Bondarenko, J.S.

Chmyga, A.A.

Dreval, M.B.

Eliseev, L.G.

Khrebtov, S.M.

Komarov, A.D.

Kozachok, A.S.

Krupnik, L.I.

Melnikov, A.V.

Nedzelskij, I.S.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

ACCURACY OF ELECTROSTATIC ENERGY ANALYZER IN HIBP DIAGNOSTICJ.S.Bondarenko, A.A. Chmyga, M.B.Dreval, L. G. Eliseev1, S.M.Khrebtov,A.D.Komarov,  A.S.Kozachok, L.I.Krupnik, A.V.Melnikov1,I.S. Nedzelskij2Institute of Plasma Physics, NSC"KIPT", 61108 Kharkov, UKRAINE1Institute of Nuclear Fusion, RRC “Kurchatov Institute”, 123182 Moscow, RUSSIA 2 Associaçao EURATOM/IST, Centro de Fusão Nuclear, IST, 1049-001 Lisboa, PORTUGALPACS: 52.70.NcI. INTRODUCTIONA heavy ion beam probe (HIBP) diagnostic is known as the only tool  for the electric potential  measurements in hot thermonuclear plasmas. It is obtained by comparison of the initial energy of the primary (single charged) ions with the energy of the secondary (double charged) ions created  in  collisions  with  the  plasma  electrons:  the difference  exactly  equals  to  the  value  of  the  plasma potential  inside  the  ionization  (sample)  volume.  The energy spectrographs used in these measurements present the most delicate part of the HIBP installation due to a very hard requirement to the accuracy of the beam energy determination which must be of the order of ∆ε/ε ~ 10-4. Fig.1. Schematic of a 30o electrostatic energy analyzer. For  the  traditional  30o Proca-Green electrostatic  energy analyzer,  schematically  shown  in  Fig.1,  the  plasma potential is calculated by the relation [1]:               φ  = 2Ua[GA(θ,ϕ) – F(θ,ϕ)δi] –Ub,                   (1)where  Ua is the potential applied to the top plate of the analyzer, Ub is the acceleration voltage, δi = (iU – iL)/(iU + iL)  is  the normalized  difference  of  the  secondary beam current  on the  upper (U) and lower  (L)  detector  plates (split-plates), and  GA(θ,ϕ) and  F(θ,ϕ) represent the gain and dynamic coefficients of the analyzer which are the functions of the analyzer geometry  and the in-plane (mid-plane) and out-plane incident angles  of  the beam (θ,ϕ) given by:            GA(θ,ϕ) = (XDtanθ - YD)/4dsin2θcos2ϕ                (2)        F(θ,ϕ) = w(sin30o + cos30otanθ)/8dsin2θ cos2ϕ     (3)(YD = YD1 +YD2)The  effects,  generally  influencing  the  accuracy  of  the measurements,  are  summarized in Fig.2,  and should be taken  into  account  during  calculations.  This  paper presents  a short  description of  some improvements  and procedures which have been elaborated and successfully applied in recent HIBP experiments on TJ-II stellarator (Spain), T-10 and TUMAN-3M tokamaks (Russia), and allow  minimizing  the  influence  of  the  most  of  these effects. Fig.2. Diagram of the effects, influencing operation of the electrostatic energy analyzer.II. IMPROVEMENTS IN ANALYZER DESIGNBecause  Eq.  (1)  was  derived  in  the  assumption  of homogeneous electric field between the analyzer plates, any disturbance will introduce an error in plasma potential determination.  Except  clear  effects  of  the  mechanical accuracies  during  analyzer  manufacturing,  the  intrinsic edge  none-uniformity  exists  and  can  strongly  influence the measurements if the trajectory of the analyzed particle inclines or shifts out of the mid-plane of the analyzer. In particular, such a situation is usual in HIBP applications on stellarators.  In early HIBP experiments it was believed that  introducing  of  the  guard  rings  with  externally distributed  potential  should  improve  the  edge  electric field  uniformity.   However,  further  experience  showed that the problem had still remained due to charging of the guard ring insulators, and not controllable change of the characteristics  of  the  resistive  divider  in  analyzer operational environment (vacuum, plasma loading). Fig.3 196                    Problems of Atomic Science and Technology. 2002. № 4. Series: Plasma Physics (7). P. 196-198shows the photo of the energy analyzer elaborated for the HIBP experiments on TJ-II.  The minimal dimension of the uniform electric field (i.e. the minimal width of the analyzer) was estimated by trajectory calculations of the secondary  ions  in  standard  regimes  of  TJ-II  operation. Rejection of the guard rings and numerical optimization of  the  analyzer  design by special  shaping of  the  plates allowed obtaining the uniformity  ∆E/E = 10-5 inside the region  occupied  by  the  trajectories.  The  additional possibility of the analyzer shift inside vacuum tank in the range  of  ±10  cm  foresees  the  spreading  of  the measurements onto the rest of extreme regimes of TJ-II. A smaller version of such an energy analyzer is used in HIBP experiments on TUMAN-3M.   Fig.3. Photo of the electrostatic energy analyzer elaborated for the HIBP experiments on TJ-II.III. CANCELLATION OF SECONDARY ELECTRON EMISSION EFFECT ON DETECTORThe actual currents on the split detector are a combination of  both  the  ion  and  electron  currents.  The  uniform secondary electron emission from the split detector does not influence analyzer operation. However, the different geometry and  surface  conditions  of  the  split-plates  and the asymmetric secondary electron exchange between the plates due to stray magnetic field of a plasma device in the  vicinity  of  the  detector  break  the  uniformity  of secondary  electron  flux  and  can  strongly  disturb  the measurements [2]. Fig.4. Schematic of the modified split-plate detector and the lines of equal potential.(1) collector plates;(2) bias box with extended suppression electrodes;(3) grounded box.Fig.4  shows  the  modified  split-plate  detector  with complete  suppression  of  the  secondary  electrons.  The appropriate  bias  geometry  has  been  simulated  by  the SIMION code. In this geometry two collector plates are enclosed inside the bias box, which has two extensions between the plates. Ua3,853,83,753,73,653,63,553,53,453,41  9 001  8 001  7 001  6 001  5 001  4 001  3 001  2 001  1 001  0 009 008 007 006 005 004 003 002 001 000Ua3,853,83 ,753,73 ,653,63,553,53 ,453,41 0 0095 090 085 080 075 070 065 060 055 050 045 040 035 030 025 020 015 010 0500Fig.5. Dynamic curves obtained with usual (left) and modified (right) split-plate detectors.Fig.5 presents the analyzer dynamic curves obtained on a test  facility  with  the  usual  (without  suppression)  and modified  (with  complete  suppression)  split-plate detectors.  The complete suppression results  in the ideal dynamic curve. Fig. 6.  Suppression of the secondary electrons created by plasma radiation.Fig.6 demonstrates the effectiveness of the new detector for  the  suppression  of  secondary  electrons  created  by plasma radiation on TUMAN-3M.IV. IMPROVEMENTS IN CALIBRATION PROCEDUREThe calibration of the energy analyzer on a test facility allows obtaining the G and F functions in a wide range of the  incident  angles.  Comparison  with  the  theoretically predicted ones gives an idea about quality of the analyzer manufacturing. In principal, the obtained data can be used in  plasma experiments  for  the  potential  calculations  by Eq. (1). However, the conditions of the analyzer operation on  a  test  facility  and  a  plasma  device  are  sufficiently different.  Therefore,  the  additional  calibration  of  the energy  analyzer  in  a  plasma  device  real  installation  is very desirable. The most straightforward way of such  in situ calibration is the use of a gas puffing into the vacuum vessel of a plasma device and the energy analysis of the secondary ions created in collisions with the gas particles. In a case of stellarators, in which the vacuum magnetic fields  are  not  strongly  disturbed  by  the  plasma,  this calibration may be considered as an absolute, simplifying the calculations of plasma potential value by the relation:                           φ  = 2UaF(δigas –δiplasma)                      (4)197 In HIBP experiments on TJ-II the additional gas puffing directly  after  plasma shot  is  used to  suppress  the post-generation  of  the  runaway  electrons.  This  procedure significantly  facilitates  the  plasma  potential measurements and increases the reliability of the data. Fig 7. The signal of secondary ions Cs2+ created both by plasma electrons and He gas in one operational cycle of  TJ-II.As the illustration, Fig.7 shows the signal  of secondary ions Cs2+ created both by the plasma electrons and He gas in  one  operational  cycle  of  TJ-II.  Fig.8  presents  the profiles of δigas and δplasma obtained in experiments on TJ-II. Fig.8. The profiles of δigas (down) and δplasma (top).V. EVALUATION OF THE IN-PLANE INCIDENT ANGLE INTO ELECTROSTATIC ENERGY ANALYZER DIRECTLY DURING PLASMA EXPERIMENTContrary to the stellarators, the calibration on a gas target is  not  so  useful  in  tokamaks  due  to  presence  of  the magnetic field created by plasma current. This magnetic field  influences  the  beam  trajectory  and,  as  a  result, changes  the  incident  angles  of  the  beam  into  energy analyzer. Eqs. (1-3) show that the resolution of the plasma potential measurements is strongly related to the accuracy of the knowledge of the incident angles of the beam θ, ϕ. The out-plane scanning of the beam by electrostatic plates arranged  just  before  the  analyzer  entrance  slit  was proposed  as  the  method  to  fix  the  out-plane  entrance angle at  ϕ = 0 [3]. The method for the evaluation of the in-plane  incident  angle  θ directly  during  plasma experiment has been elaborated recently [4]. This method is based on the addition of a second split-plate detector along  a  30o exit  axis  of  the  analyzer,  and  on  the comparison of the respective normalized beam currents δi1,  δi2.  The incident angle,  θ, is determined by a simple relation:            tanθ = [(2l/w + ∆i)/(2l/w - ∆i)]tan30o,                (5)where ∆i =  δi2 - δi1.The method has been checked on a test facility with the simplest cutting up of the detector in the mid-plane of the analyzer and shifting of the one part along a 30o axis. The incident  angle  of  the  beam was  changed  externally  by mechanical rotation of the analyzer around the entrance slit. For every incident angle the measurements have been performed in the full dynamic range of ∆i by changing the beam energy. Fig.9. The measured (θ - 30o)meas versus actual (θ - 30o)act obtained during θ = 0.1o steps rotation of the analyzer near a 30o entrance angle.Fig. 9 presents the measured (θ - 30o)meas versus actual (θ - 30o)act obtained  during  θ =  0.1o steps  rotation  of  the analyzer near a 30o entrance angle, and demonstrates the high accuracy of the method.VI. CONCLUSIONSThe  recently  elaborated  improvements  in  the  analyzer design,  complete  suppression  of  the  secondary electron emission from detector,  the use of the post-plasma-shot gas puffing for analyzer calibration, and the possibility of the  beam  incident  angles  evaluation  directly  during plasma  experiment  facilitate  the  plasma  potential measurements  by  HIBP  diagnostic  and  significantly increase  the  reliability  of  the  obtained  data.  These improvements  are  successfully  applied  in  the  current HIBP  experiments  on  TJ-II  stellarator  and  T-10, TUMAN-3M tokamaks.REFERENCES[1] L. Solensten and K.A. Connor, Rev. Sci. Instrum., 38, 516 (1987).[2] I.S. Nedzelskiy, N.B. Dreval, et al, Rev. Sci. Instrum., 72, 575 (2001).[3]  Y.  Hamada,  A.  Nishizava,  et  al,  Report  NIFS-273 (1994).[4]  I.S.  Nedzelskiy,  N.B.  Dreval  et  al,  14th Topical Conference on HTPD, Madison, USA (2002).198

Accuracy of electrostatic energy analyzer in HIBP diagnostic

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Accuracy of electrostatic energy analyzer in HIBP diagnostic

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Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)