended central electrodeis examined by changing the length of the inner conducting cylinder.It is revealed that the SCT penetration depth is shorterthan that estimated from the conventional conducting sphere modeland that the SCTs are further decelerated by extending the inner conducting cylinder.These results are consistent with the resultsof the compact toroid injection experiment performed on the TEXT Upgrade tokamak.Finally, the deceleration mechanism of the SCTs is discussedby comparing the simulation result with the proposed theoretical model.

The “Compact Toroid Injection Experiment” tomographic imaging system

The design, construction and operation of a tomographic imaging system on the Compact Toroid Injection Experiment is described. The system measures the total radiated power over energies from visible light up into the extreme ultraviolet. It then reconstructs two dimensional profiles from the data. The reconstruction routine is based on a method known as second order regularization which finds a compromise between smoothness and fit to the data. This method was found to have the best overall fidelity to test images. The hardware and overall reconstruction were calibrated using two different sources. First results from the system under real experimental conditions are presented.

Compact toroid injection experiment in JFT-2M

Compact toroid (CT) injection experiments with H mode plasmas were carried out for the first time in the JFT-2M tokamak. The soft X ray emission profile shows deep CT penetration into the core region of H mode plasmas heated by 1.2 MW NBI as well as in OH plasmas, with a toroidal magnetic field of 0.8 T. The line averaged electron density rapidly increased by Δn̄e ≈ 0.2 × 1019 m-3 at a rate of 4 × 1021 m-3·s-1 in H mode and the fuelling efficiency was roughly 20%. An asymmetric radial profile in the soft X ray emission was produced for ∼50 μs by deep penetration of the CT.