The ISTTOK Heavy Ion Beam Diagnostic

HIBD concept Multiple cell detector (Secondary ions) Toroidal direction Primary beam detector

HIBP concept

ISTTOK magnetic configuration Plan view Toroidal field inside the coil aperture = x z

Side section ISTTOK magnetic configuration Toroidal field outside the coil aperture (x) = xcxc Position coils field

Plasma current magnetic field Asymmetric current profiles are generated by a combination of symmetric profiles For radial profiles

Beam trajectories i  x,y,z In numerical simulation equations uses constant B and E; B and E are updated on each iteration cycle (toroidal curvature negleted); Electric field are modelled freely (core and edge) – normally an inverted parabola with peak eV ~ 3/2 KTe

Cs+ 20 keV Cs++ plasma coils

Cross sectionfront view Detector arrangement

Detector configurations A, B, C

Beam attenuation MOST IMPORTANT IONIZATION REACTIONS:

Density and temperature profiles Primary beam attenuation Secondary beam generation (dl is the projection of detector cell height into the primary beam path) Secondary beam attenuation (to integrate along secondary beam path dl’)

Espécie n e (m -3 ) T e (eV) K + (E=5 keV) ~10 19 (L=2 cm)   Tl + (E=100 keV) ~10 19 (L=30 cm)   10 -3

Plasma source (Cs+) Child-Langmuir perveance condition in ISTTOK HIBD

Plasma source (Xe+/Hg+)

Determination of Beam attenuation Simplified

Simplified version A1=0 (week primary beam attenuation due to tertiary ion production) B=1 (weak secondary beam attenuation) The generation factor is related to the secondary currents by: The current at the detector cell j is obtained by integration over the element dl obtained by the projection from the detector cell height length into the primary beam path.

Detector currents per row (linha) Detector rows

The beam attenuation in ISTTOK induces only first order effects to consider on the computation of the profile of n e (0) = 1  T e (0) = 200 eVEb=22 keVI 0 =1 uA

Including the effect of attenuation on the secondary beams A1=0 (week primary beam attenuation due to tertiary ion production) B≠1 (moderate secondary beam attenuation (ISTTOK)) The difference between injected primary beam current and detected primary beam current is equal to ½ of the all secondary beam current generated along the primary beam The total secondaries current lost by ionization to terciaries is given by the difference between the initial secondaries currents generated by the primary beam and the secondaries currents detected at the detector

Determination of the corrent lost by a single secondaries’ beam For ISTTOK Expanding the exponencial and taking only linear terms becomes:

So, one could find the fraction of each secondary beam current lost to terciary ionization by the ratio to the total lost current of all secondaries’ beams:

Therefore have similar shape to Calculations show that: Primary beam and secondary beams have similar radial trajectories in the plasma Effective cross sections have similar shapes for I+  I2+ and I2+  I3+ Confirmed by calculations for several ISTTOK plasma temperature and density profiles

Experimentaly determined Replacing integrals by discret sums

A7 =

Absolute value of secondaries current at the ionization volume on the primary beam The recovery of the absolute value of the generation factor is now more acurate The remaining difference is due to ionization from primary to tertiary ions Correction: see following slides

Generation of tertiaries from the primary beam (on the same volumes of generation of secondaries) And the tertiaries generation factor can be given by

We aproximate:

Using the aproximation And obtain the tertiary currents at te ionization volume using: unknown Experimentaly determined

Estimation of the total current of the tertiaries: For a given temperature the ratio between production of secondaries and tertiaries is constant (only depends on the cross sections’ ratio) In the plasma the temperature varies along the radius, but sensivity of the currents’ ratio to temperature profile changes is low (Current ratio of terciaries to secondaries)

a) T e = 100 eV e n e = 5  m -3, b) T e = 200 eV e n e = 1  m -3 e c) T e = 300 eV e n e = 1.5  m -3. Excellent recovery of absolute values of

Example of effect of negleting atenuation factors Detected currents Using full version of algorithm A1=0 B=1 A1=0 B≠1 A1 ≠0 B≠1

Determination of electron density and electron temperature Ratio between detected currents from 2 different ionization processes on the same volume Table of effective cross sections

In ISTTOK two ion species are used Hg +  Hg 3+ and Xe +  Xe 2+ (E = 22 keV,  Hg = 32.0º,  Xe = 33.3º).

Xe +  Xe 2+, Xe +  Xe 3+ e Xe 2+  Xe 3+

Hg +  Hg 2+, Hg +  Hg 3+, Hg 2+  Hg 3+ Hg +  Hg 2+, Hg +  Hg 3+ e Hg 2+  Hg 3+ I p =6 kA.

The two beams have overlaped trajectories Detector currents

The Hg2+ currents are not detected, they can be estimated from the Xe2+ currents using the average of the ionization ratio between the limits of temperature of ISTTOK (quasi constant function with temperature) Therefore the generation of Hg2+ can now be estimated from And the Hg+ primary beam current at each ionization point given by

(  ) TOF =  TOF (t 2 ) -  TOF (t 1 ) = ½(v 0 ) -1  {e{  (l tr,t 2 ) -  (l tr,t 1 )}/E 0 } dl tr +  add Accounts for shifts on trajectories due to external and internal magnetic field changes Distinguishing 2 ns delay resolution (  /  ) TOF ~3  for  =7.2  s of time-of-flight of the beam pulse from modulator to detector. Plasma potential measurements using the primary beam

Average plasma potential measurements Absolute plasma potential measurements Plasma potential measuremenst using the secondaries K 1F + eV F = K 1P + eV P K 2P + 2eV P = K 2D + 2eV D V P = (K 2D - K 1F ) + (2V D - V F ) Energy conservation

x y z Ch1 Ch2 Ch3 Ch4 MCAD “Start”“Stop” Control module Cylindrical plates XY-alignment plates Z-plates 620 mm TOF-path module

Plasma Poloidal Field Measurements Important: In ISTTOK all ion trajectories are very close to radial

Poloidal magnetic field outside the plasma: Primary beam initial position secondary beam increment from plasma bondary to detector position Velocity of primary beam at ionization point X secondary time of flight Secondary ions average acceleration from ionization point to plasma perifery Secondary beam position at cell (1)

Recursive calculation allows to determine the accelerations at ionization point poloidal field module can be obtained from

DETECTORRESOLUTION (mm) BorderCenter A C0.60.3

EXPERIMENTAL RESULTS

Plasma current Average density - comparison with interferometer HIBD Beam off

T e, n e Plasma current for 2 discharges Radial average density for two discharges interferometer HIBD

The change in average plasma potential is -450 V, Vp

Bp

references 33. ” Time – of-flight energy analyser for the plasma potential measurements by a heavy ion beam diagnostic ” S. Nedzelskiy, A. Malaquias, B. Gon ç alves, C. Silva, C. A. F. Varandas, and J. A. C. Cabral, REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 75, NUMBER 10 OCTOBER 2004, pp in Proceedings of 15th Topical Conference High-Temperature Plasma Diagnostics, San Diego, California, April 19-22, "Developement of a new type of Cs Plasma Ion Gun for application in a heavy ion beam tokamak diagnostic" J.A.C. Cabral, O.J. Hancock, A.J.T. Holmes, M. Inman, C.M.D. Mahony, A. Malaquias, A. Praxedes, W. van Toledo and C.A.F. Varandas. Plasma Sources Science and Technology, 3, 1994, pp "The Heavy Ion Beam Diagnostic for the Tokamak ISTTOK" J.A.C. Cabral, A. Malaquias, A. Praxedes, W. van Toledo, and C.A.F. Varandas. IEEE Transations on Plasma Science, vol 22 nº4, August "The Control and Data Acquisition System for the ISTTOK Heavy Ion Beam Diagnostic" C.A.F. Varandas, A.J. Baptista, C. Coreia, A. Malaquias, J. C. Mata, A Praxedes, A. P. Rodrigues, J. Sousa, W. Toledo and J.A.C. Cabral Meas. Sci. Technol. 6 (1995) "Analysis of the ISTTOK plasma density profile evolution in sawtooth discharges by heavy-ion beam probing" J.A.C. Cabral, C.A.F. Varandas, A. Malaquias, A. Praxedes, M. P. Alonso, P. Belo, R. Can á rio, H. Fernandes, J. Ferreira, C. J. Freitas, R. Gomes, J. Pires, C. Silva, A. Soares, J. Sousa and P.H.M. Vassen Plasma Phys. Control. Fusion 38 (1996) “ Engineering Aspects of an Advanced Heavy Ion Beam Diagnostic for the TJ-II Stellarator ” A. Malaquias, C. Varandas, J.A.C. Cabral, L.I. Krupnik, S.M. Khrebtov, I.S. Nedzelskij, Yu. V. Trofimenko, A. Melnikov, C. Hidalgo, I. Garcia-Cortes Fusion Technology V.1 pp. 869, "Evolution of the poloidal magnetic field profile of the ISTTOK plasma by heavy ion beam probing" A. Malaquias, J. A. C. Cabral, C.A.F. Varandas and R. Canário Fusion Engineering and Design, (1997) “Evolution of the tokamak ISTTOK plasma density and electron temperature radial profiles determined by heavy ion beam probing” A. Malaquias, I.S. Nedzelskii, C.A.F. Varandas, and J.A.C. Cabral Review of Scientific Instruments, V70, N1, Jan 1999, Part II. Pp