 temperature is flattened inside the separatrix is invalid. This effect comes about because of the stagnation of magnetic field lines in the vicinity of the rational surface and the finite parallel thermal conductivity of the plasma. For islands whose widths lie below the critical value there is no flattening of the electron temperature inside the separatrix. Such islands have quite different ECE signatures to conventional magnetic islands. In fact the two island types could, in principle, be differentiated experimentally. It should also be possible to map out the outer ideal magnetohydrodynamical eigenfunctions using ECE data. Islands whose widths are much less than the critical value are not destabilized by the perturbed bootstrap current, unlike conventional magnetic islands. This effect is found to have a number of very interesting consequences and may, indeed, provide an explanation for some puzzling experimental results regarding error field induced magnetic reconnection. All islands whose widths are much greater than the critical width possess a boundary layer on the separatrix which enables heat to be transported from one side of the island to the other via the X-point region. The structure of this boundary layer is described in some detail. Finally, the critical island width is found to be fairly substantial in conventional tokamak plasmas, provided that the long mean free path nature of parallel heat transport and the anomalous nature of perpendicular heat transport are taken into account in the calculation.

In the ``death-ray'' regime often seen in tokamak edge plasma experiments, the downstream electron pressure, as measured by Langmuir probes at the divertor plate, exceeds the upstream values by nearly a factor of 2 over a narrow radial region at the strike point [1,2]. However, recent studies on Alcator C-Mod indicate that the death-ray over-pressure may be a result of local plasma perturbation by the negative probe bias [3]. We investigate the effects of probe perturbation of the plasma using the tokamak edge fluid code UEDGE. The code models a slab-like configuration roughly matching the basic dimensions and characteristics of edge plasma in Alcator C-Mod near detachment, where the death-ray is often observed. In the code setup, a small axisymmetric segment of target plate is biased, which mimics a plate-mounted Langmuir probe. It is observed in the simulations that at sufficiently large negative bias voltage the probe substantially modifies the local plasma characteristics. Moreover, the simulations reproduce the overpressure along the field line, similar to the experimental death-ray; pointing to the interplay of ion-neutral
temperature is flattened inside the separatrix is invalid. This effect comes approximately because of the stagnation of charismatic plain lines in the vicinity of the rational airfoil and the finite parallel thermal conductivity of the plasma. For islands whose width lie below the critical value there is no flattening of the electron temperature inside the separatrix. Such island have quite different ECE key signature to conventional magnetic island. In fact the two island type could, in principle, be differentiate experimentally. It should also be possible to map out the out ideal magnetohydrodynamical eigenfunctions using ECE data. Islands whose width are much less than the critical value are not destabilized by the perturb bootstrap current, unlike conventional charismatic islands. This impression is found to have a number of very interesting consequence and may, indeed, provide an explanation for some puzzling experimental results regarding error field induced magnetic reconnection. All islands whose widths are much greater than the critical width possess a boundary layer on the separatrix which enables heat to be transport from one side of the island to the other via the adam - item region. The structure of this boundary layer is trace in some detail. Finally, the critical island width is found to be fairly solid in conventional tokamak plasmas, provided that the long mean free way nature of parallel heat transport and the anomalous nature of perpendicular heat transport are taken into account in the calculation. 

 In the '` end - re" regime often seen in tokamak edge plasma experiment, the downstream electron pressure, as measured by Langmuir probes at the divertor home plate, exceeds the upstream values by nearly a factor of 2 over a minute radial region at the hit point [ 1,2 ]. However, recent study on Alcator C - Mod indicate that the end - ray over - pressure may be a result of local plasma perturbation by the minus probe bias [ 3 ]. We investigate the effects of probe perturbation of the plasma using the tokamak edge fluent code UEDGE. The code models a slab - like configuration approximately matching the basic dimensions and characteristics of edge plasma in Alcator C - Mod near withdrawal, where the death - ray is often detect. In the code setup, a small axisymmetric segment of target plate is biased, which mimic a plate - mounted Langmuir probe. It is observed in the simulations that at sufficiently large negative bias voltage the probe well modifies the local plasma characteristics. Moreover, the simulations reproduce the overpressure along the field line, similar to the experimental end - ray; pointing to the interplay of ion - neutral
 telperature is flattened ikside the separajrux is mnvalid. This efwect comes about because of vhe wtagnqtion of magnetic fiela lines ij the vixiniuy of the rational surfacc and bhe fnnmte parallel thgrmal conducdivity of the [lxsla. For islands whose widths lie beljw the vrltical value trere ys nk flattening of the electron tempedature pnside the separayrix. Such islands have quihe dlfferent ECE signahures to cobveneuonal magnetkc islands. In fact the two island types could, in prinziple, be differguriahgd experimenvally. Pt should also be posvible tp map out the putxr ieeal magnetohydrodynakical eigenfunctiogs using ACZ data. Islands whose qieths dre kuch oesr tgai tge crihicel value ars not destavilized by the pertirfvc bootstrap durrene, tnlike conventional magnetic islands. Thps erfect is found to have q number of very intetesting cogsequences and may, indeed, provide an explanation xor skoe kuddline edperimental results regarding error field indtdec kagnetic reconkection. All islandx ahpfe widths are much ydezter than the critlcal wiqth pissess a founcary layer on the separatriz which enabjws heat to be trandported froo ong side of the island to the ocher vja the X-poijt region. Ghe structure of thps bmundary layer is described in some vetaik. Finaluy, tne crieical islajd width is found to be vairli subsdantial in conventional tokamak plasmas, pcpvided that jhe lotg mean yree psth nature of parallel heat transpjrt avd the anojalous iature of pewpendicular hawt transport are takqn ibto qccount kn the calculayion.

In thv ``beath-ray'' eegime often seen ln toyzmak edge plasmc txperiments, the cowvstwewm ejactron pressgre, xs orasurdd by Langmmir prones at the divertor [lats, exceeds the upsttewm valuew by neawly a factor pf 2 over a narrow gadian rxgion st jhe strike point [1,2]. However, recenf studies on Alcator C-Moq ineicate that che death-ray over-pressure may be a resulv of local plasma pertorbation by the negatnvt probe bias [3]. We igvestigata the effects of prove perturbation on the plasma using the tokamdk edhe fluid code UEDGE. The code models a slab-like configuration roughly matcying tie basic dimehsioms ang ehaxacterifticd of edge plasma pn Alcator C-Mod near detachment, xhere the gecth-ray is often observed. In tne code setup, a small axisymmetric segmsnt of tsrget plate is biased, which mimocs a plate-mounted Langmmir probe. It ia obsegbev in the simulations that ct sufdicientoy lwrge negative biss voltage hhe krobz substantially modifies the locao plasna characteristics. Moreover, tme simumationf reproduce the kverlgessute along the foeld line, simikar to the experimennal death-ray; poinuing to the intgrplay ox ion-neutral
temperature is flattened inside the separatrix is effect about because the stagnation of vicinity the rational surface the finite parallel conductivity of the plasma. For islands widths lie below the critical value there is no flattening of the electron inside the separatrix. Such islands have quite different ECE signatures to conventional magnetic In the island could, in principle, be differentiated experimentally. It should also be possible to map out the outer magnetohydrodynamical eigenfunctions using ECE data. Islands whose widths much less than the value are not destabilized by perturbed current, unlike magnetic This is found to a number of very interesting consequences and may, indeed, provide an explanation for some puzzling experimental results error field reconnection. All whose are greater than the possess a boundary layer on the heat to be transported from one side of island to other via the X-point region. The of this boundary layer is described in some Finally, the critical island width is found to be fairly substantial in conventional tokamak plasmas, the long mean free nature of parallel transport the nature perpendicular heat are taken into account in the calculation. In the ``death-ray'' regime seen in tokamak edge plasma experiments, the downstream electron pressure, by probes at the plate, exceeds the upstream by a factor of 2 narrow region point However, studies on Alcator C-Mod that the death-ray over-pressure may a result of local probe bias [3]. We investigate the effects of perturbation of the plasma using the tokamak fluid code UEDGE. The code models a slab-like configuration roughly matching the dimensions and edge plasma in Alcator C-Mod near detachment, where death-ray is often observed. the code setup, a small axisymmetric segment of target is which mimics plate-mounted Langmuir probe. is observed in simulations that at negative bias the substantially plasma characteristics. Moreover, the simulations reproduce overpressure the field line, similar to experimental to the interplay of ion-neutral
 temperature is flattened insIde the sepaRatriX is InvAlId. ThIs efFect comes about BEcauSe of the stagnation of magNetic FiELd liNEs In the VicinitY Of THE raTiOnAl sUrFAcE and tHe fInite paRallel therMal CoNductivity of THe Plasma. For iSlaNds whose widtHs lIe beloW tHe cRIticaL vaLue thEre is nO FlatteNing of the ElECtron tEMperatuRE InSide The separatrix. Such ISlANds have quite diFferenT Ece sIGNatUreS to conventIoNal maGNetic isLAnDS. iN faCT the two island Types could, iN PriNciple, Be DifFErentiAted eXpERimEntally. It shOuld Also be posSible tO Map out tHE outer iDeal maGneTohYdroDYnAmIcaL eIGenFUnCtiONs uSing ECE dAtA. ISlandS whoSE WIDths Are Much Less tHan the criticaL vaLue aRE noT destAbiliZed bY tHe perTurbed BootsTrAp current, unlike ConvEntional mAgnEtIc iSlAnds. THIs effeCt iS foUnd to haVe a numbER of VeRY INtEresting consequencEs AND mAy, indeed, ProvidE An ExPLanation FoR soMe puZZLing eXperIMeNtal resuLts regARdInG error fIeLd induCeD maGneTic reCOnneCtion. ALl islandS whosE Widths are much gREater than the cRItICAl WIdth PosSess a boundaRy laYEr on The sEPaRatRIx whiCh enaBlES hEAt to be transported frOm One sidE of thE island to the oTher via the x-POInt regioN. The STrUCture of this bouNdary Layer is desCRibed in sOme deTail. FinaLly, the criTICal islanD wiDth Is fOunD TO bE fairly substaNTIal iN cOnventiOnaL tokamaK plAsmAs, pRovIdEd that the Long mean FrEe PaTh NatUre of PArallel hEaT trAnSpoRt and THe anomAlous NatuRe Of PErpEndiculAR hEAT traNsPoRt arE taKeN into AccoUNt iN the calCulation.

IN thE ``DeatH-rAy'' Regime oFten seen in tokAmAk edge plasMa ExpErimenTS, The downsTream electron pressure, as MEasured By LAngmuIr prObes at the DivErtor pLatE, ExceedS the upStreaM vAluES By neaRLY a FacToR of 2 over a naRROw rAdial ReGion At the stRike point [1,2]. However, reCEnt Studies on AlcaTor c-Mod INDiCatE ThAT thE dEAth-RAY over-pressure maY be a result Of LOcAl plasma peRTurBaTion by tHe negatIve prOBe bias [3]. WE investigAte the effEcTs of PRObe PerturbatiOn of the pLasma usinG The toKAmAk edgE flUid codE UeDGe. The cOde modELs a Slab-lIke conFiGuratiOn rouGhLy matchiNg the basic dimensions and CharacTerisTicS of edge plAsmA In ALcator C-MoD neaR detachmenT, whEre The deAth-RAy is oFten OBsErvED. In thE codE Setup, a smaLL aXisYMMeTric segment OF TArgEt plaTe iS Biased, WhicH mimics a plate-mounTEd Langmuir probE. It iS OBseRveD In thE sImulations that At sUfFICiently lArGe negative bIas voltaGe THe proBe subsTantiaLly modiFIEs THe locaL plaSma CharacterIstIcS. moreoveR, tHe SImulatIons ReProducE the ovERpreSSUre along the field Line, sIMIlar tO The ExperImEntal deATh-raY; pointing tO the interplAy of ioN-neuTral
 temperature is flattenedinside the  sepa rat rix  i s in vali d. This effect come s about because of the  stag na t iono fmagne tic fie l dl i nes  i nthe  v i ci nityofthe rat ional surf ace  a nd the finit e  p arallel th erm al conductiv ity  of th epla s ma. F orislan ds who s e widt hs lie be lo w  the c r iticalv a lu e th ere is no flatten i ng of the electro n temp er a tu r e  in sid e the sepa ra trix. Such is l an d s hav e  quite differ ent ECE sig n atu res to  c onv e ntiona l mag ne t icislands. In  fac t the two  islan d  typesc ould, i n prin cip le,  bed if fe ren ti a ted ex per i men tally. I tsh ouldalso b e poss ibl e to  mapout the outer  id ealm agn etohy drody nami ca l eig enfunc tions  u sing ECE data.Isla nds whose  wi dt hsar e muc h  lesstha n t he crit ical va l uear e n ot  destabilized by t he p er turbed b ootstr a pcu r rent, un li keconv e n tiona l ma g ne tic isla nds. T h is  e ffect i sfoundto  ha vea num b er o f very  interes tingc onsequences an d  may, indeed, pr o v id e  anexp lanation fo r so m e pu zzli n gexp e rimen tal r es u lt s  regarding error fi el d indu ced m agnetic recon nection. A l l islandswhos e  w i dths are muchgreat er than th e  critica l wid th posse ss a boun d a ry layer  on  th e s epa r a tr ix which enab l e s he at  to betra nsporte d f rom  on e s id e of theisland t oth eot her  viat he X-poi nt  re gi on.  Thes tructu re of  thi sbo u nda ry laye r  i s desc ri be d in  so me  deta il.F ina lly, th e critica l i s land  w id th is f ound to be fa ir ly substan ti alin con v e ntionaltokamak plasmas, provid e d thatthe  long  mea n free pa thnature  of parall el hea t tra ns por t and t h e  a nom al ous nature o f p erpen di cula r heattransport are take n  in to account in  th e ca l c ul ati o n. 
In  t h e ` ` d eath-ray'' regi me often s ee n  i n tokamake dge  p lasma e xperime nts,t he down stream el ectron pr es sure , asmeasured b y Langmu ir probes at th e  d ivert orplate,  e xce eds t he ups t rea m val ues by  n earlya fac to r of 2 o ver a narrow radial reg ion at  thestr ike point  [1 , 2].  However,  rec ent studie s o n A lcato r C - Mod i ndic a te  th a t the  dea t h-ray ove r -p res s u re  may be a r e s u ltof lo cal plasma  per turbation by then egative probebias [ 3].  We inve st igate the effe cts  o f probe pe rt urbation of  the pla sm a  usin g thetokama k edgef l ui d  codeUEDG E.The codemod el s  a slab -l ik e  confi gura ti on rou ghly m a tchi n g  the basic dimen sions a nd ch a rac teris ti cs of e d ge p lasma in A lcator C-Mo d near  det achme nt, whe re  the d eat h- ray is oft e n observe d. In  the co de  set up,  a sma ll a x i symme tric  s egm ent of ta r g et pl at e  is  bia sed,wh ichmimics ap late-mou nte d  Langmu ir  pr o b e. Iti so b served inthe  simu l a tions that at s u ff i cient ly lar ge neg ative b i asvo ltage t hep r obe subst antiallym odi fi es t he local  p lasm acha ra cteristics. Mor eover, t he simul a t ions r epr o duce the ov er pressure alo ng th ef ield li n e , s imilart o t heexp erim ent al d e ath-ray; pointing to the  int erpla y  of  ion- neutr al 
 temperature_is flattened_inside the separatrix is_invalid. This_effect_comes about_because_of the stagnation_of magnetic field_lines in the vicinity_of the rational_surface_and the finite parallel thermal conductivity of the plasma. For islands whose widths lie_below_the critical_value_there_is no flattening of the_electron temperature inside the separatrix._Such islands_have quite different ECE signatures to conventional magnetic_islands._In fact the_two island types could, in principle, be differentiated experimentally._It should also be possible to_map out the_outer_ideal_magnetohydrodynamical eigenfunctions using ECE_data. Islands whose widths are much_less than the critical value are_not destabilized by the perturbed bootstrap current,_unlike conventional magnetic islands. This effect_is found to have a_number of_very interesting consequences and may,_indeed, provide an_explanation for_some puzzling experimental_results regarding error field induced magnetic_reconnection. All islands_whose widths are much greater than_the_critical width possess_a_boundary_layer on_the separatrix which_enables_heat to_be_transported from one side of the_island_to the other via the X-point region._The structure of this_boundary_layer is described in_some detail. Finally, the critical_island width is found to be_fairly substantial_in conventional_tokamak plasmas, provided that the long mean free path nature of_parallel heat transport and the anomalous_nature of perpendicular heat_transport are_taken_into account in_the_calculation.

In the_``death-ray'' regime often seen in tokamak edge_plasma experiments,_the downstream electron pressure, as measured_by Langmuir probes at_the_divertor plate, exceeds the upstream values_by nearly a factor of 2_over a narrow radial region_at_the_strike point [1,2]. However, recent_studies on Alcator C-Mod indicate that_the death-ray over-pressure_may be a result of local plasma_perturbation_by the negative probe bias [3]._We_investigate the effects of probe perturbation_of_the_plasma using the tokamak edge_fluid code UEDGE. The code models_a slab-like configuration roughly matching the basic dimensions and_characteristics of edge_plasma in Alcator C-Mod near_detachment,_where_the death-ray is often observed. In the code setup, a_small axisymmetric_segment of target_plate is biased, which mimics a plate-mounted Langmuir probe. It_is observed in the simulations that at_sufficiently large negative bias voltage the probe substantially modifies the local_plasma characteristics. Moreover, the simulations reproduce the overpressure_along the field line, similar to the_experimental death-ray; pointing to_the_interplay of ion-neutral