The behaviour of fast ions in tokamak experiments

Heidbrink, W. W.; Sadler, G.

doi:10.1088/0029-5515/34/4/i07

Cited by 468 publications

(499 citation statements)

References 174 publications

(332 reference statements)

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“…In magnetic-fusion tokamaks, wall desorption is dominated by physical and chemical sputtering from low-energy ion bombardment [17]. However, some instabilities carry energetic ions to the wall [18], resulting in the more copious electronic desorption. In astrophysics, cosmic ray desorption of gas condensed on dust surfaces can affect gas densities in nebulae, and increase the temperature range over which the normally nonreactive H 2 and CO, on iron dust may form CH 4 and other prebiotic molecules [19], as are observed in the Orion nebula [20].…”

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confidence: 99%

Heavy-Ion-Induced Electronic Desorption of Gas from Metals

et al. 2007

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During heavy-ion operation in several particle accelerators worldwide, dynamic pressure rises of orders of magnitude were triggered by lost beam ions that bombarded the vacuum chamber walls. This ioninduced molecular desorption, observed at CERN, GSI, and BNL, can seriously limit the ion beam lifetime and intensity of the accelerator. From dedicated test stand experiments we have discovered that heavy-ion-induced gas desorption scales with the electronic energy loss (dE e =dx) of the ions slowing down in matter; but it varies only little with the ion impact angle, unlike electronic sputtering. DOI: 10.1103/PhysRevLett.98.064801 PACS numbers: 41.75.Ak, 34.50.Dy, 79.20.Rf Energetic ions incident on matter sputter target material and also desorb gas from the target surface. The sputter and desorption yields (number of sputtered or desorbed particles per ion impact) are known to be linked to the energy loss of the projectile inside the target. Two energy loss regimes, nuclear and electronic, have been known for decades. An example for potassium ions impacting onto stainless steel, calculated with the SRIM code [1], is shown in Fig. 1. Here for low projectile energies the nuclear energy loss dominates and for higher energies the electronic energy loss dominates the total energy loss. Sputter and desorption yield measurements from 1 m thick targets in the regime of electronic energy loss have shown that both scale with the electronic energy loss dE e =dx n to the power of n 1-3. This was observed for targets of frozen gases [2 -5], for sputtering from micron-thick coatings of protein [6,7], and for desorption of nitrogen from a conductor (carbon) by 6-13 MeV=u ions (u is the nucleon mass) [8,9]. Electronic sputtering and desorption from 1 m thick targets is found to vary with the ion impact angle from normal, , as 1=cos m to the first or higher power of m [5,10].Our research was motivated by the copious gas desorption that results from lost heavy ions striking particle accelerator vacuum chambers leading to dynamic pressure rises which limit the beam intensity in a number of heavyion accelerators [11]. Related work dates back more than 30 years, when a vacuum instability in the Intersecting Storage Rings (ISR) at CERN was identified above a critical beam current [12]. Recent requirements for orders of magnitude increase in beam intensity have motivated our search for further understanding and mitigation mechanisms. In preparation for the heavy-ion program of the Large Hadron Collider at CERN, beam-loss induced molecular desorption was intensively studied in ultra-highvacuum chambers at CERN's Heavy-Ion Accelerator (LINAC 3) [13] and the Super Proton Synchrotron (SPS) [14]. Large effective desorption yields of up to 2 10 4 molecules=Pb 53 ion (4:2 MeV=u) and 3:7 10 4 molecules=In 49 ion (158 GeV=u) were measured for ions impacting under various angles [ 0 (perpendicular), 84.8 , 89.2 at LINAC 3, and 88.3 at the SPS] onto stainless steel samples which were chemically cleaned, 950 C vacuum fired, and in situ baked at...

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Heavy-Ion-Induced Electronic Desorption of Gas from Metals

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“…An initial check on the quality of the data is to test that the average deceleration rate agrees with classical Coulomb theory, since this dependence has been verified in numerous previous experiments. 9 Figure 6͑a͒ compares the measured exponential decay time obtained from fits to the neutron signal following a beam blip to the exponential decay time calculated by TRANSP. ͑The same least-squares fitting procedure is used for both ''signals.…”

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confidence: 99%

“…Experimentally, the deceleration of test populations of fast ions is properly described by classical Fokker-Planck theory to within ϳ10% in tokamak plasmas [9][10][11][12] and shows the expected functional dependence on v in a Q-machine. 13 Less is known about velocity-space diffusion, however.…”

Section: Introductionmentioning

confidence: 99%

“…18 In tokamaks, the parallel diffusion of beam ions above the injection energy is consistent with classical theory within uncertainties estimated at ϳ20%. 9 There have also been numerous fast-ion measurements that depend on the pitch-angle scattering rate and appear consistent with classical theory 9,19,20 but the uncertainties, which appear to be of order unity, have not been quantified.…”

Section: Introductionmentioning

confidence: 99%

“…Dozens of papers have compared the charge exchange spectrum during neutral beam injection with simulations; several of these resolved several different pitch angles simultaneously. 9 Two difficulties have prevented accurate quantitative tests of classical theory with these data. First, many early investigations of pitch-angle scattering were hampered by poor knowledge of the background plasma parameters.…”

Section: Introductionmentioning

confidence: 99%

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Accurate measurements of the pitch-angle scattering of beam ions

Heidbrink

2002

Physics of Plasmas

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The pitch-angle scattering rate of a dilute population of 75 keV deuterium ions is measured in a well-diagnosed, relatively quiet, magnetically-confined deuterium plasma. Neutral particle diagnostics detect the fast-ion density in velocity space following a short 10 ms pulse of injected beam ions. The data are compared to the classical theory of diffusion in velocity space caused by many, small-angle, Coulomb-scattering events. Within uncertainties of Շ15%, the data confirm the classical theory.

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On the wanderings of a quantum chemist in the world of fusion power and politics

Monkhorst

2000

Int. J. Quant. Chem.

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ABSTRACT:Ruminations by a quantum chemist are offered on his forays into the treacherous territory of the global fusion power research community. His and a colleague's proposal for a colliding beam fusion reactor is explained. His experiences hold general lessons about the treatment an innocent scientist can expect when he challenges the entrenched modus operandi of a large scientific/engineering establishment.

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The behaviour of fast ions in tokamak experiments

Cited by 468 publications

References 174 publications

Heavy-Ion-Induced Electronic Desorption of Gas from Metals

Heavy-Ion-Induced Electronic Desorption of Gas from Metals

Accurate measurements of the pitch-angle scattering of beam ions

On the wanderings of a quantum chemist in the world of fusion power and politics

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