#
57th Annual Meeting of the APS Division of Plasma Physics

## Volume 60, Number 19

##
Monday–Friday, November 16–20, 2015;
Savannah, Georgia

### Session GP12: Poster Session III (Plasma Accelerators and Radiation: NSTX-U, ST, and International MFE)

Tuesday, November 17, 2015

Room: Exhibit Hall A

Abstract ID: BAPS.2015.DPP.GP12.117

### Abstract: GP12.00117 : Physics of Plasma Cathode Current Injection During LHI*

Preview Abstract
Abstract

####
Authors:

E.T. Hinson

(UW-Madison)

J. Barr

(UW-Madison)

M. Bongard

(UW-Madison)

M.G. Burke

(UW-Madison)

R. Fonck

(UW-Madison)

J. Perry

(UW-Madison)

Localized helicity injection (LHI) ST startup employs current sources at the
tokamak edge. Max $I_{\mathrm{p}}$ in LHI scales with injection voltage
$V_{\mathrm{inj}}$, requiring an understanding of injector impedance. For the
arc-plasma cathode electron injectors in Pegasus, impedance is
plasma-determined, and typically $V_{\mathrm{inj}}$\textgreater 1kV for
$I_{\mathrm{inj}}=$2kA. At low $I_{\mathrm{inj}}$,
$I_{\mathrm{inj}}\propto V_{\mathrm{inj}}^{\mathrm{3/2}}$, an indication
of a double layer (DL) common to such devices. However, at
$I_{\mathrm{inj}}$\textgreater $\sim $1kA, $I_{\mathrm{inj}}\propto
V_{\mathrm{inj}}^{\mathrm{1/2}}$ occurs, a scaling expected for limited
launched beam density, $n_{b} \equiv {I_{inj} } \mathord{\left/ {\vphantom
{{I_{inj} } {(e\sqrt {{2eV_{inj} } \mathord{\left/ {\vphantom {{2eV_{inj} }
{m_{e} }}} \right. \kern-\nulldelimiterspace} {m_{e} }} A_{inj} )}}} \right.
\kern-\nulldelimiterspace} {(e\sqrt {{2eV_{inj} } \mathord{\left/ {\vphantom
{{2eV_{inj} } {m_{e} }}} \right. \kern-\nulldelimiterspace} {m_{e} }}
A_{inj} )}\sim I_{\mathrm{inj}}$/$V_{\mathrm{inj}}^{\mathrm{1/2}}$. An
ohmic discharge injection target was created to test this hypothesis.
Langmuir probe data showed
$I_{\mathrm{inj}}$/$V_{\mathrm{inj}}^{\mathrm{1/2}}\propto
n_{\mathrm{edge}}$ at low $n_{\mathrm{edge}}$, consistent with a limit
($n_{\mathrm{edge}}\ge n_{\mathrm{e,b}})$ imposed by quasineutrality. If
edge fueling maintained $n_{\mathrm{edge}}\ge n_{\mathrm{e,b}}$,
spectroscopic measurements of source density $n_{\mathrm{arc}}$ indicated
$I_{\mathrm{inj}}$/$V_{\mathrm{inj}}^{\mathrm{1/2}}\propto
n_{\mathrm{arc}}$, as expected from DL expansion. Thus $n_{\mathrm{b}}$
established by $n_{\mathrm{arc}}$ or $n_{\mathrm{edge}}$ determines
$V_{\mathrm{inj}}$ up to the onset of cathode spot (CS) arcing. Technology
development has increased obtainable V$_{\mathrm{inj}}$ and reduced CS
damage using new ring shielding and a cathode design drawing CS's away from
insulators. This involved a novel optimization of conical frustum geometry.
Finally, consistent with NIMROD predictions of coherent streams in the edge
during LHI, pairwise triangulation of outboard Mirnov data assuming beam
m$=$1 motion has allowed an estimate of beam R(t), Z(t) location that is
near the injector R, and consistent across the array.

*Supported by U.S. DOE Grant DE-FG02-96ER54375

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2015.DPP.GP12.117