Helium exhaust and transport studies with the ALT-II pump limiter in the TEXTOR tokamak

Abstract

The first encouraging experiments demonstrating direct, explicit control of the He 2+ density in a tokamak plasma have been performed in the TEXTOR tokamak with the Advanced Limiter Test-II pump limiter. Helium is injected in a short gas puff from the outside of the plasma, is observed to reach the plasma core, and then is readily removed from the plasma. An exhaust efficiency of -8% is obtained. Active charge-exchange spectroscopy is used to study the exhaust and transport of He 2 * within the plasma, and the density evolution is modeled with a diffusive-convective transport code.PACS numbers: 52.25.Fi, 34.70.+e, 52.55.Fa, 52.70.Kz In future burning fusion devices, helium (He) ash must be continuously removed from the core to prevent dilution of the deuterium-tritium (D-T) fuel and concomitant quenching of the burn. Thus, He ash removal is fundamental to the operation of any fusion reactor, since the rate at which the a-particle by-products of the fusion reaction are purged from the core plasma will determine the pulse length available before the burn is quenched. For proposed steady-state tokamaks, such as the International Thermonuclear Experimental Reactor (ITER), continuous purging of the He ash is essential. Recent estimates 1 show that newly created He ions must be removed within 7 to 15 energy confinement times to maintain continuous reactor operation.The recycling of injected He from the wall and the lack of direct diagnostic capability to measure He concentrations have complicated previous efforts to determine the He removal rate. 2 " 4 Estimating this rate requires knowledge of the recycling coefficient, which can only be determined indirectly and depends on the wall conditioning status and history of the device. The experiments reported here are the first in which direct, explicit removal of injected He has been demonstrated. This is made possible by the Advanced Limiter Test-II (ALT-II) system, a toroidal belt limiter 5-7 that uses turbomolecular pumps (TMPs). Most existing particle exhaust schemes use gettering materials or cryopumping systems, which do not pump He. We simulate the presence of recycled He ash in a tokamak by puffing concentrations of 3%-5% He (relative to n e ) into the TEXTOR plasma just before or during neutral-beam injection (NBI). The transport of the He into the plasma core and its subsequent pump-out phase using the ALT-II system are followed with spectroscopic techniques by observing the He in three locations: the plasma core, the plasma edge at the ALT-II limiter, and the ALT-II pumping duct. By combining the results from these measurements, the exhaust efficiency for the He found in the plasma core is obtained during ALT-II pumping.In the plasma core, the He density is measured by charge-exchange excitation (CXE) spectroscopy, in combination with NBI. Measuring spatially and temporally resolved ion temperatures and absolute densities using CXE line intensities is a well-established technique on many tokamaks. 8 " 11 We use CXE spectroscopy to obtain the local He 2+ densi...

Edge Physics, Divertors, Pump Limiters

“…These design values of ALT-II are in agreement with the measurements. We have also verified experimentally the expected helium removal efficiency [10]. The removal efficiencies we obtained for TEXTOR would just be sufficient for the requirements of helium removal for a burning reactor.…”

Resonant coupling of ion-cyclotron waves to energetic helium ions

“…Experiments have been performed on the helium distribution in the plasma, the helium transport, and accumulation of helium in the divertor chamber [5][6][7][8][9][10][11][12][13][14]. Investigations with full pumping capabilities have been executed by the toroidal pump limiter ALT-II on TEXTOR [15][16][17][18][19][20]. Also, proposals were carried out to remove the helium by implantation in nickel [21].…”

TEXTOR: A Pioneering Device for New Concepts in Plasma-Wall Interaction, Exhaust, and Confinement

“…in 1987, which is a full toroidal belt limiter equipped with turbopumps, efficient He exhaust has been demonstrated for the first time in a tokamak. 5 These studies also led to the derivation of an important figure of merit for He exhaust: The ratio t He * 0t E of the effective helium exhaust time and the energy confinement must stay below a certain maximum value to allow for a steady-state burning fusion plasma. 6 The maximum value depends on the presence and amount of other impurities in the plasma.…”